Fix threads left stopped after fork+thread spawn (PR threads/18600)

diff --git a/gdb/linux-nat.c b/gdb/linux-nat.c
index be429f8..4a0391e 100644
--- a/gdb/linux-nat.c
+++ b/gdb/linux-nat.c
@@ -2000,8 +2000,7 @@  linux_handle_syscall_trap (struct lwp_info *lp, int stopping)
    true, the new LWP remains stopped, otherwise it is continued.  */
 
 static int
-linux_handle_extended_wait (struct lwp_info *lp, int status,
-			    int stopping)
+linux_handle_extended_wait (struct lwp_info *lp, int status)
 {
   int pid = ptid_get_lwp (lp->ptid);
   struct target_waitstatus *ourstatus = &lp->waitstatus;
@@ -2071,7 +2070,7 @@  linux_handle_extended_wait (struct lwp_info *lp, int status,
 	ourstatus->kind = TARGET_WAITKIND_FORKED;
       else if (event == PTRACE_EVENT_VFORK)
 	ourstatus->kind = TARGET_WAITKIND_VFORKED;
-      else
+      else if (event == PTRACE_EVENT_CLONE)
 	{
 	  struct lwp_info *new_lp;
 
@@ -2086,43 +2085,7 @@  linux_handle_extended_wait (struct lwp_info *lp, int status,
 	  new_lp = add_lwp (ptid_build (ptid_get_pid (lp->ptid), new_pid, 0));
 	  new_lp->cloned = 1;
 	  new_lp->stopped = 1;
-
-	  if (WSTOPSIG (status) != SIGSTOP)
-	    {
-	      /* This can happen if someone starts sending signals to
-		 the new thread before it gets a chance to run, which
-		 have a lower number than SIGSTOP (e.g. SIGUSR1).
-		 This is an unlikely case, and harder to handle for
-		 fork / vfork than for clone, so we do not try - but
-		 we handle it for clone events here.  We'll send
-		 the other signal on to the thread below.  */
-
-	      new_lp->signalled = 1;
-	    }
-	  else
-	    {
-	      struct thread_info *tp;
-
-	      /* When we stop for an event in some other thread, and
-		 pull the thread list just as this thread has cloned,
-		 we'll have seen the new thread in the thread_db list
-		 before handling the CLONE event (glibc's
-		 pthread_create adds the new thread to the thread list
-		 before clone'ing, and has the kernel fill in the
-		 thread's tid on the clone call with
-		 CLONE_PARENT_SETTID).  If that happened, and the core
-		 had requested the new thread to stop, we'll have
-		 killed it with SIGSTOP.  But since SIGSTOP is not an
-		 RT signal, it can only be queued once.  We need to be
-		 careful to not resume the LWP if we wanted it to
-		 stop.  In that case, we'll leave the SIGSTOP pending.
-		 It will later be reported as GDB_SIGNAL_0.  */
-	      tp = find_thread_ptid (new_lp->ptid);
-	      if (tp != NULL && tp->stop_requested)
-		new_lp->last_resume_kind = resume_stop;
-	      else
-		status = 0;
-	    }
+	  new_lp->resumed = 1;
 
 	  /* If the thread_db layer is active, let it record the user
 	     level thread id and status, and add the thread to GDB's
@@ -2136,19 +2099,23 @@  linux_handle_extended_wait (struct lwp_info *lp, int status,
 	    }
 
 	  /* Even if we're stopping the thread for some reason
-	     internal to this module, from the user/frontend's
-	     perspective, this new thread is running.  */
+	     internal to this module, from the perspective of infrun
+	     and the user/frontend, this new thread is running until
+	     it next reports a stop.  */
 	  set_running (new_lp->ptid, 1);
-	  if (!stopping)
-	    {
-	      set_executing (new_lp->ptid, 1);
-	      /* thread_db_attach_lwp -> lin_lwp_attach_lwp forced
-		 resume_stop.  */
-	      new_lp->last_resume_kind = resume_continue;
-	    }
+	  set_executing (new_lp->ptid, 1);
 
-	  if (status != 0)
+	  if (WSTOPSIG (status) != SIGSTOP)
 	    {
+	      /* This can happen if someone starts sending signals to
+		 the new thread before it gets a chance to run, which
+		 have a lower number than SIGSTOP (e.g. SIGUSR1).
+		 This is an unlikely case, and harder to handle for
+		 fork / vfork than for clone, so we do not try - but
+		 we handle it for clone events here.  */
+
+	      new_lp->signalled = 1;
+
 	      /* We created NEW_LP so it cannot yet contain STATUS.  */
 	      gdb_assert (new_lp->status == 0);
 
@@ -2162,7 +2129,6 @@  linux_handle_extended_wait (struct lwp_info *lp, int status,
 	      new_lp->status = status;
 	    }
 
-	  new_lp->resumed = !stopping;
 	  return 1;
 	}
 
@@ -2308,6 +2274,20 @@  wait_lwp (struct lwp_info *lp)
       /* Check if the thread has exited.  */
       if (WIFEXITED (status) || WIFSIGNALED (status))
 	{
+	  if (ptid_get_pid (lp->ptid) == ptid_get_lwp (lp->ptid))
+	    {
+	      if (debug_linux_nat)
+		fprintf_unfiltered (gdb_stdlog, "WL: Process %d exited.\n",
+				    ptid_get_pid (lp->ptid));
+
+	      /* This is the leader exiting, it means the whole
+		 process is gone.  Store the status to report to the
+		 core.  Store it in the lp->waitstatus, because
+		 W_EXITCODE(0,0) == 0.  */
+	      store_waitstatus (&lp->waitstatus, status);
+	      return 0;
+	    }
+
 	  thread_dead = 1;
 	  if (debug_linux_nat)
 	    fprintf_unfiltered (gdb_stdlog, "WL: %s exited.\n",
@@ -2353,7 +2333,7 @@  wait_lwp (struct lwp_info *lp)
 	fprintf_unfiltered (gdb_stdlog,
 			    "WL: Handling extended status 0x%06x\n",
 			    status);
-      linux_handle_extended_wait (lp, status, 1);
+      linux_handle_extended_wait (lp, status);
       return 0;
     }
 
@@ -3155,7 +3135,7 @@  linux_nat_filter_event (int lwpid, int status)
 	fprintf_unfiltered (gdb_stdlog,
 			    "LLW: Handling extended status 0x%06x\n",
 			    status);
-      if (linux_handle_extended_wait (lp, status, 0))
+      if (linux_handle_extended_wait (lp, status))
 	return NULL;
     }
 
@@ -3673,9 +3653,31 @@  resume_stopped_resumed_lwps (struct lwp_info *lp, void *data)
 {
   ptid_t *wait_ptid_p = data;
 
-  if (lp->stopped
-      && lp->resumed
-      && !lwp_status_pending_p (lp))
+  if (!lp->stopped)
+    {
+      if (debug_linux_nat)
+	fprintf_unfiltered (gdb_stdlog,
+			    "RSRL: NOT resuming stopped-resumed LWP %s, "
+			    "not stopped\n",
+			    target_pid_to_str (lp->ptid));
+    }
+  else if (!lp->resumed)
+    {
+      if (debug_linux_nat)
+	fprintf_unfiltered (gdb_stdlog,
+			    "RSRL: NOT resuming stopped-resumed LWP %s, "
+			    "not resumed\n",
+			    target_pid_to_str (lp->ptid));
+    }
+  else if (lwp_status_pending_p (lp))
+    {
+      if (debug_linux_nat)
+	fprintf_unfiltered (gdb_stdlog,
+			    "RSRL: NOT resuming stopped-resumed LWP %s, "
+			    "has pending status\n",
+			    target_pid_to_str (lp->ptid));
+    }
+  else
     {
       struct regcache *regcache = get_thread_regcache (lp->ptid);
       struct gdbarch *gdbarch = get_regcache_arch (regcache);
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..576e79d
--- /dev/null
+++ b/gdb/testsuite/gdb.threads/fork-plus-threads.c
@@ -0,0 +1,94 @@ 
+#include <assert.h>
+#include <pthread.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;
+
+  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..cbb6a90
--- /dev/null
+++ b/gdb/testsuite/gdb.threads/fork-plus-threads.exp
@@ -0,0 +1,53 @@ 
+# 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 child fork are properly
+# handled.
+#
+# See https://sourceware.org/bugzilla/show_bug.cgi?id=18600
+
+# The problem was originally seen on Linux, but the test could be
+# generalized to all targets that support forks and threads.
+if ![istarget *-*-linux*] then {
+    return
+}
+
+# When using gdbserver, even on Linux, we don't get notifications
+# about new threads.  This is expected, so don't test for that.
+if [is_remote target] then {
+    return
+}
+
+standard_testfile
+
+if {[gdb_compile_pthreads "${srcdir}/${subdir}/${srcfile}" "${binfile}" executable debug] != "" } {
+    return -1
+}
+
+clean_restart ${binfile}
+
+gdb_test_no_output "set non-stop on"
+
+if ![runto_main] then {
+   fail "Can't run to main"
+   return 0
+}
+
+gdb_test_no_output "set detach-on-fork off"
+send_gdb "continue &\n"
+
+sleep 2
+
+gdb_test "info threads" "No threads.*"