[2/3] PR remote/19496, interrupted syscall in forking-threads-plus-bkpt

  This patch addresses "fork:Interrupted system call" (or wait:) failures
in gdb.threads/forking-threads-plus-breakpoint.exp.

The test program spawns ten threads, each of which do ten fork/waitpid
sequences.  The cause of the problem was that when one of the fork
children exited before the corresponding fork parent could initiate its
waitpid for that child, a SIGCHLD was delivered and interrupted a fork
or waitpid in another thread.

The fix was to wrap the system calls in a loop to retry the call if
it was interrupted, like:

do
  {
    pid = fork ();
  }
while (pid == -1 && errno == EINTR);

Since this is a Linux-only test I figure it is OK to use errno and EINTR.

Tested on Nios II Linux target with x86 Linux host.

Thanks,
--Don

gdb/testsuite/ChangeLog:
2016-01-27  Don Breazeal  <donb@codesourcery.com>

	PR remote/19496
	* gdb.threads/forking-threads-plus-breakpoint.c (thread_forks):
	Retry fork and waitpid on interrupted system call errors.

---
 .../gdb.threads/forking-threads-plus-breakpoint.c  |   14 ++++++++++++--
 1 files changed, 12 insertions(+), 2 deletions(-)
  

On 01/28/2016 12:48 AM, Don Breazeal wrote:
> This patch addresses "fork:Interrupted system call" (or wait:) failures
> in gdb.threads/forking-threads-plus-breakpoint.exp.
> 
> The test program spawns ten threads, each of which do ten fork/waitpid
> sequences.  The cause of the problem was that when one of the fork
> children exited before the corresponding fork parent could initiate its
> waitpid for that child, a SIGCHLD was delivered and interrupted a fork
> or waitpid in another thread.
> 
> The fix was to wrap the system calls in a loop to retry the call if
> it was interrupted, like:
> 
> do
>   {
>     pid = fork ();
>   }
> while (pid == -1 && errno == EINTR);
> 
> Since this is a Linux-only test I figure it is OK to use errno and EINTR.
> 
> Tested on Nios II Linux target with x86 Linux host.

I'd prefer to avoid this if possible.  These loops potentially hide
bugs like ERESTARTSYS escaping out of a syscall and mishandling of
signals.  See bc9540e842eb5639ca59cb133adef211d252843c for example:
   https://sourceware.org/ml/gdb-patches/2015-02/msg00654.html

How about setting SIGCHLD to SIG_IGN, or making SIGCHLD be SA_RESTART?

Thanks,
Pedro Alves