NAME

Atomic::Pipe - Send atomic messages from multiple writers across a POSIX pipe.

DESCRIPTION

Normally if you write to a pipe from multiple processes/threads, the messages will come mixed together unpredictably. Some messages may be interrupted by parts of messages from other writers. This module takes advantage of some POSIX specifications to allow multiple writers to send arbitrary data down a pipe in atomic chunks to avoid the issue.

NOTE: This only works for POSIX compliant pipes on POSIX compliant systems. Also some features may not be available on older systems, or some platforms.

Also: https://man7.org/linux/man-pages/man7/pipe.7.html

    POSIX.1 says that write(2)s of less than PIPE_BUF bytes must be
    atomic: the output data is written to the pipe as a contiguous
    sequence.  Writes of more than PIPE_BUF bytes may be nonatomic: the
    kernel may interleave the data with data written by other processes.
    POSIX.1 requires PIPE_BUF to be at least 512 bytes.  (On Linux,
    PIPE_BUF is 4096 bytes.) [...]

Under the hood this module will split your message into small sections of slightly smaller than the PIPE_BUF limit. Each message will be sent as 1 atomic chunk with a 4 byte prefix indicating what process id it came from, what thread id it came from, a chunk ID (in descending order, so if there are 3 chunks the first will have id 2, the second 1, and the final chunk is always 0 allowing a flush as it knows it is done) and then 1 byte with the length of the data section to follow.

On the receiving end this module will read chunks and re-assemble them based on the header data. So the reader will always get complete messages. Note that message order is not guarenteed when messages are sent from multiple processes or threads. Though all messages from any given thread/process should be in order.

SYNOPSIS

    use Atomic::Pipe;

    my ($r, $w) = Atomic::Pipe->pair;

    # Chunks will be set to the number of atomic chunks the message was split
    # into. It is fine to ignore the value returned, it will always be an
    # integer 1 or larger.
    my $chunks = $w->send_message("Hello");

    # $msg now contains "Hello";
    my $msg = $r->read_message;

    # Note, you can set the reader to be non-blocking:
    $r->blocking(0);

    # $msg2 will be undef as no messages were sent, and blocking is turned off.
    my $msg2 = $r->read_message;

Fork example from tests:

    use Test2::V0;
    use Test2::Require::RealFork;
    use Test2::IPC;
    use Atomic::Pipe;

    my ($r, $w) = Atomic::Pipe->pair;

    # For simplicty
    $SIG{CHLD} = 'IGNORE';

    # Forks and runs your coderef, then exits.
    sub worker(&) { ... }

    worker { is($w->write_message("aa" x $w->PIPE_BUF), 3, "$$ Wrote 3 chunks") };
    worker { is($w->write_message("bb" x $w->PIPE_BUF), 3, "$$ Wrote 3 chunks") };
    worker { is($w->write_message("cc" x $w->PIPE_BUF), 3, "$$ Wrote 3 chunks") };

    my @messages = ();
    push @messages => $r->read_message for 1 .. 3;

    is(
        [sort @messages],
        [sort(('aa' x PIPE_BUF), ('bb' x PIPE_BUF), ('cc' x PIPE_BUF))],
        "Got all 3 long messages, not mangled or mixed, order not guarenteed"
    );

    done_testing;

METHODS

CLASS METHODS

$bytes = Atomic::Pipe->PIPE_BUF

Get the maximum number of bytes for an atomic write to a pipe.

($r, $w) = Atomic::Pipe->pair

Create a pipe, returns a list consisting of a reader and a writer.

$p = Atomic::Pipe->new

If you really must have a new() method it is here for you to abuse. The returned pipe has both handles, it is your job to then turn it into 2 clones one with the reader and one with the writer. It is also your job to make you do not have too many handles floating around preventing an EOF.

OBJECT METHODS

PRIMARY INTERFACE

$p->write_message($msg)

Send a message in atomic chunks.

$msg = $p->read_message

Get the next message. This will block until a message is received unless you set $p->blocking(0). If blocking is turned off, and no message is ready, this will return undef. This will also return undef when the pipe is closed (EOF).

$p->blocking($bool)

$bool = $p->blocking

Get/Set blocking status.

$bool = $p->eof

True if EOF (all writers are closed).

$p->close

Close this end of the pipe (or both ends if this is not yet split into reader/writer pairs).

RESIZING THE PIPE BUFFER

On some newer linux systems it is possible to get/set the pipe size. On supported systems these allow you to do that, on other systems they are no-ops, and any that return a value will return undef.

Note: This has nothing to do with the similarly named PIPE_BUF which cannot be changed. This simply effects how much data can sit in a pipe before the writers block, it does not effect the max size of atomic writes.

$bytes = $p->size

Current size of the pipe buffer.

$bytes = $p->max_size

Maximum size, or undef if that cannot be determined. (Linux only for now).

$p->resize($bytes)

Attempt to set the pipe size in bytes. It may not work, so check $p->size.

$p->resize_or_max($bytes)

Attempt to set the pipe to the specified size, but if the size is larger than the maximum fall back to the maximum size instead.

SPLITTING THE PIPE INTO READER AND WRITER

If you used Atomic::Pipe->new() you need to now split the one object into readers and writers. These help you do that.

$bool = $p->is_reader

This returns true if this instance is ONLY a reader.

$p->is_writer

This returns true if this instance is ONLY a writer.

$p->clone_reader

This copies the object into a reader-only copy.

$p->clone_writer

This copies the object into a writer-only copy.

$p->reader

This turnes the object into a reader-only. Note that if you have no writer-copies then effectively makes it impossible to write to the pipe as you cannot get a writer anymore.

$p->writer

This turnes the object into a writer-only. Note that if you have no reader-copies then effectively makes it impossible to read from the pipe as you cannot get a reader anymore.

SOURCE

The source code repository for Atomic-Pipe can be found at http://github.com/exodist/Atomic-Pipe.

MAINTAINERS

Chad Granum <exodist@cpan.org>

AUTHORS

Chad Granum <exodist@cpan.org>

COPYRIGHT

Copyright 2020 Chad Granum <exodist7@gmail.com>.

This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

See http://dev.perl.org/licenses/

cpanm Atomic::Pipe

perl -MCPAN -e shell
install Atomic::Pipe