Re: [PATCH] Set SA_RESTART flag on SIGCHLD handler

[Anders Kaseorg <andersk@MIT.EDU>]

I wrote this greatly simplified skeleton of the failing Git test, and 
found that I can reproduce what looks like the same problem, fairly 
reliably:

$ rm -f fifo
$ mkfifo fifo
$ dash -c 'while echo -n .; do : < fifo & : > fifo; wait $!; done'
...............................dash: 1: cannot create fifo: Interrupted 
system call

After running this through strace, it seems clear what went wrong.  The 
parent’s open("fifo", O_WRONLY) is what causes the child’s open("fifo", 
O_RDONLY) to return.  If the child exits immediately, the parent may 
receive a SIGCHLD before its own open() returns.  Then when its SIGCHLD 
handler exits, open() returns EINTR.

That also explains why the SA_RESTART and restart-on-EINTR patches only 
turned it into a deadlock.  If the open() were restarted now, it would 
just block to wait for a _second_ reader of the FIFO, which will never 
arrive.

But if that’s what happened, it seems like a kernel bug to me! 
Shouldn’t an open() call only be restarted or return EINTR in the case 
that it hasn’t had any side effects yet and can safely be restarted?

Anders