[PATCH] Fix "PC register is not available" issue

[Eli Zaretskii <eliz@gnu.org>]

This problem was raised and mentioned several times over the last few
years.  It was discussed here in the thread which started with this
message:

  https://sourceware.org/ml/gdb-patches/2013-06/msg00237.html

In the ensuing discussion, there was a consensus that these failures
shouldn't be fatal, and perhaps we should even ignore them entirely.

The most annoying problem with this is that after the message about
failure to suspend a thread, there's another message:

  warning: SuspendThread (tid=0x720) failed. (winerr 5)
  PC register is not available  <<<<<<<<<<<<<<<<<<<<<<<<<

and the debug session becomes useless after that: the debuggee stops,
and you cannot continue it.  You can only examine memory.

A very similar, perhaps the same, problem is described here:

  https://sourceware.org/bugzilla/show_bug.cgi?id=14018

There you can find a short test program that demonstrates the issue,
and some analysis.  After hitting this problem myself many times, I
came to the same conclusion: namely, that GDB is trying to suspend a
thread for which it doesn't have the necessary privileges.  My
hypothesis is that those are the threads that Windows starts in the
context of the process being debugged, perhaps for the purposes of
handling some debug events.  (I can see the "New thread" messages from
time to time, although I know for certain that the inferior didn't
start any application threads.)

So I came up with the patch below.  The idea is that setting
th->suspended to -1 just signals to GDB that the thread isn't
suspended, and shouldn't be resumed; otherwise, we simply ignore the
failure to suspend the thread, although the warning is still printed.

I am running with this patch for almost a month, and the dreaded "PC
register is not available" message didn't appear even once.  No more
botched debugging sessions!  The test program in PR/14018 also runs
indefinitely until interrupted, instead of stopping.

So I suggest to install this.  OK?  Should I also close the Bugzilla
PR?

--- gdb/windows-nat.c~0	2014-02-06 04:21:29.000000000 +0200
+++ gdb/windows-nat.c	2014-02-26 22:27:10.225625000 +0200
@@ -313,9 +313,10 @@ thread_rec (DWORD id, int get_context)
 		    warning (_("SuspendThread (tid=0x%x) failed."
 			       " (winerr %u)"),
 			     (unsigned) id, (unsigned) err);
-		    return NULL;
+		    th->suspended = -1;
 		  }
-		th->suspended = 1;
+		else
+		  th->suspended = 1;
 	      }
 	    else if (get_context < 0)
 	      th->suspended = -1;