[Octave-bug-tracker] [bug #30477] "help" command hangs because SIGCHLD handler is not async-signal-safe

[Matt McCutchen]

Follow-up Comment #8, bug #30477 (project octave):

I tried to repeat the test with the current development Octave, but I got a
build failure; I submitted bug #31074.

> OR, it would help if someone could explain how Octave's sigchld_handler
function in src/sighandlers.cc should be changed so that it will work properly
on your system.

Suppose the main program is in the middle of a call to __libc_malloc (the
common backend to malloc, operator new, etc.).  If a SIGCHLD arrives,
sigchld_handler is invoked, and it may invoke __libc_malloc again via
octave_child_list::instance_ok.  __libc_malloc takes a lock, so the reentrant
call causes a hang.  If it weren't for that lock, attempting to allocate on a
heap already in an intermediate state from the outer __libc_malloc would cause
corruption.

See the discussion of async-signal-safe functions in the POSIX standard:

http://www.opengroup.org/onlinepubs/9699919799/xrat/V4_xsh_chap02.html#tag_22_02_04_04

The typical solution would be to redesign the sigchld_handler so that it
never calls __libc_malloc.  It would also work to block SIGCHLD in any part of
the main program where a SIGCHLD could arrive during a call to __libc_malloc,
or switch to handling SIGCHLD via signalfd (but signalfd is only available on
Linux, and this problem probably affects other unices too).

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