linux-nat.c: no need to block child signals so aggressively.

linux-nat.c: no need to block child signals so aggressively.

In http://sourceware.org/ml/gdb-patches/2013-08/msg00174.html , the
issue of child signal handling around ptrace option support discovery
being different between GDB and GDBserver came up.

I recalled adding these block_child_signals calls, and the "We don't
want those ptrace calls to be interrupted" comment, but not exactly
why.  So I looked into it.  My first guess is that I got confused.
The patch that added this
<http://sourceware.org/ml/gdb-patches/2009-04/msg00125.html> rewrote
the linux native async support completely, and the old async support
code had the SIGCHLD handler itself do waitpid, so in places that we'd
want a blocking waitpid, we'd have to have the signal handler blocked.
That was probably the mindset I had at the time.  Anyway, whatever the
case, looks like I was wrong on the need for this blocking.
Given GDBserver doesn't block like this, I investigated why this is
currently needed on GDB but not on GDBserver.

I removed the block_child_signals (and restore) calls, and hacked
linux-nat.c to call linux_test_for_tracefork in a loop, like:

 @@ -534,7 +534,10 @@ static int
  linux_supports_tracefork (int pid)
  {
    if (linux_supports_tracefork_flag == -1)
 -    linux_test_for_tracefork (pid);
 +    {
 +      while (1)
 +       linux_test_for_tracefork (pid);
 +    }
    return linux_supports_tracefork_flag;
  }

Running the resulting GDB, I then saw bad things happening.
Specifically, I'd end up with a bunch of zombies, and eventually, the
machine would refuse to spawn new processes, claming insufficient
resources.

The issue is that linux_test_for_tracefork test forks, and has the
child fork again.  If we don't block SIGCHLD on entry to the function,
the children will inherit SIGCHLD's action/disposition (meaning,
SIGCHLD will be unblocked in the child).  When the first child forks
again a second child, and that child exits, the first child gets a
SIGCHLD.  Now, when we try to wrap up for the whole options test, we
kill the first child, and collect the waitstatus.  Here, when SIGCHLD
isn't blocked, GDB will first see the child reporting a stop with
SIGCHLD.  gdbserver's ptrace options test does a PTRACE_KILL loop at
the end, which catches the SIGCHLD, and retries the kill.  The GDB
version did not do that.  So the GDB version would proceed, leaving
the child zombie (until GDB exists), as nothing collected its final
waitstatus.

So this patch makes the GDB version of linux_test_for_tracefork do the
exact same as the GDBserver version, removes all this unnecessary
blocking throughout, and adds a couple comments at places that do need
it -- namely: places where we'll use sleep with sigsuspend; and
linux_async_pipe, as that destroys the pipe the signal handler
touches.

Tested on x86_64 Fedora 17, sync and async.

gdb/
2013-08-19  Pedro Alves  <palves@redhat.com>

	* linux-nat.c (linux_test_for_tracefork)
	(linux_test_for_tracesysgood, linux_child_follow_fork)
	(lin_lwp_attach_lwp, linux_nat_resume): Don't block child signals.
	(linux_nat_wait_1): Extend comment.
	(linux_async_pipe): Add comment.