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 "on_drain" callback will be invoked once.

$handle->on_drain ($cb)

Sets the "on_drain" callback or clears it (see the description of "on_drain" in the constructor).

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

$handle->push_write ($data)

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

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

$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 "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):

$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 "on_drain" handler by a callback that shuts down the socket (and set "low_water_mark" to 0). This method is a shorthand for just that, and replaces the "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 "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).

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 "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 "type" is used, "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 "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
   }
    

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 "on_read" and the "complex" way, using a queue.

In the simple case, you just install an "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 ("$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. "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 "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 "unshift" another line-read. This line-read will be queued 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];
      ...
   });

$handle->on_read ($cb)

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

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

$handle->rbuf

Returns the read buffer (as a modifiable lvalue). You can also access the read buffer directly as the "->{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 "on_read" callback or when "push_read" or "unshift_read" are used with a single callback (i.e. untyped). Typed "push_read" and "unshift_read" methods will manage the read buffer on their own.

$handle->push_read ($cb)

$handle->unshift_read ($cb)

Append the given callback to the end of the queue ("push_read") or prepend it ("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).

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

$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 "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):

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 "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, "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 "->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 $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 (perldoc -m AnyEvent::Handle, search for "register_read_type")).

$handle->stop_read

$handle->start_read

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

Note that AnyEvent::Handle will automatically "start_read" for you when you change the "on_read" callback or push/unshift a read callback, and it will automatically "stop_read" for you when neither "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.

$handle->starttls ($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 "starttls". See the "tls" constructor argument for general info.

Starting TLS is currently an asynchronous operation - when you push some write data and then call "->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 "tls" constructor argument (either "connect", "accept" or an existing Net::SSLeay object).

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

The TLS connection object will end up in "$handle->{tls}", the TLS context in "$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).

$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).

$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.

$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 NOT work in a read or write callback, so when you want to destroy the AnyEvent::Handle object from within such an callback. You MUST call "->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 "linger" option, however.

$handle->destroyed

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

Can be useful to decide whether the handle is still valid after some callback possibly destroyed the handle. For example, "->push_write", "->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 "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.

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 AnyEvent::TLS without any arguments.