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);
    
        # Writer too (but buffers unwritten items until your next write_burst(),
        # write_message(), or flush(), or will do a writing block when the pipe
        # instance is destroyed.
        $w->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;

MIXED DATA MODE

    Mixed data mode is a special use-case for Atomic::Pipe. In this mode
    the assumption is that the writer end of the pipe uses the pipe as
    STDOUT or STDERR, and as such a lot of random non-atomic prints can
    happen on the writer end of the pipe. The special case is when you want
    to send atomic-chunks of data inline with the random prints, and in the
    end extract the data from the noise. The atomic nature of messages and
    bursts makes this possible.

    Please note that mixed data mode makes use of 3 ASCII control
    characters:

    SHIFT OUT (^N or \x0E)

      Used to start a burst

    SHIFT IN (^O or \x0F)

      Used to terminate a burst

    DATA LINK ESCAPE (^P or \x10)

      If this directly follows a SHIFT-OUT it marks the burst as being part
      of a data-message.

    If the random prints include SHIFT OUT then they will confuse the
    read-side parser and it will not be possible to extract data, in fact
    reading from the pipe will become quite unpredictable. In practice this
    is unlikely to cause issues, but printing a binary file or random noise
    could do it.

    A burst may not include SHIFT IN as the SHIFT IN control+character
    marks the end of a burst. A burst may also not start with the DATA LINK
    ESCAPE control character as that is used to mark the start of a
    data-message.

    data-messages may contain any data/characters/bytes as they messages
    include a length so an embedded SHIFT IN will not terminate things
    early.

        # Create a pair in mixed-data mode
        my ($r, $w) = Atomic::Pipe->pair(mixed_data_mode => 1);
    
        # Open STDOUT to the write handle
        open(STDOUT, '>&', $w->{wh}) or die "Could not clone write handle: $!";
    
        # For sanity
        $wh->autoflush(1);
    
        print "A line!\n";
    
        print "Start a line ..."; # Note no "\n"
    
        # Any number of newlines is fine the message will send/recieve as a whole.
        $w->write_burst("This is a burst message\n\n\n");
    
        # Data will be broken into atomic chunks and sent
        $w->write_message($data);
    
        print "... Finish the line we started earlier\n";
    
        my ($type, $data) = $r->get_line_burst_or_data;
        # Type: 'line'
        # Data: "A line!\n"
    
        ($type, $data) = $r->get_line_burst_or_data;
        # Type: 'burst'
        # Data: "This is a burst message\n\n\n"
    
        ($type, $data) = $r->get_line_burst_or_data;
        # Type: 'message'
        # Data: $data
    
        ($type, $data) = $r->get_line_burst_or_data;
        # Type: 'line'
        # Data: "Start a line ...... Finish the line we started earlier\n"
    
        # mixed-data mode is always non-blocking
        ($type, $data) = $r->get_line_burst_or_data;
        # Type: undef
        # Data: undef

    You can also turn mixed-data mode after construction, but you must do
    so on both ends:

        $r->set_mixed_data_mode();
        $w->set_mixed_data_mode();

    Doing so will make the pipe non-blocking, and will make all
    bursts/messages include the necessary control characters.

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.

    $r = Atomic::Pipe->read_fifo($FIFO_PATH)

    $w = Atomic::Pipe->write_fifo($FIFO_PATH)

      These 2 constructors let you connect to a FIFO by filesystem path.

      The interface difference (read_fifo and write_fifo vs specifying a
      mode) is because the modes to use for fifo's are not obvious ('+<'
      for reading).

      NOTE: THERE IS NO EOF for the read-end in the process that created
      the fifo. You need to figure out when the last message is received on
      your own somehow. If you use blocking reads in a loop with no loop
      exit condition then the loop will never end even after all writers
      are gone.

    $p = Atomic::Pipe->from_fh($fh)

    $p = Atomic::Pipe->from_fh($mode, $fh)

      Create an instance around an existing filehandle (A clone of the
      handle will be made and kept internally).

      This will fail if the handle is not a pipe.

      If no mode is provided this constructor will determine the mode
      (reader or writer) for you from the given handle. Note: This works on
      linux, but not BSD or Solaris, on most platforms your must provide a
      mode.

      Valid modes:

      '>&'

	Write-only.

      '>&='

	Write-only and reuse fileno.

      '<&'

	Read-only.

      '<&='

	Read-only and reuse fileno.

    $p = Atomic::Pipe->from_fd($mode, $fd)

      $fd must be a file descriptor number.

      This will fail if the fd is not a pipe.

      You must specify one of these modes (as a string):

      '>&'

	Write-only.

      '>&='

	Write-only and reuse fileno.

      '<&'

	Read-only.

      '<&='

	Read-only and reuse fileno.

 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. This works on read and write handles. On
      writers this will write as many chunks/bursts as it can, then buffer
      any remaining until your next write_message(), write_burst(), or
      flush(), at which point it will write as much as it can again. If the
      instance is garbage collected with chunks/bursts in the buffer it
      will block until all can be written.

    $bool = $p->pending_output

      True if the pipe is a non-blocking writer and there is pending output
      waiting for a flush (and for the pipe to have room for the new data).

    $w->flush()

      Write any buffered items. This is only useful on writers that are in
      non-blocking mode, it is a no-op everywhere else.

    $bool = $r->eof()

      True if all writers are closed, and the buffers do not contain any
      usable data.

      Usable data means raw data that has yet to be processed, complete
      messages, or complete data bursts. Any of these can still be
      retreieved using read_message(), or get_line_burst_or_data().

    $p->close

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

    $undef_or_bytes = $p->fits_in_burst($data)

      This will return undef if the data DES NOT fit in a burst. This will
      return the size of the data in bytes if it will fit in a burst.

    $undef_or_true = $p->write_burst($data)

      Attempt to write $data in a single atomic burst. If the data is too
      big to write atomically this method will not write any data and will
      return undef. If the data does fit in an atomic write then a true
      value will be returned.

      Note: YOU MUST NOT USE read_message() when writing bursts. This
      method sends the data as-is with no data-header or modification. This
      method should be used when the other side is reading the pipe
      directly without an Atomic::Pipe on the receiving end.

      The primary use case of this is if you have multiple writers sending
      short plain-text messages that will not exceed the atomic pipe buffer
      limit (minimum of 512 bytes on systems that support atomic pipes
      accoring to POSIX).

    $fh = $p->rh

    $fh = $p->wh

      Get the read or write handles.

    $read_size = $p->read_size()

    $p->read_size($read_size)

      Get/set the read size. This is how much data to ATTEMPT to read each
      time fill_buffer() is called. The default is 65,536 which is the
      default pipe size on linux, though the value is hardcoded currently.

    $bytes = $p->fill_buffer

      Read a chunk of data from the pipe and store it in the internal
      buffer. Bytes read are returned. This is only useful if you want to
      pull data out of the pipe (maybe to unblock the writer?) but do not
      want to process any of the data yet.

      This is automatically called as needed by other methods, usually you
      do not need to use it directly.

  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.

  MIXED DATA MODE METHODS

    $p->set_mixed_data_mode

      Enable mixed-data mode. Also makes read-side non-blocking.

    ($type, $data) = $r->get_line_burst_or_data()

      Get a line, a burst, or a message from the pipe. Always non-blocking,
      will return (undef, undef) if no complete line/burst/message is
      ready.

      $type will be one of: undef, 'line', 'burst', or 'message'.

      $data will either be undef, or a complete line, burst, or message.

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/

