[Xapian-devel] Problems with /bin/cat and flintlock?

[Olly Betts]

On Sat, May 28, 2011 at 02:04:32AM +1200, Samuel Williams wrote:
> In my synthetic test, I got the following behavior when attached to /bin/cat:
> 
> # strace -p 2091
> Process 2091 attached - interrupt to quit
> read(0, 0x89e7000, 32768)               = ? ERESTARTSYS (To be restarted)
> --- SIGHUP (Hangup) @ 0 (0) ---
> read(0, "", 32768)                      = 0
> close(0)                                = 0
> close(1)                                = 0
> close(2)                                = -1 EBADF (Bad file descriptor)
> exit_group(0)                           = ?
> Process 2091 detached
> 
> This seems like the desired behavior.

So in this case, the restarted read() is reporting EOF on stdin.

> However, in the case where the WritableDatabase hung, I got the
> following behavior:
> 
> # strace -p 25694
> Process 25694 attached - interrupt to quit
> read(0, 0x859b000, 32768)               = ? ERESTARTSYS (To be restarted)
> --- SIGHUP (Hangup) @ 0 (0) ---
> read(0, 
> ^C <unfinished ...>
> Process 25694 detached

Whereas here it doesn't.

> Well, I'm not sure why there is a difference. Both processes were
> ignoring SIGHUP according to /proc/$pid/status

I tried using sigaction() to set SIGHUP to SIG_IGN, with SA_RESTART in
sa_flags, but that makes no difference for me even rerunning the test
more than 100 times.  This is the amended testcase patch:

http://oligarchy.co.uk/xapian/patches/xapian-ignoresighup1-testcase-with-SA_RESTART.patch

I also tried with 0 in sa_flags.

I wonder if there's some sort of race in the kernel and/or libc here
which results in different behaviour sometimes, depending on the
exact timing of things happening.

Cheers,
    Olly