PR threads/18600: Threads left stopped after fork+thread spawn

When a program forks and another process start threads while gdb is
handling the fork event, newly created threads are left stuck stopped
by gdb, even though gdb presents them as "running", to the user.

This can be seen with the test added by this patch.  The test has the
inferior fork a certain number of times and waits for all children to
exit.  Each fork child spawns a number of threads that do nothing and
joins them immediately.  Normally, the program should run unimpeded
(from the point of view of the user) and exit very quickly.  Without
this fix, it doesn't because of some threads left stopped by gdb, so
inferior 1 never exits.

The program triggers when a new clone thread is found while inside the
linux_stop_and_wait_all_lwps call in linux-thread-db.c:

      linux_stop_and_wait_all_lwps ();

      ALL_LWPS (lp)
	if (ptid_get_pid (lp->ptid) == pid)
	  thread_from_lwp (lp->ptid);

      linux_unstop_all_lwps ();

Within linux_stop_and_wait_all_lwps, we reach
linux_handle_extended_wait with the "stopping" parameter set to 1, and
because of that we don't mark the new lwp as resumed.  As consequence,
the subsequent resume_stopped_resumed_lwps, called from
linux_unstop_all_lwps, never resumes the new LWP.

There's lots of cruft in linux_handle_extended_wait that no longer
makes sense.  On systems with CLONE events support, we don't rely on
libthread_db for thread listing anymore, so the code that preserves
stop_requested and the handling of last_resume_kind is all dead.

So the fix is to remove all that, and simply always mark the new LWP
as resumed, so that resume_stopped_resumed_lwps re-resumes it.

gdb/ChangeLog:
2015-07-30  Pedro Alves  <palves@redhat.com>
	    Simon Marchi  <simon.marchi@ericsson.com>

	PR threads/18600
	* linux-nat.c (linux_handle_extended_wait): On CLONE event, always
	mark the new thread as resumed.  Remove STOPPING parameter.
	(wait_lwp): Adjust call to linux_handle_extended_wait.
	(linux_nat_filter_event): Adjust call to
	linux_handle_extended_wait.
	(resume_stopped_resumed_lwps): Add debug output.

gdb/testsuite/ChangeLog:
2015-07-30  Simon Marchi  <simon.marchi@ericsson.com>
	    Pedro Alves  <palves@redhat.com>

	PR threads/18600
	* gdb.threads/fork-plus-threads.c: New file.
	* gdb.threads/fork-plus-threads.exp: New file.

2 files changed, 184 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.threads/fork-plus-threads.c b/gdb/testsuite/gdb.threads/fork-plus-threads.c
new file mode 100644
index 0000000..780a4b8
--- /dev/null
+++ b/gdb/testsuite/gdb.threads/fork-plus-threads.c
@@ -0,0 +1,115 @@
+/* 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 <assert.h>
+#include <pthread.h>
+#include <unistd.h>
+#include <stdio.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+
+
+/* Number of times the main process forks.  */
+#define NFORKS 10
+
+/* Number of threads by each fork child.  */
+#define NTHREADS 10
+
+static void *
+thread_func (void *arg)
+{
+  /* Empty.  */
+}
+
+static void
+fork_child (void)
+{
+  pthread_t threads[NTHREADS];
+  int i;
+  int ret;
+
+  for (i = 0; i < NTHREADS; i++)
+    {
+      ret = pthread_create (&threads[i], NULL, thread_func, NULL);
+      assert (ret == 0);
+    }
+
+  for (i = 0; i < NTHREADS; i++)
+    {
+      ret = pthread_join (threads[i], NULL);
+      assert (ret == 0);
+    }
+}
+
+int
+main (void)
+{
+  pid_t childs[NFORKS];
+  int i;
+  int status;
+  int num_exited = 0;
+
+  /* Don't run forever if the wait loop below gets stuck.  */
+  alarm (180);
+
+  for (i = 0; i < NFORKS; i++)
+    {
+      pid_t pid;
+
+      pid = fork ();
+
+      if (pid > 0)
+	{
+	  /* Parent.  */
+	  childs[i] = pid;
+	}
+      else if (pid == 0)
+	{
+	  /* Child.  */
+	  fork_child ();
+	  return 0;
+	}
+      else
+	{
+	  perror ("fork");
+	  return 1;
+	}
+    }
+
+  while (num_exited != NFORKS)
+    {
+      pid_t pid = wait (&status);
+
+      if (pid == -1)
+	{
+	  perror ("wait");
+	  return 1;
+	}
+
+      if (WIFEXITED (status))
+	{
+	  num_exited++;
+	}
+      else
+	{
+	  printf ("Hmm, unexpected wait status 0x%x from child %d\n", status,
+		  pid);
+	}
+    }
+
+  return 0;
+}
diff --git a/gdb/testsuite/gdb.threads/fork-plus-threads.exp b/gdb/testsuite/gdb.threads/fork-plus-threads.exp
new file mode 100644
index 0000000..53d1102
--- /dev/null
+++ b/gdb/testsuite/gdb.threads/fork-plus-threads.exp
@@ -0,0 +1,69 @@
+# Copyright (C) 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/>.
+
+# This test verifies that threads created by the child fork are
+# properly handled.  Specifically, GDB used to have a bug where it
+# would leave child fork threads stuck stopped, even though "info
+# threads" would show them running.
+#
+# See https://sourceware.org/bugzilla/show_bug.cgi?id=18600
+
+standard_testfile
+
+proc do_test { detach_on_fork } {
+    global GDBFLAGS
+    global srcfile testfile
+    global gdb_prompt
+
+    set saved_gdbflags $GDBFLAGS
+    set GDBFLAGS [concat $GDBFLAGS " -ex \"set non-stop on\""]
+
+    if {[prepare_for_testing "failed to prepare" \
+	     $testfile $srcfile {debug pthreads}] == -1} {
+	set GDBFLAGS $saved_gdbflags
+	return -1
+    }
+
+    set GDBFLAGS $saved_gdbflags
+
+    if ![runto_main] then {
+	fail "Can't run to main"
+	return 0
+    }
+
+    gdb_test_no_output "set detach-on-fork $detach_on_fork"
+    set test "continue &"
+    gdb_test_multiple $test $test {
+	-re "$gdb_prompt " {
+	    pass $test
+	}
+    }
+
+    set test "inferior 1 exited"
+    gdb_test_multiple "" $test {
+	-re "Inferior 1 \(\[^\r\n\]+\) exited normally" {
+	    pass $test
+	}
+    }
+
+    gdb_test "info threads" "No threads\." \
+	"no threads left"
+}
+
+foreach detach_on_fork {"on" "off"} {
+    with_test_prefix "detach-on-fork=$detach_on_fork" {
+	do_test $detach_on_fork
+    }
+}