port threadsafety redux

[Andy Wingo <wingo@pobox.com>]

Hi!

So, threads and ports again.  We didn't really come to a resolution in
this thread:

  http://article.gmane.org/gmane.lisp.guile.devel/17023

To recap, in Guile 2.0 a port has mutable internal state that can be
corrupted when when multiple threads write to it at once.  I ran into
this when doing some multithreaded server experiments, and fixed it in
the same way that libc fixes the issue for stdio streams:

  https://www.gnu.org/software/libc/manual/html_node/Streams-and-Threads.html#Streams-and-Threads

Namely, ports can have associated recursive mutexes.  They can be in a
mode in which every operation on a port grabs the mutex.  The interface
to set a port into unlocked mode (à la fsetlocking) is unimplemented,
but the machinery is there.

This change fixed the crashes I was seeing, but it slows down port
operations.  For an intel chip from a couple years ago the slowdown was
something on the order of 3x, for a tight putchar() loop; for Loongson
it could be as bad as 26x.  Mark was unhappy with this.

Mark also made the argument that locking on port operations doesn't
always make sense.  Indeed I quote from the libc documentation:

  But there are situations where this is not enough and there are also
  situations where this is not wanted. The implicit locking is not
  enough if the program requires more than one stream function call to
  happen atomically. One example would be if an output line a program
  wants to generate is created by several function calls. The functions
  by themselves would ensure only atomicity of their own operation, but
  not atomicity over all the function calls. For this it is necessary to
  perform the stream locking in the application code.

So we don't yet expose the equivalent of flockfile, but at this point
since there are still concerns out there I wanted to ask if the current
solution still makes sense.

I hope this is a fair summary of the issue.

My perspective on this is that crashes are unacceptable, and also that
it does make sense to log to stderr from multiple threads at once.  When
writing to ports under error conditions you don't always have the luxury
of being able to coordinate access in some nicer way.  I sympathize with
the desire to make put-char etc faster, as that means that more code can
be written in Scheme.

One possible alternate solution would be to expose ports more to Scheme
and so to make it easier and safer for Scheme to manipulate port data.
This would also make it possible to implement coroutines in Scheme that
yield when IO would block.

Or, we could just make stdio/stderr be locked by default, and some other
things not.  Seems squirrely to me though.

Dunno.  I would add that although there is a solution to this issue in
master, it might not make it into 2.2.  There will probably be a dozen
prereleases before 2.2.0, so even if a 2.1.1 manages to make it out the
door before we come to a solution, that doesn't mean that the choices in
such a release are the right or final ones.

Regards,

Andy
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