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IO::Async::Stream.3pm

NAME

"IO::Async::Stream" - read and write buffers around an IO handle

SYNOPSIS

  use IO::Socket::INET;
  use IO::Async::Stream;
 
  use IO::Async::Loop;
  my $loop = IO::Async::Loop->new();
 
  my $socket = IO::Socket::INET->new(
     PeerHost => "some.other.host",
     PeerPort => 12345,
     Blocking => 0,                   # This line is very important
  );
 
  my $stream = IO::Async::Stream->new(
     handle => $socket,
 
     on_read => sub {
        my ( $self, $buffref, $closed ) = @_;
 
        if( $$buffref =~ s/^(.*\n)// ) {
           print "Received a line $1";
 
           return 1;
        }
 
        if( $closed ) {
           print "Closed; last partial line is $$buffref\n";
        }
 
        return 0;
     }
  );
 
  $stream->write( "An initial line here\n" );
 
 

Or

  my $record_stream = IO::Async::Stream->new(
     handle => ...,
 
     on_read => sub {
        my ( $self, $buffref, $closed ) = @_;
 
        if( length $$buffref >= 16 ) {
           my $record = substr( $$buffref, 0, 16, "" );
           print "Received a 16-byte record: $record\n";
 
           return 1;
        }
 
        if( $closed and length $$buffref ) {
           print "Closed: a partial record still exists\n";
        }
 
        return 0;
     }
  );
 
 

Or

  use IO::Handle;
 
  my $stream = IO::Async::Stream->new(
     read_handle  => \*STDIN,
     write_handle => \*STDOUT,
     ...
  );
 
 

DESCRIPTION

This module provides a subclass of "IO::Async::Handle" which implements asynchronous communications buffers around stream handles. It provides buffering for both incoming and outgoing data, which are transferred to or from the actual OS-level filehandle as controlled by the containing Loop.

Data can be added to the outgoing buffer at any time using the "write()" method, and will be flushed whenever the underlying handle is notified as being write-ready. Whenever the handle is notified as being read-ready, the data is read in from the handle, and the "on_read" code is called to indicate the data is available. The code can then inspect the buffer and possibly consume any input it considers ready.

This object may be used in one of two ways; with a callback function, or as a base class.

Callbacks

If certain keys are supplied to the constructor, they should contain CODE references to callback functions that will be called in the following manner:

  $ret = $on_read->( $self, \$buffer, $handleclosed )
 
  $on_read_error->( $self, $errno )
 
  $on_outgoing_empty->( $self )
 
  $on_write_error->( $self, $errno )
 
 

A reference to the calling "IO::Async::Stream" object is passed as the first argument, so that the callback can access it.

Base Class

If a subclass is built, then it can override the "on_read" or "on_outgoing_empty" methods, which will be called in the following manner:

  $ret = $self->on_read( \$buffer, $handleclosed )
 
  $self->on_read_error( $errno )
 
  $self->on_outgoing_empty()
 
  $self->on_write_error( $errno )
 
 

The first argument to the "on_read()" callback is a reference to a plain perl string. The code should inspect and remove any data it likes, but is not required to remove all, or indeed any of the data. Any data remaining in the buffer will be preserved for the next call, the next time more data is received from the handle.

In this way, it is easy to implement code that reads records of some form when completed, but ignores partially-received records, until all the data is present. If the method is confident no more useful data remains, it should return 0. If not, it should return 1, and the method will be called again. This makes it easy to implement code that handles multiple incoming records at the same time. See the examples at the end of this documentation for more detail.

The second argument to the "on_read()" method is a scalar indicating whether the handle has been closed. Normally it is false, but will become true once the handle closes. A reference to the buffer is passed to the method in the usual way, so it may inspect data contained in it. Once the method returns a false value, it will not be called again, as the handle is now closed and no more data can arrive.

The "on_read()" code may also dynamically replace itself with a new callback by returning a CODE reference instead of 0 or 1. The original callback or method that the object first started with may be restored by returning "undef". Whenever the callback is changed in this way, the new code is called again; even if the read buffer is currently empty. See the examples at the end of this documentation for more detail.

The "on_read_error" and "on_write_error" callbacks are passed the value of $! at the time the error occured. (The $! variable itself, by its nature, may have changed from the original error by the time this callback runs so it should always use the value passed in).

If an error occurs when the corresponding error callback is not supplied, and there is not a subclass method for it, then the "close()" method is called instead.

The "on_outgoing_empty" callback is not passed any arguments.

PARAMETERS

The following named parameters may be passed to "new" or "configure":

read_handle => IO

The IO handle to read from. Must implement "fileno" and "sysread" methods.

write_handle => IO

The IO handle to write to. Must implement "fileno" and "syswrite" methods.

handle => IO

Shortcut to specifying the same IO handle for both of the above.

on_read => CODE

A CODE reference for when more data is available in the internal receiving buffer.

on_read_error => CODE

A CODE reference for when the "sysread()" method on the read handle fails.

on_outgoing_empty => CODE

A CODE reference for when the writing data buffer becomes empty.

on_write_error => CODE

A CODE reference for when the "syswrite()" method on the write handle fails.

autoflush => BOOL

Optional. If true, the "write" method will attempt to write data to the operating system immediately, without waiting for the loop to indicate the filehandle is write-ready. This is useful, for example, on streams that should contain up-to-date logging or console information.

It currently defaults to false for any file handle, but future versions of "IO::Async" may enable this by default on STDOUT and STDERR.

read_all => BOOL

Optional. If true, attempt to read as much data from the kernel as possible when the handle becomes readable. By default this is turned off, meaning at most a fixed-size buffer of 8 KiB is read. If there is still more data in the kernel's buffer, the handle will still be readable, and will be read from again. This behaviour allows multiple streams to be multiplexed simultaneously, meaning that a large bulk transfer on one stream cannot starve other filehandles of processing time. Turning this option on may improve bulk data transfer rate, at the risk of delaying or stalling processing on other filehandles.

write_all => BOOL

Optional. Analogous to the "read_all" option, but for writing to filehandles into the kernel buffer. When "autoflush" is enabled, option only affects deferred writing if the initial attempt failed due to buffer space.

If a read handle is given, it is required that either an "on_read" callback reference is passed, or that the object provides an "on_read" method. It is optional whether either is true for "on_outgoing_empty"; if neither is supplied then no action will be taken when the writing buffer becomes empty.

An "on_read" callback may be supplied even if no read handle is yet given, to be used when a read handle is eventually provided by the "set_handles" method.

METHODS

$stream->close

A synonym for "close_when_empty". This should not be used when the deferred wait behaviour is required, as the behaviour of "close" may change in a future version of "IO::Async". Instead, call "close_when_empty" directly.

$stream->close_when_empty

If the write buffer is empty, this method calls "close" on the underlying IO handles, and removes the stream from its containing loop. If the write buffer still contains data, then this is deferred until the buffer is empty. This is intended for ``write-then-close'' one-shot streams.

  $stream->write( "Here is my final data\n" );
  $stream->close_when_empty;
 
 

Because of this deferred nature, it may not be suitable for error handling. See instead the "close_now" method.

$stream->close_now

This method immediately closes the underlying IO handles and removes the stream from the containing loop. It will not wait to flush the remaining data in the write buffer.

$stream->write( $data )

This method adds data to the outgoing data queue. The data is not yet sent to the handle; this will be done later in the "on_write_ready()" method.

$data

A scalar containing data to write

If the "autoflush" option is set, this method will try immediately to write the data to the underlying filehandle. If this completes successfully then it will have been written by the time this method returns. If it fails to write completely, then the data is queued as if "autoflush" were not set, and will be flushed later as normal by the "on_write_ready()" method.

EXAMPLES

A line-based on_read() method

The following "on_read()" method accepts incoming "\n"-terminated lines and prints them to the program's "STDOUT" stream.

  sub on_read
  {
     my $self = shift;
     my ( $buffref, $handleclosed ) = @_;
 
     if( $$buffref =~ s/^(.*\n)// ) {
        print "Received a line: $1";
        return 1;
     }
 
     return 0;
  }
 
 

Because a reference to the buffer itself is passed, it is simple to use a "s///" regular expression on the scalar it points at, to both check if data is ready (i.e. a whole line), and to remove it from the buffer. If no data is available then 0 is returned, to indicate it should not be tried again. If a line was successfully extracted, then 1 is returned, to indicate it should try again in case more lines exist in the buffer.

Dynamic replacement of on_read()

Consider the following protocol (inspired by IMAP), which consists of "\n"-terminated lines that may have an optional data block attached. The presence of such a data block, as well as its size, is indicated by the line prefix.

  sub on_read
  {
     my $self = shift;
     my ( $buffref, $handleclosed ) = @_;
 
     if( $$buffref =~ s/^DATA (\d+):(.*)\n// ) {
        my $length = $1;
        my $line   = $2;
 
        return sub {
           my $self = shift;
           my ( $buffref, $handleclosed ) = @_;
 
           return 0 unless length $$buffref >= $length;
 
           # Take and remove the data from the buffer
           my $data = substr( $$buffref, 0, $length, "" );
 
           print "Received a line $line with some data ($data)\n";
 
           return undef; # Restore the original method
        }
     }
     elsif( $$buffref =~ s/^LINE:(.*)\n// ) {
        my $line = $1;
 
        print "Received a line $line with no data\n";
 
        return 1;
     }
     else {
        print STDERR "Unrecognised input\n";
        # Handle it somehow
     }
  }
 
 

In the case where trailing data is supplied, a new temporary "on_read()" callback is provided in a closure. This closure captures the $length variable so it knows how much data to expect. It also captures the $line variable so it can use it in the event report. When this method has finished reading the data, it reports the event, then restores the original method by returning "undef".

SEE ALSO

*

IO::Handle - Supply object methods for I/O handles

AUTHOR

Paul Evans <leonerd@leonerd.org.uk>