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Felipe Gasper
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NAME

IO::Framed - Convenience wrapper for frame-based I/O

SYNOPSIS

Reading:

    #See below about seed bytes.
    my $iof = IO::Framed->new( $fh, 'seed bytes' );

    #This returns undef if the $in_fh doesn’t have at least
    #the given length (5 in this case) of bytes to read.
    $frame = $iof->read(5);

    #Don’t call this after an incomplete read().
    $line_or_undef = $iof->read_until("\x0a");

Writing, unqueued (i.e., for blocking writes):

    #The second parameter (if given) is executed immediately after the final
    #byte of the payload is written. For blocking I/O this happens
    #before the following method returns.
    $iof->write('hoohoo', sub { print 'sent!' } );

Writing, queued (for non-blocking writes):

    $iof->enable_write_queue();

    #This just adds to a memory queue:
    $iof->write('hoohoo', sub { print 'sent!' } );

    #This will be 1, since we have 1 message/frame queued to send.
    $iof->get_write_queue_count();

    #Returns 1 if it empties out the queue; 0 otherwise.
    #Partial frame writes are accommodated; the callback given as 2nd
    #argument to write() only fires when the queue item is sent completely.
    my $empty = $iof->flush_write_queue();

You can also use IO::Framed::Read and IO::Framed::Write, which contain just the read and write features. (IO::Framed is actually a subclass of them both.)

DESCRIPTION

While writing Net::WAMP I noticed that I was reimplementing some of the same patterns I’d used in Net::WebSocket to parse frames from a stream:

  • Only read() entire frames, with a read queue for any partials.

  • Continuance when a partial frame is delivered.

  • Write queue with callbacks for non-blocking I/O

  • Signal resilience: resume read/write after Perl receives a trapped signal rather than throwing/giving EINTR. (cf. IO::SigGuard)

These are now made available in this distribution.

ABOUT READS

The premise here is that you expect a given number of bytes at a given time and that a partial read should be continued once it is sensible to do so.

As a result, read() will throw an exception if the number of bytes given for a continuance is not the same number as were originally requested. read_until() will throw a similar exception if called between an incomplete read() and its completion.

Example:

    #This reads only 2 bytes, so read() will return undef.
    $iof->read(10);

    #… wait for readiness if non-blocking …

    #XXX This die()s because we’re in the middle of trying to read
    #10 bytes, not 4.
    $iof->read(4);

    #If this completes the read (i.e., takes in 8 bytes), then it’ll
    #return the full 10 bytes; otherwise, it’ll return undef again.
    $iof->read(10);

EINTR prompts a redo of the read operation. EAGAIN and EWOULDBLOCK (the same error generally, but not always) prompt an undef return. Any other failures prompt an instance of IO::Framed::X::ReadError to be thrown.

End-Match Reads

Reader modules now implement a read_until() method, which reads arbitrarily many bytes until a given sequence of bytes appears then returns those bytes (plus the looked-for sequence in the return). An obvious application for this feature is line-by-line reads, e.g., to implement HTTP or other line-based protocols.

Empty Reads

This class’s read() and read_until() methods will, by default, throw an instance of IO::Framed::X::EmptyRead on an empty read. This is normal and logical behavior in contexts (like Net::WebSocket) where the data stream itself indicates when no more data will come across. In such cases an empty read is genuinely an error condition: it either means you’re reading past when you should, or the other side prematurely went away.

In some other cases, though, that empty read is the normal and expected way to know that a filehandle/socket has no more data to read.

If you prefer, then, you can call the allow_empty_read() method to switch to a different behavior, e.g.:

    $framed->allow_empty_read();

    my $frame = $framed->read(10);

    if (length $frame) {
        #yay, we got a frame!
    }
    elsif (defined $frame) {
        #no more data will come in, so let’s close up shop
    }
    else {
        #undef means we just haven’t gotten as much data as we want yet;
        #in this case, that means fewer than 10 bytes are available.
    }

    #----------------------------------------------------------------------
    # The same example as above with line-oriented input …

    my $line = $framed->read_until("\x0a");

    if (length $line) {
        #yay, we got a line!
    }
    elsif (defined $line) {
        #no more data will come in, so let’s close up shop
    }
    else {
        #undef means we just haven’t gotten a full line yet.
    }

Instead of throwing the aforementioned exception, read() now returns empty-string on an empty read. That means that you now have to distinguish between multiple “falsey” states: undef for when the requested number of bytes hasn’t yet arrived, and empty string for when no more bytes will ever arrive. But it is also true now that the only exceptions thrown are bona fide errors, which will suit some applications better than the default behavior.

NB: If you want to be super-light, you can bring in IO::Framed::Read instead of the full IO::Framed. (IO::Framed is already pretty lightweight, though.)

ABOUT WRITES

Writes for blocking I/O are straightforward: the system will always send the entire buffer. The OS’s write() won’t return until everything meant to be written is written. Life is pleasant; life is simple. :)

Non-blocking I/O is trickier. Not only can the OS’s write() write a subset of the data it’s given, but we also can’t know that the output filehandle is ready right when we want it. This means that we have to queue up our writes then write them once we know (e.g., through select()) that the filehandle is ready. Each write() call, then, enqueues one new buffer to write.

Since it’s often useful to know when a payload has been sent, write() accepts an optional callback that will be executed immediately after the last byte of the payload is written to the output filehandle.

Empty out the write queue by calling flush_write_queue() and looking for a truthy response. (A falsey response means there is still data left in the queue.) get_write_queue_count() gives you the number of queue items left to write. (A partially-written item is treated the same as a fully-unwritten one.)

Note that, while it’s acceptable to activate and deactive the write queue, the write queue must be empty in order to deactivate it. (You’ll get a nasty, untyped exception otherwise!)

write() returns undef on EAGAIN and EWOULDBLOCK. It retries on EINTR, so you should never actually see this error from this module. Other errors prompt a thrown exception.

NB: enable_write_queue() and disable_write_queue() return the object, so you can instantiate thus:

    my $nb_writer = IO::Framed::Write->new($fh)->enable_write_queue();

NB: If you want to be super-light, you can bring in IO::Framed::Write instead of the full IO::Framed. (IO::Framed is already pretty lightweight, though.)

CUSTOM READ & WRITE LOGIC

As of version 0.04, you can override READ and WRITE methods with your preferred logic. For example, in Linux you might prefer send() rather than syswrite() to avoid SIGPIPE, thus:

    package My::Framed;

    use parent qw( IO::Framed::Write );

    #Only these two arguments are given.
    sub WRITE {
        return send( $_[0], $_[1], Socket::MSG_NOSIGNAL );
    }

(NB: In *BSD OSes you can set SO_SIGNOPIPE on the filehandle instead.)

You can likewise set READ() to achieve the same effect for reads. (READ() receives all four arguments that sysread() can consume.)

IMPORTANT: Unlike most inherited methods, READ() and WRITE() do NOT receive the object instance. They must follow the same semantics as Perl’s sysread() and syswrite(): i.e., they must return the number of bytes read/written, or return undef and set $! appropriately on error.

ERROR RESPONSES

An empty read or any I/O error besides the ones mentioned previously are indicated via an instance of one of the following exceptions.

All exceptions subclass IO::Framed::X::Base, which itself subclasses X::Tiny::Base.

IO::Framed::X::ReadError
IO::Framed::X::WriteError

These both have an OS_ERROR property (cf. X::Tiny::Base’s accessor method).

IO::Framed::X::EmptyRead

No properties. If this is thrown, your peer has probably closed the connection. Unless you have called allow_empty_read() to set an alternate behavior, you might want to trap this exception if you call read().

NOTE: This distribution doesn’t write to $!. EAGAIN and EWOULDBLOCK on flush_write_queue() are ignored; all other errors are converted to thrown exceptions.

LEGACY CLASSES

This distribution also includes the following DEPRECATED legacy classes:

  • IO::Framed::Write::Blocking

  • IO::Framed::Write::NonBlocking

  • IO::Framed::ReadWrite

  • IO::Framed::ReadWrite::Blocking

  • IO::Framed::ReadWrite::NonBlocking

I’ll keep these in for the time being but eventually WILL remove them. Please adjust any calling code that you might have.

REPOSITORY

https://github.com/FGasper/p5-IO-Framed

AUTHOR

Felipe Gasper (FELIPE)

COPYRIGHT

Copyright 2017 by Gasper Software Consulting, LLC

LICENSE

This distribution is released under the same license as Perl.