=head1 NAME

AnyEvent::Handle - non-blocking I/O on streaming handles via AnyEvent

=head1 SYNOPSIS

   use AnyEvent;
   use AnyEvent::Handle;

   my $cv = AnyEvent->condvar;

   my $hdl; $hdl = new AnyEvent::Handle
      fh => \*STDIN,
      on_error => sub {
         my ($hdl, $fatal, $msg) = @_;
         AE::log error => $msg;
         $hdl->destroy;
         $cv->send;
      };

   # send some request line
   $hdl->push_write ("getinfo\015\012");

   # read the response line
   $hdl->push_read (line => sub {
      my ($hdl, $line) = @_;
      say "got line <$line>";
      $cv->send;
   });

   $cv->recv;

=head1 DESCRIPTION

This is a helper module to make it easier to do event-based I/O
on stream-based filehandles (sockets, pipes, and other stream
things). Specifically, it doesn't work as expected on files, packet-based
sockets or similar things.

The L<AnyEvent::Intro> tutorial contains some well-documented
AnyEvent::Handle examples.

In the following, where the documentation refers to "bytes", it means
characters. As sysread and syswrite are used for all I/O, their
treatment of characters applies to this module as well.

At the very minimum, you should specify C<fh> or C<connect>, and the
C<on_error> callback.

All callbacks will be invoked with the handle object as their first
argument.

=cut

package AnyEvent::Handle;

use Scalar::Util ();
use List::Util ();
use Carp ();
use Errno qw(EAGAIN EWOULDBLOCK EINTR);

use AnyEvent (); BEGIN { AnyEvent::common_sense }
use AnyEvent::Util qw(WSAEWOULDBLOCK);

our $VERSION = $AnyEvent::VERSION;

sub _load_func($) {
   my $func = $_[0];

   unless (defined &$func) {
      my $pkg = $func;
      do {
         $pkg =~ s/::[^:]+$//
            or return;
         eval "require $pkg";
      } until defined &$func;
   }

   \&$func
}

sub MAX_READ_SIZE() { 131072 }

=head1 METHODS

=over 4

=item $handle = B<new> AnyEvent::Handle fh => $filehandle, key => value...

The constructor supports these arguments (all as C<< key => value >> pairs).

=over 4

=item fh => $filehandle     [C<fh> or C<connect> MANDATORY]

The filehandle this L<AnyEvent::Handle> object will operate on.
NOTE: The filehandle will be set to non-blocking mode (using
C<AnyEvent::fh_unblock>) by the constructor and needs to stay in
that mode.

=item connect => [$host, $service]      [C<fh> or C<connect> MANDATORY]

Try to connect to the specified host and service (port), using
C<AnyEvent::Socket::tcp_connect>. The C<$host> additionally becomes the
default C<peername>.

You have to specify either this parameter, or C<fh>, above.

It is possible to push requests on the read and write queues, and modify
properties of the stream, even while AnyEvent::Handle is connecting.

When this parameter is specified, then the C<on_prepare>,
C<on_connect_error> and C<on_connect> callbacks will be called under the
appropriate circumstances:

=over 4

=item on_prepare => $cb->($handle)

This (rarely used) callback is called before a new connection is
attempted, but after the file handle has been created (you can access that
file handle via C<< $handle->{fh} >>). It could be used to prepare the
file handle with parameters required for the actual connect (as opposed to
settings that can be changed when the connection is already established).

The return value of this callback should be the connect timeout value in
seconds (or C<0>, or C<undef>, or the empty list, to indicate that the
default timeout is to be used).

=item on_connect => $cb->($handle, $host, $port, $retry->())

This callback is called when a connection has been successfully established.

The peer's numeric host and port (the socket peername) are passed as
parameters, together with a retry callback. At the time it is called the
read and write queues, EOF status, TLS status and similar properties of
the handle will have been reset.

If, for some reason, the handle is not acceptable, calling C<$retry> will
continue with the next connection target (in case of multi-homed hosts or
SRV records there can be multiple connection endpoints). The C<$retry>
callback can be invoked after the connect callback returns, i.e. one can
start a handshake and then decide to retry with the next host if the
handshake fails.

In most cases, you should ignore the C<$retry> parameter.

=item on_connect_error => $cb->($handle, $message)

This callback is called when the connection could not be
established. C<$!> will contain the relevant error code, and C<$message> a
message describing it (usually the same as C<"$!">).

If this callback isn't specified, then C<on_error> will be called with a
fatal error instead.

=back

=item on_error => $cb->($handle, $fatal, $message)

This is the error callback, which is called when, well, some error
occured, such as not being able to resolve the hostname, failure to
connect, or a read error.

Some errors are fatal (which is indicated by C<$fatal> being true). On
fatal errors the handle object will be destroyed (by a call to C<< ->
destroy >>) after invoking the error callback (which means you are free to
examine the handle object). Examples of fatal errors are an EOF condition
with active (but unsatisfiable) read watchers (C<EPIPE>) or I/O errors. In
cases where the other side can close the connection at will, it is
often easiest to not report C<EPIPE> errors in this callback.

AnyEvent::Handle tries to find an appropriate error code for you to check
against, but in some cases (TLS errors), this does not work well.

If you report the error to the user, it is recommended to always output
the C<$message> argument in human-readable error messages (you don't need
to report C<"$!"> if you report C<$message>).

If you want to react programmatically to the error, then looking at C<$!>
and comparing it against some of the documented C<Errno> values is usually
better than looking at the C<$message>.

Non-fatal errors can be retried by returning, but it is recommended
to simply ignore this parameter and instead abondon the handle object
when this callback is invoked. Examples of non-fatal errors are timeouts
C<ETIMEDOUT>) or badly-formatted data (C<EBADMSG>).

On entry to the callback, the value of C<$!> contains the operating
system error code (or C<ENOSPC>, C<EPIPE>, C<ETIMEDOUT>, C<EBADMSG> or
C<EPROTO>).

While not mandatory, it is I<highly> recommended to set this callback, as
you will not be notified of errors otherwise. The default just calls
C<croak>.

=item on_read => $cb->($handle)

This sets the default read callback, which is called when data arrives
and no read request is in the queue (unlike read queue callbacks, this
callback will only be called when at least one octet of data is in the
read buffer).

To access (and remove data from) the read buffer, use the C<< ->rbuf >>
method or access the C<< $handle->{rbuf} >> member directly. Note that you
must not enlarge or modify the read buffer, you can only remove data at
the beginning from it.

You can also call C<< ->push_read (...) >> or any other function that
modifies the read queue. Or do both. Or ...

When an EOF condition is detected, AnyEvent::Handle will first try to
feed all the remaining data to the queued callbacks and C<on_read> before
calling the C<on_eof> callback. If no progress can be made, then a fatal
error will be raised (with C<$!> set to C<EPIPE>).

Note that, unlike requests in the read queue, an C<on_read> callback
doesn't mean you I<require> some data: if there is an EOF and there
are outstanding read requests then an error will be flagged. With an
C<on_read> callback, the C<on_eof> callback will be invoked.

=item on_eof => $cb->($handle)

Set the callback to be called when an end-of-file condition is detected,
i.e. in the case of a socket, when the other side has closed the
connection cleanly, and there are no outstanding read requests in the
queue (if there are read requests, then an EOF counts as an unexpected
connection close and will be flagged as an error).

For sockets, this just means that the other side has stopped sending data,
you can still try to write data, and, in fact, one can return from the EOF
callback and continue writing data, as only the read part has been shut
down.

If an EOF condition has been detected but no C<on_eof> callback has been
set, then a fatal error will be raised with C<$!> set to <0>.

=item on_drain => $cb->($handle)

This sets the callback that is called once when the write buffer becomes
empty (and immediately when the handle object is created).

To append to the write buffer, use the C<< ->push_write >> method.

This callback is useful when you don't want to put all of your write data
into the queue at once, for example, when you want to write the contents
of some file to the socket you might not want to read the whole file into
memory and push it into the queue, but instead only read more data from
the file when the write queue becomes empty.

=item timeout => $fractional_seconds

=item rtimeout => $fractional_seconds

=item wtimeout => $fractional_seconds

If non-zero, then these enables an "inactivity" timeout: whenever this
many seconds pass without a successful read or write on the underlying
file handle (or a call to C<timeout_reset>), the C<on_timeout> callback
will be invoked (and if that one is missing, a non-fatal C<ETIMEDOUT>
error will be raised).

There are three variants of the timeouts that work independently of each
other, for both read and write (triggered when nothing was read I<OR>
written), just read (triggered when nothing was read), and just write:
C<timeout>, C<rtimeout> and C<wtimeout>, with corresponding callbacks
C<on_timeout>, C<on_rtimeout> and C<on_wtimeout>, and reset functions
C<timeout_reset>, C<rtimeout_reset>, and C<wtimeout_reset>.

Note that timeout processing is active even when you do not have any
outstanding read or write requests: If you plan to keep the connection
idle then you should disable the timeout temporarily or ignore the
timeout in the corresponding C<on_timeout> callback, in which case
AnyEvent::Handle will simply restart the timeout.

Zero (the default) disables the corresponding timeout.

=item on_timeout => $cb->($handle)

=item on_rtimeout => $cb->($handle)

=item on_wtimeout => $cb->($handle)

Called whenever the inactivity timeout passes. If you return from this
callback, then the timeout will be reset as if some activity had happened,
so this condition is not fatal in any way.

=item rbuf_max => <bytes>

If defined, then a fatal error will be raised (with C<$!> set to C<ENOSPC>)
when the read buffer ever (strictly) exceeds this size. This is useful to
avoid some forms of denial-of-service attacks.

For example, a server accepting connections from untrusted sources should
be configured to accept only so-and-so much data that it cannot act on
(for example, when expecting a line, an attacker could send an unlimited
amount of data without a callback ever being called as long as the line
isn't finished).

=item wbuf_max => <bytes>

If defined, then a fatal error will be raised (with C<$!> set to C<ENOSPC>)
when the write buffer ever (strictly) exceeds this size. This is useful to
avoid some forms of denial-of-service attacks.

Although the units of this parameter is bytes, this is the I<raw> number
of bytes not yet accepted by the kernel. This can make a difference when
you e.g. use TLS, as TLS typically makes your write data larger (but it
can also make it smaller due to compression).

As an example of when this limit is useful, take a chat server that sends
chat messages to a client. If the client does not read those in a timely
manner then the send buffer in the server would grow unbounded.

=item autocork => <boolean>

When disabled (the default), C<push_write> will try to immediately
write the data to the handle if possible. This avoids having to register
a write watcher and wait for the next event loop iteration, but can
be inefficient if you write multiple small chunks (on the wire, this
disadvantage is usually avoided by your kernel's nagle algorithm, see
C<no_delay>, but this option can save costly syscalls).

When enabled, writes will always be queued till the next event loop
iteration. This is efficient when you do many small writes per iteration,
but less efficient when you do a single write only per iteration (or when
the write buffer often is full). It also increases write latency.

=item no_delay => <boolean>

When doing small writes on sockets, your operating system kernel might
wait a bit for more data before actually sending it out. This is called
the Nagle algorithm, and usually it is beneficial.

In some situations you want as low a delay as possible, which can be
accomplishd by setting this option to a true value.

The default is your operating system's default behaviour (most likely
enabled). This option explicitly enables or disables it, if possible.

=item keepalive => <boolean>

Enables (default disable) the SO_KEEPALIVE option on the stream socket:
normally, TCP connections have no time-out once established, so TCP
connections, once established, can stay alive forever even when the other
side has long gone. TCP keepalives are a cheap way to take down long-lived
TCP connections when the other side becomes unreachable. While the default
is OS-dependent, TCP keepalives usually kick in after around two hours,
and, if the other side doesn't reply, take down the TCP connection some 10
to 15 minutes later.

It is harmless to specify this option for file handles that do not support
keepalives, and enabling it on connections that are potentially long-lived
is usually a good idea.

=item oobinline => <boolean>

BSD majorly fucked up the implementation of TCP urgent data. The result
is that almost no OS implements TCP according to the specs, and every OS
implements it slightly differently.

If you want to handle TCP urgent data, then setting this flag (the default
is enabled) gives you the most portable way of getting urgent data, by
putting it into the stream.

Since BSD emulation of OOB data on top of TCP's urgent data can have
security implications, AnyEvent::Handle sets this flag automatically
unless explicitly specified. Note that setting this flag after
establishing a connection I<may> be a bit too late (data loss could
already have occured on BSD systems), but at least it will protect you
from most attacks.

=item read_size => <bytes>

The initial read block size, the number of bytes this module will try
to read during each loop iteration. Each handle object will consume
at least this amount of memory for the read buffer as well, so when
handling many connections watch out for memory requirements). See also
C<max_read_size>. Default: C<2048>.

=item max_read_size => <bytes>

The maximum read buffer size used by the dynamic adjustment
algorithm: Each time AnyEvent::Handle can read C<read_size> bytes in
one go it will double C<read_size> up to the maximum given by this
option. Default: C<131072> or C<read_size>, whichever is higher.

=item low_water_mark => <bytes>

Sets the number of bytes (default: C<0>) that make up an "empty" write
buffer: If the buffer reaches this size or gets even samller it is
considered empty.

Sometimes it can be beneficial (for performance reasons) to add data to
the write buffer before it is fully drained, but this is a rare case, as
the operating system kernel usually buffers data as well, so the default
is good in almost all cases.

=item linger => <seconds>

If this is non-zero (default: C<3600>), the destructor of the
AnyEvent::Handle object will check whether there is still outstanding
write data and will install a watcher that will write this data to the
socket. No errors will be reported (this mostly matches how the operating
system treats outstanding data at socket close time).

This will not work for partial TLS data that could not be encoded
yet. This data will be lost. Calling the C<stoptls> method in time might
help.

=item peername => $string

A string used to identify the remote site - usually the DNS hostname
(I<not> IDN!) used to create the connection, rarely the IP address.

Apart from being useful in error messages, this string is also used in TLS
peername verification (see C<verify_peername> in L<AnyEvent::TLS>). This
verification will be skipped when C<peername> is not specified or is
C<undef>.

=item tls => "accept" | "connect" | Net::SSLeay::SSL object

When this parameter is given, it enables TLS (SSL) mode, that means
AnyEvent will start a TLS handshake as soon as the connection has been
established and will transparently encrypt/decrypt data afterwards.

All TLS protocol errors will be signalled as C<EPROTO>, with an
appropriate error message.

TLS mode requires Net::SSLeay to be installed (it will be loaded
automatically when you try to create a TLS handle): this module doesn't
have a dependency on that module, so if your module requires it, you have
to add the dependency yourself. If Net::SSLeay cannot be loaded or is too
old, you get an C<EPROTO> error.

Unlike TCP, TLS has a server and client side: for the TLS server side, use
C<accept>, and for the TLS client side of a connection, use C<connect>
mode.

You can also provide your own TLS connection object, but you have
to make sure that you call either C<Net::SSLeay::set_connect_state>
or C<Net::SSLeay::set_accept_state> on it before you pass it to
AnyEvent::Handle. Also, this module will take ownership of this connection
object.

At some future point, AnyEvent::Handle might switch to another TLS
implementation, then the option to use your own session object will go
away.

B<IMPORTANT:> since Net::SSLeay "objects" are really only integers,
passing in the wrong integer will lead to certain crash. This most often
happens when one uses a stylish C<< tls => 1 >> and is surprised about the
segmentation fault.

Use the C<< ->starttls >> method if you need to start TLS negotiation later.

=item tls_ctx => $anyevent_tls

Use the given C<AnyEvent::TLS> object to create the new TLS connection
(unless a connection object was specified directly). If this
parameter is missing (or C<undef>), then AnyEvent::Handle will use
C<AnyEvent::Handle::TLS_CTX>.

Instead of an object, you can also specify a hash reference with C<< key
=> value >> pairs. Those will be passed to L<AnyEvent::TLS> to create a
new TLS context object.

=item on_starttls => $cb->($handle, $success[, $error_message])

This callback will be invoked when the TLS/SSL handshake has finished. If
C<$success> is true, then the TLS handshake succeeded, otherwise it failed
(C<on_stoptls> will not be called in this case).

The session in C<< $handle->{tls} >> can still be examined in this
callback, even when the handshake was not successful.

TLS handshake failures will not cause C<on_error> to be invoked when this
callback is in effect, instead, the error message will be passed to C<on_starttls>.

Without this callback, handshake failures lead to C<on_error> being
called as usual.

Note that you cannot just call C<starttls> again in this callback. If you
need to do that, start an zero-second timer instead whose callback can
then call C<< ->starttls >> again.

=item on_stoptls => $cb->($handle)

When a SSLv3/TLS shutdown/close notify/EOF is detected and this callback is
set, then it will be invoked after freeing the TLS session. If it is not,
then a TLS shutdown condition will be treated like a normal EOF condition
on the handle.

The session in C<< $handle->{tls} >> can still be examined in this
callback.

This callback will only be called on TLS shutdowns, not when the
underlying handle signals EOF.

=item json => L<JSON>, L<JSON::PP> or L<JSON::XS> object

This is the json coder object used by the C<json> read and write types.

If you don't supply it, then AnyEvent::Handle will create and use a
suitable one (on demand), which will write and expect UTF-8 encoded
JSON texts (either using L<JSON::XS> or L<JSON>). The written texts are
guaranteed not to contain any newline character.

For security reasons, this encoder will likely I<not> handle numbers and
strings, only arrays and objects/hashes. The reason is that originally
JSON was self-delimited, but Dougles Crockford thought it was a splendid
idea to redefine JSON incompatibly, so this is no longer true.

For protocols that used back-to-back JSON texts, this might lead to
run-ins, where two or more JSON texts will be interpreted as one JSON
text.

For this reason, if the default encoder uses L<JSON::XS>, it will default
to not allowing anything but arrays and objects/hashes, at least for the
forseeable future (it will change at some point). This might or might not
be true for the L<JSON> module, so this might cause a security issue.

If you depend on either behaviour, you should create your own json object
and pass it in explicitly.

=item cbor => L<CBOR::XS> object

This is the cbor coder object used by the C<cbor> read and write types.

If you don't supply it, then AnyEvent::Handle will create and use a
suitable one (on demand), which will write CBOR without using extensions,
if possible.

Note that you are responsible to depend on the L<CBOR::XS> module if you
want to use this functionality, as AnyEvent does not have a dependency on
it itself.

=back

=cut

sub new {
   my $class = shift;
   my $self = bless { @_ }, $class;

   if ($self->{fh}) {
      $self->_start;
      return unless $self->{fh}; # could be gone by now

   } elsif ($self->{connect}) {
      require AnyEvent::Socket;

      $self->{peername} = $self->{connect}[0]
         unless exists $self->{peername};

      $self->{_skip_drain_rbuf} = 1;

      {
         Scalar::Util::weaken (my $self = $self);

         $self->{_connect} =
            AnyEvent::Socket::tcp_connect (
               $self->{connect}[0],
               $self->{connect}[1],
               sub {
                  my ($fh, $host, $port, $retry) = @_;

                  delete $self->{_connect}; # no longer needed

                  if ($fh) {
                     $self->{fh} = $fh;

                     delete $self->{_skip_drain_rbuf};
                     $self->_start;

                     $self->{on_connect}
                        and $self->{on_connect}($self, $host, $port, sub {
                               delete @$self{qw(fh _tw _rtw _wtw _ww _rw _eof _queue rbuf _wbuf tls _tls_rbuf _tls_wbuf)};
                               $self->{_skip_drain_rbuf} = 1;
                               &$retry;
                            });

                  } else {
                     if ($self->{on_connect_error}) {
                        $self->{on_connect_error}($self, "$!");
                        $self->destroy if $self;
                     } else {
                        $self->_error ($!, 1);
                     }
                  }
               },
               sub {
                  local $self->{fh} = $_[0];

                  $self->{on_prepare}
                     ? $self->{on_prepare}->($self)
                     : ()
               }
            );
      }

   } else {
      Carp::croak "AnyEvent::Handle: either an existing fh or the connect parameter must be specified";
   }

   $self
}

sub _start {
   my ($self) = @_;

   # too many clueless people try to use udp and similar sockets
   # with AnyEvent::Handle, do them a favour.
   my $type = getsockopt $self->{fh}, Socket::SOL_SOCKET (), Socket::SO_TYPE ();
   Carp::croak "AnyEvent::Handle: only stream sockets supported, anything else will NOT work!"
      if Socket::SOCK_STREAM () != (unpack "I", $type) && defined $type;

   AnyEvent::fh_unblock $self->{fh};

   $self->{_activity}  =
   $self->{_ractivity} =
   $self->{_wactivity} = AE::now;

   $self->{read_size} ||= 2048;
   $self->{max_read_size} = $self->{read_size}
      if $self->{read_size} > ($self->{max_read_size} || MAX_READ_SIZE);

   $self->timeout   (delete $self->{timeout}  ) if $self->{timeout};
   $self->rtimeout  (delete $self->{rtimeout} ) if $self->{rtimeout};
   $self->wtimeout  (delete $self->{wtimeout} ) if $self->{wtimeout};

   $self->no_delay  (delete $self->{no_delay} ) if exists $self->{no_delay}  && $self->{no_delay};
   $self->keepalive (delete $self->{keepalive}) if exists $self->{keepalive} && $self->{keepalive};

   $self->oobinline (exists $self->{oobinline} ? delete $self->{oobinline} : 1);

   $self->starttls  (delete $self->{tls}, delete $self->{tls_ctx})
      if $self->{tls};

   $self->on_drain  (delete $self->{on_drain} ) if $self->{on_drain};

   $self->start_read
      if $self->{on_read} || @{ $self->{_queue} };

   $self->_drain_wbuf;
}

sub _error {
   my ($self, $errno, $fatal, $message) = @_;

   $! = $errno;
   $message ||= "$!";

   if ($self->{on_error}) {
      $self->{on_error}($self, $fatal, $message);
      $self->destroy if $fatal;
   } elsif ($self->{fh} || $self->{connect}) {
      $self->destroy;
      Carp::croak "AnyEvent::Handle uncaught error: $message";
   }
}

=item $fh = $handle->fh

This method returns the file handle used to create the L<AnyEvent::Handle> object.

=cut

sub fh { $_[0]{fh} }

=item $handle->on_error ($cb)

Replace the current C<on_error> callback (see the C<on_error> constructor argument).

=cut

sub on_error {
   $_[0]{on_error} = $_[1];
}

=item $handle->on_eof ($cb)

Replace the current C<on_eof> callback (see the C<on_eof> constructor argument).

=cut

sub on_eof {
   $_[0]{on_eof} = $_[1];
}

=item $handle->on_timeout ($cb)

=item $handle->on_rtimeout ($cb)

=item $handle->on_wtimeout ($cb)

Replace the current C<on_timeout>, C<on_rtimeout> or C<on_wtimeout>
callback, or disables the callback (but not the timeout) if C<$cb> =
C<undef>. See the C<timeout> constructor argument and method.

=cut

# see below

=item $handle->autocork ($boolean)

Enables or disables the current autocork behaviour (see C<autocork>
constructor argument). Changes will only take effect on the next write.

=cut

sub autocork {
   $_[0]{autocork} = $_[1];
}

=item $handle->no_delay ($boolean)

Enables or disables the C<no_delay> setting (see constructor argument of
the same name for details).

=cut

sub no_delay {
   $_[0]{no_delay} = $_[1];

   setsockopt $_[0]{fh}, Socket::IPPROTO_TCP (), Socket::TCP_NODELAY (), int $_[1]
      if $_[0]{fh};
}

=item $handle->keepalive ($boolean)

Enables or disables the C<keepalive> setting (see constructor argument of
the same name for details).

=cut

sub keepalive {
   $_[0]{keepalive} = $_[1];

   eval {
      local $SIG{__DIE__};
      setsockopt $_[0]{fh}, Socket::SOL_SOCKET (), Socket::SO_KEEPALIVE (), int $_[1]
         if $_[0]{fh};
   };
}

=item $handle->oobinline ($boolean)

Enables or disables the C<oobinline> setting (see constructor argument of
the same name for details).

=cut

sub oobinline {
   $_[0]{oobinline} = $_[1];

   eval {
      local $SIG{__DIE__};
      setsockopt $_[0]{fh}, Socket::SOL_SOCKET (), Socket::SO_OOBINLINE (), int $_[1]
         if $_[0]{fh};
   };
}

=item $handle->on_starttls ($cb)

Replace the current C<on_starttls> callback (see the C<on_starttls> constructor argument).

=cut

sub on_starttls {
   $_[0]{on_starttls} = $_[1];
}

=item $handle->on_stoptls ($cb)

Replace the current C<on_stoptls> callback (see the C<on_stoptls> constructor argument).

=cut

sub on_stoptls {
   $_[0]{on_stoptls} = $_[1];
}

=item $handle->rbuf_max ($max_octets)

Configures the C<rbuf_max> setting (C<undef> disables it).

=item $handle->wbuf_max ($max_octets)

Configures the C<wbuf_max> setting (C<undef> disables it).

=cut

sub rbuf_max {
   $_[0]{rbuf_max} = $_[1];
}

sub wbuf_max {
   $_[0]{wbuf_max} = $_[1];
}

#############################################################################

=item $handle->timeout ($seconds)

=item $handle->rtimeout ($seconds)

=item $handle->wtimeout ($seconds)

Configures (or disables) the inactivity timeout.

The timeout will be checked instantly, so this method might destroy the
handle before it returns.

=item $handle->timeout_reset

=item $handle->rtimeout_reset

=item $handle->wtimeout_reset

Reset the activity timeout, as if data was received or sent.

These methods are cheap to call.

=cut

for my $dir ("", "r", "w") {
   my $timeout    = "${dir}timeout";
   my $tw         = "_${dir}tw";
   my $on_timeout = "on_${dir}timeout";
   my $activity   = "_${dir}activity";
   my $cb;

   *$on_timeout = sub {
      $_[0]{$on_timeout} = $_[1];
   };

   *$timeout = sub {
      my ($self, $new_value) = @_;

      $new_value >= 0
         or Carp::croak "AnyEvent::Handle->$timeout called with negative timeout ($new_value), caught";

      $self->{$timeout} = $new_value;
      delete $self->{$tw}; &$cb;
   };

   *{"${dir}timeout_reset"} = sub {
      $_[0]{$activity} = AE::now;
   };

   # main workhorse:
   # reset the timeout watcher, as neccessary
   # also check for time-outs
   $cb = sub {
      my ($self) = @_;

      if ($self->{$timeout} && $self->{fh}) {
         my $NOW = AE::now;

         # when would the timeout trigger?
         my $after = $self->{$activity} + $self->{$timeout} - $NOW;

         # now or in the past already?
         if ($after <= 0) {
            $self->{$activity} = $NOW;

            if ($self->{$on_timeout}) {
               $self->{$on_timeout}($self);
            } else {
               $self->_error (Errno::ETIMEDOUT);
            }

            # callback could have changed timeout value, optimise
            return unless $self->{$timeout};

            # calculate new after
            $after = $self->{$timeout};
         }

         Scalar::Util::weaken $self;
         return unless $self; # ->error could have destroyed $self

         $self->{$tw} ||= AE::timer $after, 0, sub {
            delete $self->{$tw};
            $cb->($self);
         };
      } else {
         delete $self->{$tw};
      }
   }
}

#############################################################################

=back

=head2 WRITE QUEUE

AnyEvent::Handle manages two queues per handle, one for writing and one
for reading.

The write queue is very simple: you can add data to its end, and
AnyEvent::Handle will automatically try to get rid of it for you.

When data could be written and the write buffer is shorter then the low
water mark, the C<on_drain> callback will be invoked once.

=over 4

=item $handle->on_drain ($cb)

Sets the C<on_drain> callback or clears it (see the description of
C<on_drain> in the constructor).

This method may invoke callbacks (and therefore the handle might be
destroyed after it returns).

=cut

sub on_drain {
   my ($self, $cb) = @_;

   $self->{on_drain} = $cb;

   $cb->($self)
      if $cb && $self->{low_water_mark} >= (length $self->{wbuf}) + (length $self->{_tls_wbuf});
}

=item $handle->push_write ($data)

Queues the given scalar to be written. You can push as much data as
you want (only limited by the available memory and C<wbuf_max>), as
C<AnyEvent::Handle> buffers it independently of the kernel.

This method may invoke callbacks (and therefore the handle might be
destroyed after it returns).

=cut

sub _drain_wbuf {
   my ($self) = @_;

   if (!$self->{_ww} && length $self->{wbuf}) {

      Scalar::Util::weaken $self;

      my $cb = sub {
         my $len = syswrite $self->{fh}, $self->{wbuf};

         if (defined $len) {
            substr $self->{wbuf}, 0, $len, "";

            $self->{_activity} = $self->{_wactivity} = AE::now;

            $self->{on_drain}($self)
               if $self->{low_water_mark} >= (length $self->{wbuf}) + (length $self->{_tls_wbuf})
                  && $self->{on_drain};

            delete $self->{_ww} unless length $self->{wbuf};
         } elsif ($! != EAGAIN && $! != EINTR && $! != EWOULDBLOCK && $! != WSAEWOULDBLOCK) {
            $self->_error ($!, 1);
         }
      };

      # try to write data immediately
      $cb->() unless $self->{autocork};

      # if still data left in wbuf, we need to poll
      $self->{_ww} = AE::io $self->{fh}, 1, $cb
         if length $self->{wbuf};

      if (
         defined $self->{wbuf_max}
         && $self->{wbuf_max} < length $self->{wbuf}
      ) {
         $self->_error (Errno::ENOSPC, 1), return;
      }
   };
}

our %WH;

# deprecated
sub register_write_type($$) {
   $WH{$_[0]} = $_[1];
}

sub push_write {
   my $self = shift;

   if (@_ > 1) {
      my $type = shift;

      @_ = ($WH{$type} ||= _load_func "$type\::anyevent_write_type"
            or Carp::croak "unsupported/unloadable type '$type' passed to AnyEvent::Handle::push_write")
           ->($self, @_);
   }

   # we downgrade here to avoid hard-to-track-down bugs,
   # and diagnose the problem earlier and better.

   if ($self->{tls}) {
      utf8::downgrade $self->{_tls_wbuf} .= $_[0];
      &_dotls ($self)    if $self->{fh};
   } else {
      utf8::downgrade $self->{wbuf}      .= $_[0];
      $self->_drain_wbuf if $self->{fh};
   }
}

=item $handle->push_write (type => @args)

Instead of formatting your data yourself, you can also let this module
do the job by specifying a type and type-specific arguments. You
can also specify the (fully qualified) name of a package, in which
case AnyEvent tries to load the package and then expects to find the
C<anyevent_write_type> function inside (see "custom write types", below).

Predefined types are (if you have ideas for additional types, feel free to
drop by and tell us):

=over 4

=item netstring => $string

Formats the given value as netstring
(http://cr.yp.to/proto/netstrings.txt, this is not a recommendation to use them).

=cut

register_write_type netstring => sub {
   my ($self, $string) = @_;

   (length $string) . ":$string,"
};

=item packstring => $format, $data

An octet string prefixed with an encoded length. The encoding C<$format>
uses the same format as a Perl C<pack> format, but must specify a single
integer only (only one of C<cCsSlLqQiInNvVjJw> is allowed, plus an
optional C<!>, C<< < >> or C<< > >> modifier).

=cut

register_write_type packstring => sub {
   my ($self, $format, $string) = @_;

   pack "$format/a*", $string
};

=item json => $array_or_hashref

Encodes the given hash or array reference into a JSON object. Unless you
provide your own JSON object, this means it will be encoded to JSON text
in UTF-8.

The default encoder might or might not handle every type of JSON value -
it might be limited to arrays and objects for security reasons. See the
C<json> constructor attribute for more details.

JSON objects (and arrays) are self-delimiting, so if you only use arrays
and hashes, you can write JSON at one end of a handle and read them at the
other end without using any additional framing.

The JSON text generated by the default encoder is guaranteed not to
contain any newlines: While this module doesn't need delimiters after or
between JSON texts to be able to read them, many other languages depend on
them.

A simple RPC protocol that interoperates easily with other languages is
to send JSON arrays (or objects, although arrays are usually the better
choice as they mimic how function argument passing works) and a newline
after each JSON text:

   $handle->push_write (json => ["method", "arg1", "arg2"]); # whatever
   $handle->push_write ("\012");
 
An AnyEvent::Handle receiver would simply use the C<json> read type and
rely on the fact that the newline will be skipped as leading whitespace:

   $handle->push_read (json => sub { my $array = $_[1]; ... });

Other languages could read single lines terminated by a newline and pass
this line into their JSON decoder of choice.

=item cbor => $perl_scalar

Encodes the given scalar into a CBOR value. Unless you provide your own
L<CBOR::XS> object, this means it will be encoded to a CBOR string not
using any extensions, if possible.

CBOR values are self-delimiting, so you can write CBOR at one end of
a handle and read them at the other end without using any additional
framing.

A simple nd very very fast RPC protocol that interoperates with
other languages is to send CBOR and receive CBOR values (arrays are
recommended):

   $handle->push_write (cbor => ["method", "arg1", "arg2"]); # whatever
 
An AnyEvent::Handle receiver would simply use the C<cbor> read type:

   $handle->push_read (cbor => sub { my $array = $_[1]; ... });

=cut

sub json_coder() {
   eval { require JSON::XS; JSON::XS->new->utf8 }
      || do { require JSON::PP; JSON::PP->new->utf8 }
}

register_write_type json => sub {
   my ($self, $ref) = @_;

   ($self->{json} ||= json_coder)
      ->encode ($ref)
};

sub cbor_coder() {
   require CBOR::XS;
   CBOR::XS->new
}

register_write_type cbor => sub {
   my ($self, $scalar) = @_;

   ($self->{cbor} ||= cbor_coder)
      ->encode ($scalar)
};

=item storable => $reference

Freezes the given reference using L<Storable> and writes it to the
handle. Uses the C<nfreeze> format.

=cut

register_write_type storable => sub {
   my ($self, $ref) = @_;

   require Storable unless $Storable::VERSION;

   pack "w/a*", Storable::nfreeze ($ref)
};

=back

=item $handle->push_shutdown

Sometimes you know you want to close the socket after writing your data
before it was actually written. One way to do that is to replace your
C<on_drain> handler by a callback that shuts down the socket (and set
C<low_water_mark> to C<0>). This method is a shorthand for just that, and
replaces the C<on_drain> callback with:

   sub { shutdown $_[0]{fh}, 1 }

This simply shuts down the write side and signals an EOF condition to the
the peer.

You can rely on the normal read queue and C<on_eof> handling
afterwards. This is the cleanest way to close a connection.

This method may invoke callbacks (and therefore the handle might be
destroyed after it returns).

=cut

sub push_shutdown {
   my ($self) = @_;

   delete $self->{low_water_mark};
   $self->on_drain (sub { shutdown $_[0]{fh}, 1 });
}

=item custom write types - Package::anyevent_write_type $handle, @args

Instead of one of the predefined types, you can also specify the name of
a package. AnyEvent will try to load the package and then expects to find
a function named C<anyevent_write_type> inside. If it isn't found, it
progressively tries to load the parent package until it either finds the
function (good) or runs out of packages (bad).

Whenever the given C<type> is used, C<push_write> will the function with
the handle object and the remaining arguments.

The function is supposed to return a single octet string that will be
appended to the write buffer, so you can mentally treat this function as a
"arguments to on-the-wire-format" converter.

Example: implement a custom write type C<join> that joins the remaining
arguments using the first one.

   $handle->push_write (My::Type => " ", 1,2,3);

   # uses the following package, which can be defined in the "My::Type" or in
   # the "My" modules to be auto-loaded, or just about anywhere when the
   # My::Type::anyevent_write_type is defined before invoking it.

   package My::Type;

   sub anyevent_write_type {
      my ($handle, $delim, @args) = @_;

      join $delim, @args
   }

=cut

#############################################################################

=back

=head2 READ QUEUE

AnyEvent::Handle manages two queues per handle, one for writing and one
for reading.

The read queue is more complex than the write queue. It can be used in two
ways, the "simple" way, using only C<on_read> and the "complex" way, using
a queue.

In the simple case, you just install an C<on_read> callback and whenever
new data arrives, it will be called. You can then remove some data (if
enough is there) from the read buffer (C<< $handle->rbuf >>). Or you can
leave the data there if you want to accumulate more (e.g. when only a
partial message has been received so far), or change the read queue with
e.g. C<push_read>.

In the more complex case, you want to queue multiple callbacks. In this
case, AnyEvent::Handle will call the first queued callback each time new
data arrives (also the first time it is queued) and remove it when it has
done its job (see C<push_read>, below).

This way you can, for example, push three line-reads, followed by reading
a chunk of data, and AnyEvent::Handle will execute them in order.

Example 1: EPP protocol parser. EPP sends 4 byte length info, followed by
the specified number of bytes which give an XML datagram.

   # in the default state, expect some header bytes
   $handle->on_read (sub {
      # some data is here, now queue the length-header-read (4 octets)
      shift->unshift_read (chunk => 4, sub {
         # header arrived, decode
         my $len = unpack "N", $_[1];

         # now read the payload
         shift->unshift_read (chunk => $len, sub {
            my $xml = $_[1];
            # handle xml
         });
      });
   });

Example 2: Implement a client for a protocol that replies either with "OK"
and another line or "ERROR" for the first request that is sent, and 64
bytes for the second request. Due to the availability of a queue, we can
just pipeline sending both requests and manipulate the queue as necessary
in the callbacks.

When the first callback is called and sees an "OK" response, it will
C<unshift> another line-read. This line-read will be queued I<before> the
64-byte chunk callback.

   # request one, returns either "OK + extra line" or "ERROR"
   $handle->push_write ("request 1\015\012");

   # we expect "ERROR" or "OK" as response, so push a line read
   $handle->push_read (line => sub {
      # if we got an "OK", we have to _prepend_ another line,
      # so it will be read before the second request reads its 64 bytes
      # which are already in the queue when this callback is called
      # we don't do this in case we got an error
      if ($_[1] eq "OK") {
         $_[0]->unshift_read (line => sub {
            my $response = $_[1];
            ...
         });
      }
   });

   # request two, simply returns 64 octets
   $handle->push_write ("request 2\015\012");

   # simply read 64 bytes, always
   $handle->push_read (chunk => 64, sub {
      my $response = $_[1];
      ...
   });

=over 4

=cut

sub _drain_rbuf {
   my ($self) = @_;

   # avoid recursion
   return if $self->{_skip_drain_rbuf};
   local $self->{_skip_drain_rbuf} = 1;

   while () {
      # we need to use a separate tls read buffer, as we must not receive data while
      # we are draining the buffer, and this can only happen with TLS.
      $self->{rbuf} .= delete $self->{_tls_rbuf}
         if exists $self->{_tls_rbuf};

      my $len = length $self->{rbuf};

      if (my $cb = shift @{ $self->{_queue} }) {
         unless ($cb->($self)) {
            # no progress can be made
            # (not enough data and no data forthcoming)
            $self->_error (Errno::EPIPE, 1), return
               if $self->{_eof};

            unshift @{ $self->{_queue} }, $cb;
            last;
         }
      } elsif ($self->{on_read}) {
         last unless $len;

         $self->{on_read}($self);

         if (
            $len == length $self->{rbuf} # if no data has been consumed
            && !@{ $self->{_queue} }     # and the queue is still empty
            && $self->{on_read}          # but we still have on_read
         ) {
            # no further data will arrive
            # so no progress can be made
            $self->_error (Errno::EPIPE, 1), return
               if $self->{_eof};

            last; # more data might arrive
         }
      } else {
         # read side becomes idle
         delete $self->{_rw} unless $self->{tls};
         last;
      }
   }

   if ($self->{_eof}) {
      $self->{on_eof}
         ? $self->{on_eof}($self)
         : $self->_error (0, 1, "Unexpected end-of-file");

      return;
   }

   if (
      defined $self->{rbuf_max}
      && $self->{rbuf_max} < length $self->{rbuf}
   ) {
      $self->_error (Errno::ENOSPC, 1), return;
   }

   # may need to restart read watcher
   unless ($self->{_rw}) {
      $self->start_read
         if $self->{on_read} || @{ $self->{_queue} };
   }
}

=item $handle->on_read ($cb)

This replaces the currently set C<on_read> callback, or clears it (when
the new callback is C<undef>). See the description of C<on_read> in the
constructor.

This method may invoke callbacks (and therefore the handle might be
destroyed after it returns).

=cut

sub on_read {
   my ($self, $cb) = @_;

   $self->{on_read} = $cb;
   $self->_drain_rbuf if $cb;
}

=item $handle->rbuf

Returns the read buffer (as a modifiable lvalue). You can also access the
read buffer directly as the C<< ->{rbuf} >> member, if you want (this is
much faster, and no less clean).

The only operation allowed on the read buffer (apart from looking at it)
is removing data from its beginning. Otherwise modifying or appending to
it is not allowed and will lead to hard-to-track-down bugs.

NOTE: The read buffer should only be used or modified in the C<on_read>
callback or when C<push_read> or C<unshift_read> are used with a single
callback (i.e. untyped). Typed C<push_read> and C<unshift_read> methods
will manage the read buffer on their own.

=cut

sub rbuf : lvalue {
   $_[0]{rbuf}
}

=item $handle->push_read ($cb)

=item $handle->unshift_read ($cb)

Append the given callback to the end of the queue (C<push_read>) or
prepend it (C<unshift_read>).

The callback is called each time some additional read data arrives.

It must check whether enough data is in the read buffer already.

If not enough data is available, it must return the empty list or a false
value, in which case it will be called repeatedly until enough data is
available (or an error condition is detected).

If enough data was available, then the callback must remove all data it is
interested in (which can be none at all) and return a true value. After returning
true, it will be removed from the queue.

These methods may invoke callbacks (and therefore the handle might be
destroyed after it returns).

=cut

our %RH;

sub register_read_type($$) {
   $RH{$_[0]} = $_[1];
}

sub push_read {
   my $self = shift;
   my $cb = pop;

   if (@_) {
      my $type = shift;

      $cb = ($RH{$type} ||= _load_func "$type\::anyevent_read_type"
             or Carp::croak "unsupported/unloadable type '$type' passed to AnyEvent::Handle::push_read")
            ->($self, $cb, @_);
   }

   push @{ $self->{_queue} }, $cb;
   $self->_drain_rbuf;
}

sub unshift_read {
   my $self = shift;
   my $cb = pop;

   if (@_) {
      my $type = shift;

      $cb = ($RH{$type} ||= _load_func "$type\::anyevent_read_type"
             or Carp::croak "unsupported/unloadable type '$type' passed to AnyEvent::Handle::unshift_read")
            ->($self, $cb, @_);
   }

   unshift @{ $self->{_queue} }, $cb;
   $self->_drain_rbuf;
}

=item $handle->push_read (type => @args, $cb)

=item $handle->unshift_read (type => @args, $cb)

Instead of providing a callback that parses the data itself you can chose
between a number of predefined parsing formats, for chunks of data, lines
etc. You can also specify the (fully qualified) name of a package, in
which case AnyEvent tries to load the package and then expects to find the
C<anyevent_read_type> function inside (see "custom read types", below).

Predefined types are (if you have ideas for additional types, feel free to
drop by and tell us):

=over 4

=item chunk => $octets, $cb->($handle, $data)

Invoke the callback only once C<$octets> bytes have been read. Pass the
data read to the callback. The callback will never be called with less
data.

Example: read 2 bytes.

   $handle->push_read (chunk => 2, sub {
      say "yay " . unpack "H*", $_[1];
   });

=cut

register_read_type chunk => sub {
   my ($self, $cb, $len) = @_;

   sub {
      $len <= length $_[0]{rbuf} or return;
      $cb->($_[0], substr $_[0]{rbuf}, 0, $len, "");
      1
   }
};

=item line => [$eol, ]$cb->($handle, $line, $eol)

The callback will be called only once a full line (including the end of
line marker, C<$eol>) has been read. This line (excluding the end of line
marker) will be passed to the callback as second argument (C<$line>), and
the end of line marker as the third argument (C<$eol>).

The end of line marker, C<$eol>, can be either a string, in which case it
will be interpreted as a fixed record end marker, or it can be a regex
object (e.g. created by C<qr>), in which case it is interpreted as a
regular expression.

The end of line marker argument C<$eol> is optional, if it is missing (NOT
undef), then C<qr|\015?\012|> is used (which is good for most internet
protocols).

Partial lines at the end of the stream will never be returned, as they are
not marked by the end of line marker.

=cut

register_read_type line => sub {
   my ($self, $cb, $eol) = @_;

   if (@_ < 3) {
      # this is faster then the generic code below
      sub {
         (my $pos = index $_[0]{rbuf}, "\012") >= 0
            or return;

         (my $str = substr $_[0]{rbuf}, 0, $pos + 1, "") =~ s/(\015?\012)\Z// or die;
         $cb->($_[0], $str, "$1");
         1
      }
   } else {
      $eol = quotemeta $eol unless ref $eol;
      $eol = qr|^(.*?)($eol)|s;

      sub {
         $_[0]{rbuf} =~ s/$eol// or return;

         $cb->($_[0], "$1", "$2");
         1
      }
   }
};

=item regex => $accept[, $reject[, $skip], $cb->($handle, $data)

Makes a regex match against the regex object C<$accept> and returns
everything up to and including the match. All the usual regex variables
($1, %+ etc.) from the regex match are available in the callback.

Example: read a single line terminated by '\n'.

   $handle->push_read (regex => qr<\n>, sub { ... });

If C<$reject> is given and not undef, then it determines when the data is
to be rejected: it is matched against the data when the C<$accept> regex
does not match and generates an C<EBADMSG> error when it matches. This is
useful to quickly reject wrong data (to avoid waiting for a timeout or a
receive buffer overflow).

Example: expect a single decimal number followed by whitespace, reject
anything else (not the use of an anchor).

   $handle->push_read (regex => qr<^[0-9]+\s>, qr<[^0-9]>, sub { ... });

If C<$skip> is given and not C<undef>, then it will be matched against
the receive buffer when neither C<$accept> nor C<$reject> match,
and everything preceding and including the match will be accepted
unconditionally. This is useful to skip large amounts of data that you
know cannot be matched, so that the C<$accept> or C<$reject> regex do not
have to start matching from the beginning. This is purely an optimisation
and is usually worth it only when you expect more than a few kilobytes.

Example: expect a http header, which ends at C<\015\012\015\012>. Since we
expect the header to be very large (it isn't in practice, but...), we use
a skip regex to skip initial portions. The skip regex is tricky in that
it only accepts something not ending in either \015 or \012, as these are
required for the accept regex.

   $handle->push_read (regex =>
      qr<\015\012\015\012>,
      undef, # no reject
      qr<^.*[^\015\012]>,
      sub { ... });

=cut

register_read_type regex => sub {
   my ($self, $cb, $accept, $reject, $skip) = @_;

   my $data;
   my $rbuf = \$self->{rbuf};

   sub {
      # accept
      if ($$rbuf =~ $accept) {
         $data .= substr $$rbuf, 0, $+[0], "";
         $cb->($_[0], $data);
         return 1;
      }
      
      # reject
      if ($reject && $$rbuf =~ $reject) {
         $_[0]->_error (Errno::EBADMSG);
      }

      # skip
      if ($skip && $$rbuf =~ $skip) {
         $data .= substr $$rbuf, 0, $+[0], "";
      }

      ()
   }
};

=item netstring => $cb->($handle, $string)

A netstring (http://cr.yp.to/proto/netstrings.txt, this is not an endorsement).

Throws an error with C<$!> set to EBADMSG on format violations.

=cut

register_read_type netstring => sub {
   my ($self, $cb) = @_;

   sub {
      unless ($_[0]{rbuf} =~ s/^(0|[1-9][0-9]*)://) {
         if ($_[0]{rbuf} =~ /[^0-9]/) {
            $_[0]->_error (Errno::EBADMSG);
         }
         return;
      }

      my $len = $1;

      $_[0]->unshift_read (chunk => $len, sub {
         my $string = $_[1];
         $_[0]->unshift_read (chunk => 1, sub {
            if ($_[1] eq ",") {
               $cb->($_[0], $string);
            } else {
               $_[0]->_error (Errno::EBADMSG);
            }
         });
      });

      1
   }
};

=item packstring => $format, $cb->($handle, $string)

An octet string prefixed with an encoded length. The encoding C<$format>
uses the same format as a Perl C<pack> format, but must specify a single
integer only (only one of C<cCsSlLqQiInNvVjJw> is allowed, plus an
optional C<!>, C<< < >> or C<< > >> modifier).

For example, DNS over TCP uses a prefix of C<n> (2 octet network order),
EPP uses a prefix of C<N> (4 octtes).

Example: read a block of data prefixed by its length in BER-encoded
format (very efficient).

   $handle->push_read (packstring => "w", sub {
      my ($handle, $data) = @_;
   });

=cut

register_read_type packstring => sub {
   my ($self, $cb, $format) = @_;

   sub {
      # when we can use 5.10 we can use ".", but for 5.8 we use the re-pack method
      defined (my $len = eval { unpack $format, $_[0]{rbuf} })
         or return;

      $format = length pack $format, $len;

      # bypass unshift if we already have the remaining chunk
      if ($format + $len <= length $_[0]{rbuf}) {
         my $data = substr $_[0]{rbuf}, $format, $len;
         substr $_[0]{rbuf}, 0, $format + $len, "";
         $cb->($_[0], $data);
      } else {
         # remove prefix
         substr $_[0]{rbuf}, 0, $format, "";

         # read remaining chunk
         $_[0]->unshift_read (chunk => $len, $cb);
      }

      1
   }
};

=item json => $cb->($handle, $hash_or_arrayref)

Reads a JSON object or array, decodes it and passes it to the
callback. When a parse error occurs, an C<EBADMSG> error will be raised.

If a C<json> object was passed to the constructor, then that will be
used for the final decode, otherwise it will create a L<JSON::XS> or
L<JSON::PP> coder object expecting UTF-8.

This read type uses the incremental parser available with JSON version
2.09 (and JSON::XS version 2.2) and above.

Since JSON texts are fully self-delimiting, the C<json> read and write
types are an ideal simple RPC protocol: just exchange JSON datagrams. See
the C<json> write type description, above, for an actual example.

=cut

register_read_type json => sub {
   my ($self, $cb) = @_;

   my $json = $self->{json} ||= json_coder;

   my $data;

   sub {
      my $ref = eval { $json->incr_parse ($_[0]{rbuf}) };

      if ($ref) {
         $_[0]{rbuf} = $json->incr_text;
         $json->incr_text = "";
         $cb->($_[0], $ref);

         1
      } elsif ($@) {
         # error case
         $json->incr_skip;

         $_[0]{rbuf} = $json->incr_text;
         $json->incr_text = "";

         $_[0]->_error (Errno::EBADMSG);

         ()
      } else {
         $_[0]{rbuf} = "";

         ()
      }
   }
};

=item cbor => $cb->($handle, $scalar)

Reads a CBOR value, decodes it and passes it to the callback. When a parse
error occurs, an C<EBADMSG> error will be raised.

If a L<CBOR::XS> object was passed to the constructor, then that will be
used for the final decode, otherwise it will create a CBOR coder without
enabling any options.

You have to provide a dependency to L<CBOR::XS> on your own: this module
will load the L<CBOR::XS> module, but AnyEvent does not depend on it
itself.

Since CBOR values are fully self-delimiting, the C<cbor> read and write
types are an ideal simple RPC protocol: just exchange CBOR datagrams. See
the C<cbor> write type description, above, for an actual example.

=cut

register_read_type cbor => sub {
   my ($self, $cb) = @_;

   my $cbor = $self->{cbor} ||= cbor_coder;

   my $data;

   sub {
      my (@value) = eval { $cbor->incr_parse ($_[0]{rbuf}) };

      if (@value) {
         $cb->($_[0], @value);

         1
      } elsif ($@) {
         # error case
         $cbor->incr_reset;

         $_[0]->_error (Errno::EBADMSG);

         ()
      } else {
         ()
      }
   }
};

=item storable => $cb->($handle, $ref)

Deserialises a L<Storable> frozen representation as written by the
C<storable> write type (BER-encoded length prefix followed by nfreeze'd
data).

Raises C<EBADMSG> error if the data could not be decoded.

=cut

register_read_type storable => sub {
   my ($self, $cb) = @_;

   require Storable unless $Storable::VERSION;

   sub {
      # when we can use 5.10 we can use ".", but for 5.8 we use the re-pack method
      defined (my $len = eval { unpack "w", $_[0]{rbuf} })
         or return;

      my $format = length pack "w", $len;

      # bypass unshift if we already have the remaining chunk
      if ($format + $len <= length $_[0]{rbuf}) {
         my $data = substr $_[0]{rbuf}, $format, $len;
         substr $_[0]{rbuf}, 0, $format + $len, "";

         eval { $cb->($_[0], Storable::thaw ($data)); 1 }
            or return $_[0]->_error (Errno::EBADMSG);
      } else {
         # remove prefix
         substr $_[0]{rbuf}, 0, $format, "";

         # read remaining chunk
         $_[0]->unshift_read (chunk => $len, sub {
            eval { $cb->($_[0], Storable::thaw ($_[1])); 1 }
               or $_[0]->_error (Errno::EBADMSG);
         });
      }

      1
   }
};

=item tls_detect => $cb->($handle, $detect, $major, $minor)

Checks the input stream for a valid SSL or TLS handshake TLSPaintext
record without consuming anything. Only SSL version 3 or higher
is handled, up to the fictituous protocol 4.x (but both SSL3+ and
SSL2-compatible framing is supported).

If it detects that the input data is likely TLS, it calls the callback
with a true value for C<$detect> and the (on-wire) TLS version as second
and third argument (C<$major> is C<3>, and C<$minor> is 0..4 for SSL
3.0, TLS 1.0, 1.1, 1.2 and 1.3, respectively).  If it detects the input
to be definitely not TLS, it calls the callback with a false value for
C<$detect>.

The callback could use this information to decide whether or not to start
TLS negotiation.

In all cases the data read so far is passed to the following read
handlers.

Usually you want to use the C<tls_autostart> read type instead.

If you want to design a protocol that works in the presence of TLS
dtection, make sure that any non-TLS data doesn't start with the octet 22
(ASCII SYN, 16 hex) or 128-255 (i.e. highest bit set). The checks this
read type does are a bit more strict, but might losen in the future to
accomodate protocol changes.

This read type does not rely on L<AnyEvent::TLS> (and thus, not on
L<Net::SSLeay>).

=item tls_autostart => [$tls_ctx, ]$tls

Tries to detect a valid SSL or TLS handshake. If one is detected, it tries
to start tls by calling C<starttls> with the given arguments.

In practise, C<$tls> must be C<accept>, or a Net::SSLeay context that has
been configured to accept, as servers do not normally send a handshake on
their own and ths cannot be detected in this way.

See C<tls_detect> above for more details.

Example: give the client a chance to start TLS before accepting a text
line.

   $hdl->push_read (tls_autostart => "accept");
   $hdl->push_read (line => sub {
      print "received ", ($_[0]{tls} ? "encrypted" : "cleartext"), " <$_[1]>\n";
   });

=cut

register_read_type tls_detect => sub {
   my ($self, $cb) = @_;

   sub {
      # this regex matches a full or partial tls record
      if (
         # ssl3+: type(22=handshake) major(=3) minor(any) length_hi
         $self->{rbuf} =~ /^(?:\z| \x16 (\z| [\x03\x04] (?:\z| . (?:\z| [\x00-\x40] ))))/xs
         # ssl2 comapatible: len_hi len_lo type(1) major minor dummy(forlength)
         or $self->{rbuf} =~ /^(?:\z| [\x80-\xff] (?:\z| . (?:\z| \x01 (\z| [\x03\x04] (?:\z| . (?:\z| . ))))))/xs
      ) {
         return if 3 != length $1; # partial match, can't decide yet

         # full match, valid TLS record
         my ($major, $minor) = unpack "CC", $1;
         $cb->($self, "accept", $major, $minor);
      } else {
         # mismatch == guaranteed not TLS
         $cb->($self, undef);
      }

      1
   }
};

register_read_type tls_autostart => sub {
   my ($self, @tls) = @_;

   $RH{tls_detect}($self, sub {
      return unless $_[1];
      $_[0]->starttls (@tls);
   })
};

=back

=item custom read types - Package::anyevent_read_type $handle, $cb, @args

Instead of one of the predefined types, you can also specify the name
of a package. AnyEvent will try to load the package and then expects to
find a function named C<anyevent_read_type> inside. If it isn't found, it
progressively tries to load the parent package until it either finds the
function (good) or runs out of packages (bad).

Whenever this type is used, C<push_read> will invoke the function with the
handle object, the original callback and the remaining arguments.

The function is supposed to return a callback (usually a closure) that
works as a plain read callback (see C<< ->push_read ($cb) >>), so you can
mentally treat the function as a "configurable read type to read callback"
converter.

It should invoke the original callback when it is done reading (remember
to pass C<$handle> as first argument as all other callbacks do that,
although there is no strict requirement on this).

For examples, see the source of this module (F<perldoc -m
AnyEvent::Handle>, search for C<register_read_type>)).

=item $handle->stop_read

=item $handle->start_read

In rare cases you actually do not want to read anything from the
socket. In this case you can call C<stop_read>. Neither C<on_read> nor
any queued callbacks will be executed then. To start reading again, call
C<start_read>.

Note that AnyEvent::Handle will automatically C<start_read> for you when
you change the C<on_read> callback or push/unshift a read callback, and it
will automatically C<stop_read> for you when neither C<on_read> is set nor
there are any read requests in the queue.

In older versions of this module (<= 5.3), these methods had no effect,
as TLS does not support half-duplex connections. In current versions they
work as expected, as this behaviour is required to avoid certain resource
attacks, where the program would be forced to read (and buffer) arbitrary
amounts of data before being able to send some data. The drawback is that
some readings of the the SSL/TLS specifications basically require this
attack to be working, as SSL/TLS implementations might stall sending data
during a rehandshake.

As a guideline, during the initial handshake, you should not stop reading,
and as a client, it might cause problems, depending on your application.

=cut

sub stop_read {
   my ($self) = @_;

   delete $self->{_rw};
}

sub start_read {
   my ($self) = @_;

   unless ($self->{_rw} || $self->{_eof} || !$self->{fh}) {
      Scalar::Util::weaken $self;

      $self->{_rw} = AE::io $self->{fh}, 0, sub {
         my $rbuf = \($self->{tls} ? my $buf : $self->{rbuf});
         my $len = sysread $self->{fh}, $$rbuf, $self->{read_size}, length $$rbuf;

         if ($len > 0) {
            $self->{_activity} = $self->{_ractivity} = AE::now;

            if ($self->{tls}) {
               Net::SSLeay::BIO_write ($self->{_rbio}, $$rbuf);

               &_dotls ($self);
            } else {
               $self->_drain_rbuf;
            }

            if ($len == $self->{read_size}) {
               $self->{read_size} *= 2;
               $self->{read_size} = $self->{max_read_size} || MAX_READ_SIZE
                  if $self->{read_size} > ($self->{max_read_size} || MAX_READ_SIZE);
            }

         } elsif (defined $len) {
            delete $self->{_rw};
            $self->{_eof} = 1;
            $self->_drain_rbuf;

         } elsif ($! != EAGAIN && $! != EINTR && $! != EWOULDBLOCK && $! != WSAEWOULDBLOCK) {
            return $self->_error ($!, 1);
         }
      };
   }
}

our $ERROR_SYSCALL;
our $ERROR_WANT_READ;

sub _tls_error {
   my ($self, $err) = @_;

   return $self->_error ($!, 1)
      if $err == Net::SSLeay::ERROR_SYSCALL ();

   my $err = Net::SSLeay::ERR_error_string (Net::SSLeay::ERR_get_error ());

   # reduce error string to look less scary
   $err =~ s/^error:[0-9a-fA-F]{8}:[^:]+:([^:]+):/\L$1: /;

   if ($self->{_on_starttls}) {
      (delete $self->{_on_starttls})->($self, undef, $err);
      &_freetls;
   } else {
      &_freetls;
      $self->_error (Errno::EPROTO, 1, $err);
   }
}

# poll the write BIO and send the data if applicable
# also decode read data if possible
# this is basiclaly our TLS state machine
# more efficient implementations are possible with openssl,
# but not with the buggy and incomplete Net::SSLeay.
sub _dotls {
   my ($self) = @_;

   my $tmp;

   while (length $self->{_tls_wbuf}) {
      if (($tmp = Net::SSLeay::write ($self->{tls}, $self->{_tls_wbuf})) <= 0) {
         $tmp = Net::SSLeay::get_error ($self->{tls}, $tmp);

         return $self->_tls_error ($tmp)
            if $tmp != $ERROR_WANT_READ
               && ($tmp != $ERROR_SYSCALL || $!);

         last;
      }

      substr $self->{_tls_wbuf}, 0, $tmp, "";
   }

   while (defined ($tmp = Net::SSLeay::read ($self->{tls}))) {
      unless (length $tmp) {
         $self->{_on_starttls}
            and (delete $self->{_on_starttls})->($self, undef, "EOF during handshake"); # ???
         &_freetls;

         if ($self->{on_stoptls}) {
            $self->{on_stoptls}($self);
            return;
         } else {
            # let's treat SSL-eof as we treat normal EOF
            delete $self->{_rw};
            $self->{_eof} = 1;
         }
      }

      $self->{_tls_rbuf} .= $tmp;
      $self->_drain_rbuf;
      $self->{tls} or return; # tls session might have gone away in callback
   }

   $tmp = Net::SSLeay::get_error ($self->{tls}, -1); # -1 is not neccessarily correct, but Net::SSLeay doesn't tell us
   return $self->_tls_error ($tmp)
      if $tmp != $ERROR_WANT_READ
         && ($tmp != $ERROR_SYSCALL || $!);

   while (length ($tmp = Net::SSLeay::BIO_read ($self->{_wbio}))) {
      $self->{wbuf} .= $tmp;
      $self->_drain_wbuf;
      $self->{tls} or return; # tls session might have gone away in callback
   }

   $self->{_on_starttls}
      and Net::SSLeay::state ($self->{tls}) == 1
      and (delete $self->{_on_starttls})->($self, 1, "TLS/SSL connection established");
}

=item $handle->starttls ($tls[, $tls_ctx])

Instead of starting TLS negotiation immediately when the AnyEvent::Handle
object is created, you can also do that at a later time by calling
C<starttls>. See the C<tls> constructor argument for general info.

Starting TLS is currently an asynchronous operation - when you push some
write data and then call C<< ->starttls >> then TLS negotiation will start
immediately, after which the queued write data is then sent. This might
change in future versions, so best make sure you have no outstanding write
data when calling this method.

The first argument is the same as the C<tls> constructor argument (either
C<"connect">, C<"accept"> or an existing Net::SSLeay object).

The second argument is the optional C<AnyEvent::TLS> object that is used
when AnyEvent::Handle has to create its own TLS connection object, or
a hash reference with C<< key => value >> pairs that will be used to
construct a new context.

The TLS connection object will end up in C<< $handle->{tls} >>, the TLS
context in C<< $handle->{tls_ctx} >> after this call and can be used or
changed to your liking. Note that the handshake might have already started
when this function returns.

Due to bugs in OpenSSL, it might or might not be possible to do multiple
handshakes on the same stream. It is best to not attempt to use the
stream after stopping TLS.

This method may invoke callbacks (and therefore the handle might be
destroyed after it returns).

=cut

our %TLS_CACHE; #TODO not yet documented, should we?

sub starttls {
   my ($self, $tls, $ctx) = @_;

   Carp::croak "It is an error to call starttls on an AnyEvent::Handle object while TLS is already active, caught"
      if $self->{tls};

   unless (defined $AnyEvent::TLS::VERSION) {
      eval {
         require Net::SSLeay;
         require AnyEvent::TLS;
         1
      } or return $self->_error (Errno::EPROTO, 1, "TLS support not available on this system");
   }

   $self->{tls}     = $tls;
   $self->{tls_ctx} = $ctx if @_ > 2;

   return unless $self->{fh};

   $ERROR_SYSCALL   = Net::SSLeay::ERROR_SYSCALL     ();
   $ERROR_WANT_READ = Net::SSLeay::ERROR_WANT_READ   ();

   $tls = delete $self->{tls};
   $ctx = $self->{tls_ctx};

   local $Carp::CarpLevel = 1; # skip ourselves when creating a new context or session

   if ("HASH" eq ref $ctx) {
      if ($ctx->{cache}) {
         my $key = $ctx+0;
         $ctx = $TLS_CACHE{$key} ||= new AnyEvent::TLS %$ctx;
      } else {
         $ctx = new AnyEvent::TLS %$ctx;
      }
   }
   
   $self->{tls_ctx} = $ctx || TLS_CTX ();
   $self->{tls}     = $tls = $self->{tls_ctx}->_get_session ($tls, $self, $self->{peername});

   # basically, this is deep magic (because SSL_read should have the same issues)
   # but the openssl maintainers basically said: "trust us, it just works".
   # (unfortunately, we have to hardcode constants because the abysmally misdesigned
   # and mismaintained ssleay-module didn't offer them for a decade or so).
   # http://www.mail-archive.com/openssl-dev@openssl.org/msg22420.html
   #
   # in short: this is a mess.
   #
   # note that we do not try to keep the length constant between writes as we are required to do.
   # we assume that most (but not all) of this insanity only applies to non-blocking cases,
   # and we drive openssl fully in blocking mode here. Or maybe we don't - openssl seems to
   # have identity issues in that area.
#   Net::SSLeay::set_mode ($ssl,
#      (eval { local $SIG{__DIE__}; Net::SSLeay::MODE_ENABLE_PARTIAL_WRITE () } || 1)
#      | (eval { local $SIG{__DIE__}; Net::SSLeay::MODE_ACCEPT_MOVING_WRITE_BUFFER () } || 2));
   Net::SSLeay::set_mode ($tls, 1|2);

   $self->{_rbio} = Net::SSLeay::BIO_new (Net::SSLeay::BIO_s_mem ());
   $self->{_wbio} = Net::SSLeay::BIO_new (Net::SSLeay::BIO_s_mem ());

   Net::SSLeay::BIO_write ($self->{_rbio}, $self->{rbuf});
   $self->{rbuf} = "";

   Net::SSLeay::set_bio ($tls, $self->{_rbio}, $self->{_wbio});

   $self->{_on_starttls} = sub { $_[0]{on_starttls}(@_) }
      if $self->{on_starttls};

   &_dotls; # need to trigger the initial handshake
   $self->start_read; # make sure we actually do read
}

=item $handle->stoptls

Shuts down the SSL connection - this makes a proper EOF handshake by
sending a close notify to the other side, but since OpenSSL doesn't
support non-blocking shut downs, it is not guaranteed that you can re-use
the stream afterwards.

This method may invoke callbacks (and therefore the handle might be
destroyed after it returns).

=cut

sub stoptls {
   my ($self) = @_;

   if ($self->{tls} && $self->{fh}) {
      Net::SSLeay::shutdown ($self->{tls});

      &_dotls;

#      # we don't give a shit. no, we do, but we can't. no...#d#
#      # we, we... have to use openssl :/#d#
#      &_freetls;#d#
   }
}

sub _freetls {
   my ($self) = @_;

   return unless $self->{tls};

   $self->{tls_ctx}->_put_session (delete $self->{tls})
      if $self->{tls} > 0;
   
   delete @$self{qw(_rbio _wbio _tls_wbuf _on_starttls)};
}

=item $handle->resettls

This rarely-used method simply resets and TLS state on the handle, usually
causing data loss.

One case where it may be useful is when you want to skip over the data in
the stream but you are not interested in interpreting it, so data loss is
no concern.

=cut

*resettls = \&_freetls;

sub DESTROY {
   my ($self) = @_;

   &_freetls;

   my $linger = exists $self->{linger} ? $self->{linger} : 3600;

   if ($linger && length $self->{wbuf} && $self->{fh}) {
      my $fh   = delete $self->{fh};
      my $wbuf = delete $self->{wbuf};

      my @linger;

      push @linger, AE::io $fh, 1, sub {
         my $len = syswrite $fh, $wbuf, length $wbuf;

         if ($len > 0) {
            substr $wbuf, 0, $len, "";
         } elsif (defined $len || ($! != EAGAIN && $! != EINTR && $! != EWOULDBLOCK && $! != WSAEWOULDBLOCK)) {
            @linger = (); # end
         }
      };
      push @linger, AE::timer $linger, 0, sub {
         @linger = ();
      };
   }
}

=item $handle->destroy

Shuts down the handle object as much as possible - this call ensures that
no further callbacks will be invoked and as many resources as possible
will be freed. Any method you will call on the handle object after
destroying it in this way will be silently ignored (and it will return the
empty list).

Normally, you can just "forget" any references to an AnyEvent::Handle
object and it will simply shut down. This works in fatal error and EOF
callbacks, as well as code outside. It does I<NOT> work in a read or write
callback, so when you want to destroy the AnyEvent::Handle object from
within such an callback. You I<MUST> call C<< ->destroy >> explicitly in
that case.

Destroying the handle object in this way has the advantage that callbacks
will be removed as well, so if those are the only reference holders (as
is common), then one doesn't need to do anything special to break any
reference cycles.

The handle might still linger in the background and write out remaining
data, as specified by the C<linger> option, however.

=cut

sub destroy {
   my ($self) = @_;

   $self->DESTROY;
   %$self = ();
   bless $self, "AnyEvent::Handle::destroyed";
}

sub AnyEvent::Handle::destroyed::AUTOLOAD {
   #nop
}

=item $handle->destroyed

Returns false as long as the handle hasn't been destroyed by a call to C<<
->destroy >>, true otherwise.

Can be useful to decide whether the handle is still valid after some
callback possibly destroyed the handle. For example, C<< ->push_write >>,
C<< ->starttls >> and other methods can call user callbacks, which in turn
can destroy the handle, so work can be avoided by checking sometimes:

   $hdl->starttls ("accept");
   return if $hdl->destroyed;
   $hdl->push_write (...

Note that the call to C<push_write> will silently be ignored if the handle
has been destroyed, so often you can just ignore the possibility of the
handle being destroyed.

=cut

sub destroyed { 0 }
sub AnyEvent::Handle::destroyed::destroyed { 1 }

=item AnyEvent::Handle::TLS_CTX

This function creates and returns the AnyEvent::TLS object used by default
for TLS mode.

The context is created by calling L<AnyEvent::TLS> without any arguments.

=cut

our $TLS_CTX;

sub TLS_CTX() {
   $TLS_CTX ||= do {
      require AnyEvent::TLS;

      new AnyEvent::TLS
   }
}

=back


=head1 NONFREQUENTLY ASKED QUESTIONS

=over 4

=item I C<undef> the AnyEvent::Handle reference inside my callback and
still get further invocations!

That's because AnyEvent::Handle keeps a reference to itself when handling
read or write callbacks.

It is only safe to "forget" the reference inside EOF or error callbacks,
from within all other callbacks, you need to explicitly call the C<<
->destroy >> method.

=item Why is my C<on_eof> callback never called?

Probably because your C<on_error> callback is being called instead: When
you have outstanding requests in your read queue, then an EOF is
considered an error as you clearly expected some data.

To avoid this, make sure you have an empty read queue whenever your handle
is supposed to be "idle" (i.e. connection closes are O.K.). You can set
an C<on_read> handler that simply pushes the first read requests in the
queue.

See also the next question, which explains this in a bit more detail.

=item How can I serve requests in a loop?

Most protocols consist of some setup phase (authentication for example)
followed by a request handling phase, where the server waits for requests
and handles them, in a loop.

There are two important variants: The first (traditional, better) variant
handles requests until the server gets some QUIT command, causing it to
close the connection first (highly desirable for a busy TCP server). A
client dropping the connection is an error, which means this variant can
detect an unexpected detection close.

To handle this case, always make sure you have a non-empty read queue, by
pushing the "read request start" handler on it:

   # we assume a request starts with a single line
   my @start_request; @start_request = (line => sub {
      my ($hdl, $line) = @_;

      ... handle request

      # push next request read, possibly from a nested callback
      $hdl->push_read (@start_request);
   });

   # auth done, now go into request handling loop
   # now push the first @start_request
   $hdl->push_read (@start_request);

By always having an outstanding C<push_read>, the handle always expects
some data and raises the C<EPIPE> error when the connction is dropped
unexpectedly.

The second variant is a protocol where the client can drop the connection
at any time. For TCP, this means that the server machine may run out of
sockets easier, and in general, it means you cannot distinguish a protocl
failure/client crash from a normal connection close. Nevertheless, these
kinds of protocols are common (and sometimes even the best solution to the
problem).

Having an outstanding read request at all times is possible if you ignore
C<EPIPE> errors, but this doesn't help with when the client drops the
connection during a request, which would still be an error.

A better solution is to push the initial request read in an C<on_read>
callback. This avoids an error, as when the server doesn't expect data
(i.e. is idly waiting for the next request, an EOF will not raise an
error, but simply result in an C<on_eof> callback. It is also a bit slower
and simpler:

   # auth done, now go into request handling loop
   $hdl->on_read (sub {
      my ($hdl) = @_;

      # called each time we receive data but the read queue is empty
      # simply start read the request

      $hdl->push_read (line => sub {
         my ($hdl, $line) = @_;

         ... handle request

         # do nothing special when the request has been handled, just
         # let the request queue go empty.
      });
   });

=item I get different callback invocations in TLS mode/Why can't I pause
reading?

Unlike, say, TCP, TLS connections do not consist of two independent
communication channels, one for each direction. Or put differently, the
read and write directions are not independent of each other: you cannot
write data unless you are also prepared to read, and vice versa.

This means that, in TLS mode, you might get C<on_error> or C<on_eof>
callback invocations when you are not expecting any read data - the reason
is that AnyEvent::Handle always reads in TLS mode.

During the connection, you have to make sure that you always have a
non-empty read-queue, or an C<on_read> watcher. At the end of the
connection (or when you no longer want to use it) you can call the
C<destroy> method.

=item How do I read data until the other side closes the connection?

If you just want to read your data into a perl scalar, the easiest way
to achieve this is by setting an C<on_read> callback that does nothing,
clearing the C<on_eof> callback and in the C<on_error> callback, the data
will be in C<$_[0]{rbuf}>:

   $handle->on_read (sub { });
   $handle->on_eof (undef);
   $handle->on_error (sub {
      my $data = delete $_[0]{rbuf};
   });

Note that this example removes the C<rbuf> member from the handle object,
which is not normally allowed by the API. It is expressly permitted in
this case only, as the handle object needs to be destroyed afterwards.

The reason to use C<on_error> is that TCP connections, due to latencies
and packets loss, might get closed quite violently with an error, when in
fact all data has been received.

It is usually better to use acknowledgements when transferring data,
to make sure the other side hasn't just died and you got the data
intact. This is also one reason why so many internet protocols have an
explicit QUIT command.

=item I don't want to destroy the handle too early - how do I wait until
all data has been written?

After writing your last bits of data, set the C<on_drain> callback
and destroy the handle in there - with the default setting of
C<low_water_mark> this will be called precisely when all data has been
written to the socket:

   $handle->push_write (...);
   $handle->on_drain (sub {
      AE::log debug => "All data submitted to the kernel.";
      undef $handle;
   });

If you just want to queue some data and then signal EOF to the other side,
consider using C<< ->push_shutdown >> instead.

=item I want to contact a TLS/SSL server, I don't care about security.

If your TLS server is a pure TLS server (e.g. HTTPS) that only speaks TLS,
connect to it and then create the AnyEvent::Handle with the C<tls>
parameter:

   tcp_connect $host, $port, sub {
      my ($fh) = @_;

      my $handle = new AnyEvent::Handle
         fh  => $fh,
         tls => "connect",
         on_error => sub { ... };

      $handle->push_write (...);
   };

=item I want to contact a TLS/SSL server, I do care about security.

Then you should additionally enable certificate verification, including
peername verification, if the protocol you use supports it (see
L<AnyEvent::TLS>, C<verify_peername>).

E.g. for HTTPS:

   tcp_connect $host, $port, sub {
      my ($fh) = @_;

       my $handle = new AnyEvent::Handle
          fh       => $fh,
          peername => $host,
          tls      => "connect",
          tls_ctx  => { verify => 1, verify_peername => "https" },
          ...

Note that you must specify the hostname you connected to (or whatever
"peername" the protocol needs) as the C<peername> argument, otherwise no
peername verification will be done.

The above will use the system-dependent default set of trusted CA
certificates. If you want to check against a specific CA, add the
C<ca_file> (or C<ca_cert>) arguments to C<tls_ctx>:

       tls_ctx  => {
          verify          => 1,
          verify_peername => "https",
          ca_file         => "my-ca-cert.pem",
       },

=item I want to create a TLS/SSL server, how do I do that?

Well, you first need to get a server certificate and key. You have
three options: a) ask a CA (buy one, use cacert.org etc.) b) create a
self-signed certificate (cheap. check the search engine of your choice,
there are many tutorials on the net) or c) make your own CA (tinyca2 is a
nice program for that purpose).

Then create a file with your private key (in PEM format, see
L<AnyEvent::TLS>), followed by the certificate (also in PEM format). The
file should then look like this:

   -----BEGIN RSA PRIVATE KEY-----
   ...header data
   ... lots of base64'y-stuff
   -----END RSA PRIVATE KEY-----

   -----BEGIN CERTIFICATE-----
   ... lots of base64'y-stuff
   -----END CERTIFICATE-----

The important bits are the "PRIVATE KEY" and "CERTIFICATE" parts.  Then
specify this file as C<cert_file>:

   tcp_server undef, $port, sub {
      my ($fh) = @_;

      my $handle = new AnyEvent::Handle
         fh       => $fh,
         tls      => "accept",
         tls_ctx  => { cert_file => "my-server-keycert.pem" },
         ...

When you have intermediate CA certificates that your clients might not
know about, just append them to the C<cert_file>.

=back

=head1 SUBCLASSING AnyEvent::Handle

In many cases, you might want to subclass AnyEvent::Handle.

To make this easier, a given version of AnyEvent::Handle uses these
conventions:

=over 4

=item * all constructor arguments become object members.

At least initially, when you pass a C<tls>-argument to the constructor it
will end up in C<< $handle->{tls} >>. Those members might be changed or
mutated later on (for example C<tls> will hold the TLS connection object).

=item * other object member names are prefixed with an C<_>.

All object members not explicitly documented (internal use) are prefixed
with an underscore character, so the remaining non-C<_>-namespace is free
for use for subclasses.

=item * all members not documented here and not prefixed with an underscore
are free to use in subclasses.

Of course, new versions of AnyEvent::Handle may introduce more "public"
member variables, but that's just life. At least it is documented.

=back

=head1 AUTHOR

Robin Redeker C<< <elmex at ta-sa.org> >>, Marc Lehmann <schmorp@schmorp.de>.

=cut

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