[K42-discussion] Another problem with asynchrony

[Bryan S Rosenburg]

When you write to a pipe whose other end has been closed, the write is 
supposed to fail with an errno of EPIPE, and in addition you are supposed 
to receive a SIGPIPE.

In Linux, unless you ignore or handle the signal, it will always kill you 
before you get back from the write.  (I make this assertion on only a 
little evidence.)

In K42, the pipe server can generate the signal before returning EPIPE, 
but there's no good way to ensure that the signal is acted on before the 
write returns.  In practice, I've found that the client sometimes sees the 
write return and sometimes it doesn't.

As far as I can tell, Posix says nothing more than:

The write() and pwrite() functions shall fail if:
. . . .
[EPIPE] 
An attempt is made to write to a pipe or FIFO that is not open for reading 
by any process, or that only has one end open. A SIGPIPE signal shall also 
be sent to the thread. 

which by itself doesn't seem to imply any degree of synchronicity.  But 
there may be constraints on when a signal "sent" to a thread because of 
the thread's own action must be delivered.

In any case, I think Linux programs will be surprised if they survive to 
see an EPIPE returned from write, whether or not they should be.

I'm not proposing to do anything about this problem right now, but I 
thought I'd put it out for discussion.

- Bryan
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