NAME

Future::Buffer - a string buffer that uses Futures

SYNOPSIS

   use Future::Buffer;

   use Future::AsyncAwait;
   use Future::IO;

   my $buffer = Future::Buffer->new(
      fill => sub { Future::IO->sysread( $socket, 8192 ) }
   );

   async sub print_lines
   {
      while(1) {
         my $line = await $buffer->read_until( "\n" );
         chomp $line;

         say "Got a line: $line";
      }
   }

   await print_lines();

DESCRIPTION

Objects in this class provide a string buffer, on which operations return Future instances which will complete when data is available. Data can be inserted into the buffer either in a push-based manner by calling the write method, or in a pull-based manner by providing it with a fill callback by which it can request data itself. This flexibility allows the buffer to act as an adapter between push- and pull-based providers and consumers.

Each read-like method returns a Future which will complete once there are enough bytes in the buffer to satisfy the required condition. The buffer behaves somewhat like a pipe, where bytes provided at the writing end (either by the write method or the fill callback) are eventually consumed at the reading end by one of the read futures.

Multiple read futures can remain pending at once, and will be completed in the order they were created when more data is eventually available. Thus, any call to the write method to provide more data can potentially result in multiple futures becoming ready.

Since version 0.04 the buffer supports an end-of-file condition. The "close" method or a fill callback future yielding an empty result will mark that the buffer is now closed. Once it has exhausted the remaining stored data any further read futures will yield empty.

CONSTRUCTOR

new

   $buffer = Future::Buffer->new( %args );

Returns a new Future::Buffer instance.

Takes the following named arguments:

fill => CODE

   $data = await $fill->();

Optional callback which the buffer will invoke when it needs more data.

Any read futures which are waiting on the fill future are constructed by using the fill future as a prototype, ensuring they have the correct type.

If the result is an empty list this will be treated as an end-of-file notification and the buffer is closed.

METHODS

length

   $len = $buffer->length;

Returns the length of the currently-stored data; that is, data that has been provided by write calls or the fill callback but not yet consumed by a read future.

is_empty

   $empty = $buffer->is_empty;

Returns true if the stored length is zero.

write

   $f = $buffer->write( $data );

Appends to the stored data, invoking any pending read futures that are outstanding and can now complete.

Currently this method returns an already-completed Future. Some later version may implement a buffer maximum size, and choose not to complete this future until there is enough space to accept the new data. For now it is safe for the caller to ignore the return value, but it may become not so.

close

   $buffer->close;

Marks that the buffer is now at EOF condition. Once any remaining buffered content is consumed, any further read futures will all yield EOF condition.

read_atmost

   $data = await $buffer->read_atmost( $len );

Returns a future which will complete when there is some data available in the buffer and will yield up too the given length. Note that, analogous to calling the read IO method on a filehandle, this can still complete and yield a shorter length if less is currently available.

If the stream is closed and there is no remaining data, the returned future will yield empty.

read_exactly

   $data = await $buffer->read_exactly( $len );

Returns a future which will complete when there is enough data available in the buffer to yield exactly the length given.

If the stream is closed and there is no remaining data, the returned future will yield empty.

read_until

   $data = await $buffer->read_until( $pattern );

Returns a future which will complete when the buffer contains a match for the given pattern (which may either be a plain string or a compiled Regexp). The future will yield the contents of the buffer up to and including this match.

If the stream is closed and there is no remaining data, the returned future will yield empty.

For example, a readline-like operation can be performed by

   $f = $buffer->read_until( "\x0d\x0a" );

read_unpacked

   $data = await $buffer->read_unpacked( $pack_format );

Since version 0.03.

Returns a future which will complete when the buffer contains enough data to unpack all of the requested fields using the given pack() format. The future will yield a list of all the fields extracted by the format.

If the stream is closed and there is no remaining data, the returned future will yield empty.

Note that because the implementation is shamelessly stolen from IO::Handle::Packable the same limitations on what pack formats are recognized will apply.

unread

   $buffer->unread( $data );

Since version 0.03.

Prepends more data back into the buffer,

It is uncommon to need this method, but it may be useful in certain situations such as when it is hard to determine upfront how much data needs to be read for a single operation, and it turns out too much was read. The trailing content past what is needed can be put back for a later operation.

Note that use of this method causes an inherent race condition between outstanding read futures and existing data in the buffer. If there are no pending futures then this is safe. If there is no existing data already in the buffer this is also safe. If neither of these is true then a warning is printed indicating that the logic of the caller is not well-defined.

TODO

• An "on-read" event, taking maybe inspiration from IO::Async::Stream. This would allow both pull- and push-based consumers.

• Size limitation. Allow an upper bound of stored data, make write calls return pending futures until buffer can accept it. Needs consideration of unbounded read_until though.

• Consider extensions of the "read_unpacked" method to handle more situations. This may require building a shared CPAN module for doing streaming-unpack along with IO::Handle::Packable and other situations.

AUTHOR

Paul Evans <leonerd@leonerd.org.uk>

Inspired by Ryu::Buffer by Tom Molesworth <TEAM@cpan.org>

cpanm Future::Buffer

perl -MCPAN -e shell
install Future::Buffer