native/Linux: internal error if resume is short-circuited

If the linux_nat_resume's short-circuits the resume because the
current thread has a pending status, and, a thread with a higher
number was previously stopped for a breakpoint, GDB internal errors,
like:

 /home/pedro/gdb/mygit/src/gdb/linux-nat.c:2590: internal-error: status_callback: Assertion `lp->status != 0' failed.

Fix this by make status_callback bail out earlier.  GDBserver is
already doing the same.

New test added that exercises this.

gdb/ChangeLog:
2015-03-19  Pedro Alves  <palves@redhat.com>

	* linux-nat.c (status_callback): Return early if the LWP has no
	status pending.

gdb/testsuite/ChangeLog:
2015-03-19  Pedro Alves  <palves@redhat.com>

	* gdb.threads/continue-pending-status.c: New file.
	* gdb.threads/continue-pending-status.exp: New file.

1 files changed, 58 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.threads/continue-pending-status.c b/gdb/testsuite/gdb.threads/continue-pending-status.c
new file mode 100644
index 0000000..eea0c7e
--- /dev/null
+++ b/gdb/testsuite/gdb.threads/continue-pending-status.c
@@ -0,0 +1,58 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2015 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#include <stdio.h>
+#include <unistd.h>
+#include <stdlib.h>
+#include <pthread.h>
+#include <assert.h>
+
+pthread_barrier_t barrier;
+
+#define NUM_THREADS 2
+
+void *
+thread_function (void *arg)
+{
+  /* This ensures that the breakpoint is only hit after both threads
+     are created, so the test can always switch to the non-event
+     thread when the breakpoint triggers.  */
+  pthread_barrier_wait (&barrier);
+
+  while (1); /* break here */
+}
+
+int
+main (void)
+{
+  int i;
+
+  pthread_barrier_init (&barrier, NULL, NUM_THREADS);
+
+  for (i = 0; i < NUM_THREADS; i++)
+    {
+      pthread_t thread;
+      int res;
+
+      res = pthread_create (&thread, NULL,
+			    thread_function, NULL);
+      assert (res == 0);
+    }
+
+  sleep (300);
+  return 0;
+}