Fix PR18360 - internal error when using "interrupt -a"

If you do "interrupt -a" just while some thread is stepping over a
breakpoint, gdb trips on an internal error.

The test added by this patch manages to trigger this consistently by
spawning a few threads that are constantly tripping on a conditional
breakpoint whose condition always evaluates to false.  With current
gdb, you get:

~~~
 interrupt -a
 .../src/gdb/inline-frame.c:343: internal-error: void skip_inline_frames(ptid_t): Assertion `find_inline_frame_state (ptid) == NULL' failed.
 A problem internal to GDB has been detected,
 further debugging may prove unreliable.
 Quit this debugging session? (y or n) FAIL: gdb.threads/interrupt-while-step-over.exp: displaced-stepping=on: iter=0: interrupt -a (GDB internal error)
[...]
 .../src/gdb/inline-frame.c:343: internal-error: void skip_inline_frames(ptid_t): Assertion `find_inline_frame_state (ptid) == NULL' failed.
 A problem internal to GDB has been detected,
 further debugging may prove unreliable.
 Quit this debugging session? (y or n) FAIL: gdb.threads/interrupt-while-step-over.exp: displaced-stepping=off: iter=0: wait for stops (GDB internal error)
~~~

The assertion triggers because we're processing a stop for a thread
that had already stopped before and thus had already its inline-frame
state filled in.

Calling handle_inferior_event_1 directly within a
"thread_stop_requested" observer is something that I've wanted to get
rid of before, for being fragile.  Nowadays, infrun is aware of
threads with pending events, so we can use that instead, and let the
normal fetch_inferior_event -> handle_inferior_event code path handle
the forced stop.

The change to finish_step_over is necessary because sometimes a thread
that was told to PTRACE_SINGLESTEP reports back a SIGSTOP instead of a
SIGTRAP (i.e., we tell it to single-step, and then interrupt it quick
enough that on the kernel side the thread dequeues the SIGTOP before
ever having had a chance of executing the instruction to be stepped).
SIGSTOP gets translated to a GDB_SIGNAL_0.  And then finish_step_over
would miss calling clear_step_over_info, and thus miss restarting the
other threads (which in this case of threads with pending events,
means setting their "resumed" flag, so their pending events can be
consumed).

And now that we always restart threads in finish_step_over, we no
longer need to do that in handle_signal_stop.

Tested on x86_64 Fedora 23, native and gdbserver.

gdb/ChangeLog:
2017-03-08  Pedro Alves  <palves@redhat.com>

	PR gdb/18360
	* infrun.c (start_step_over, do_target_resume, resume)
	(restart_threads): Assert we're not resuming a thread that is
	meant to be stopped.
	(infrun_thread_stop_requested_callback): Delete.
	(infrun_thread_stop_requested): If the thread is internally
	stopped, queue a pending stop event and clear the thread's
	inline-frame state.
	(handle_stop_requested): New function.
	(handle_syscall_event, handle_inferior_event_1): Use
	handle_stop_requested.
	(handle_stop_requested): New function.
	(handle_signal_stop): Set the thread's stop_signal here instead of
	at caller.
	(finish_step_over): Clear step over info unconditionally.
	(handle_signal_stop): If the user had interrupted the event
	thread, consider the stop a random signal.
	(handle_signal_stop) <signal arrived while stepping over
	breakpoint>: Don't restart threads here.
	(stop_waiting): Don't clear step-over info here.

gdb/testsuite/ChangeLog:
2017-03-08  Pedro Alves  <palves@redhat.com>

	PR gdb/18360
	* gdb.threads/interrupt-while-step-over.c: New file.
	* gdb.threads/interrupt-while-step-over.exp: New file.

1 files changed, 102 insertions, 94 deletions

diff --git a/gdb/infrun.c b/gdb/infrun.c
index 1e5e9f1..c8c2d6e 100644
--- a/gdb/infrun.c
+++ b/gdb/infrun.c
@@ -2083,6 +2083,8 @@ start_step_over (void)
       step_over_what step_what;
       int must_be_in_line;
 
+      gdb_assert (!tp->stop_requested);
+
       next = thread_step_over_chain_next (tp);
 
       /* If this inferior already has a displaced step in process,
@@ -2331,6 +2333,8 @@ do_target_resume (ptid_t resume_ptid, int step, enum gdb_signal sig)
 {
   struct thread_info *tp = inferior_thread ();
 
+  gdb_assert (!tp->stop_requested);
+
   /* Install inferior's terminal modes.  */
   target_terminal_inferior ();
 
@@ -2395,6 +2399,7 @@ resume (enum gdb_signal sig)
      single-step).  */
   int step;
 
+  gdb_assert (!tp->stop_requested);
   gdb_assert (!thread_is_in_step_over_chain (tp));
 
   QUIT;
@@ -3276,65 +3281,6 @@ static void keep_going (struct execution_control_state *ecs);
 static void process_event_stop_test (struct execution_control_state *ecs);
 static int switch_back_to_stepped_thread (struct execution_control_state *ecs);
 
-/* Callback for iterate over threads.  If the thread is stopped, but
-   the user/frontend doesn't know about that yet, go through
-   normal_stop, as if the thread had just stopped now.  ARG points at
-   a ptid.  If PTID is MINUS_ONE_PTID, applies to all threads.  If
-   ptid_is_pid(PTID) is true, applies to all threads of the process
-   pointed at by PTID.  Otherwise, apply only to the thread pointed by
-   PTID.  */
-
-static int
-infrun_thread_stop_requested_callback (struct thread_info *info, void *arg)
-{
-  ptid_t ptid = * (ptid_t *) arg;
-
-  if ((ptid_equal (info->ptid, ptid)
-       || ptid_equal (minus_one_ptid, ptid)
-       || (ptid_is_pid (ptid)
-	   && ptid_get_pid (ptid) == ptid_get_pid (info->ptid)))
-      && is_running (info->ptid)
-      && !is_executing (info->ptid))
-    {
-      struct cleanup *old_chain;
-      struct execution_control_state ecss;
-      struct execution_control_state *ecs = &ecss;
-
-      memset (ecs, 0, sizeof (*ecs));
-
-      old_chain = make_cleanup_restore_current_thread ();
-
-      overlay_cache_invalid = 1;
-      /* Flush target cache before starting to handle each event.
-	 Target was running and cache could be stale.  This is just a
-	 heuristic.  Running threads may modify target memory, but we
-	 don't get any event.  */
-      target_dcache_invalidate ();
-
-      /* Go through handle_inferior_event/normal_stop, so we always
-	 have consistent output as if the stop event had been
-	 reported.  */
-      ecs->ptid = info->ptid;
-      ecs->event_thread = info;
-      ecs->ws.kind = TARGET_WAITKIND_STOPPED;
-      ecs->ws.value.sig = GDB_SIGNAL_0;
-
-      handle_inferior_event (ecs);
-
-      if (!ecs->wait_some_more)
-	{
-	  /* Cancel any running execution command.  */
-	  thread_cancel_execution_command (info);
-
-	  normal_stop ();
-	}
-
-      do_cleanups (old_chain);
-    }
-
-  return 0;
-}
-
 /* This function is attached as a "thread_stop_requested" observer.
    Cleanup local state that assumed the PTID was to be resumed, and
    report the stop to the frontend.  */
@@ -3344,17 +3290,51 @@ infrun_thread_stop_requested (ptid_t ptid)
 {
   struct thread_info *tp;
 
-  /* PTID was requested to stop.  Remove matching threads from the
-     step-over queue, so we don't try to resume them
-     automatically.  */
+  /* PTID was requested to stop.  If the thread was already stopped,
+     but the user/frontend doesn't know about that yet (e.g., the
+     thread had been temporarily paused for some step-over), set up
+     for reporting the stop now.  */
   ALL_NON_EXITED_THREADS (tp)
     if (ptid_match (tp->ptid, ptid))
       {
+	if (tp->state != THREAD_RUNNING)
+	  continue;
+	if (tp->executing)
+	  continue;
+
+	/* Remove matching threads from the step-over queue, so
+	   start_step_over doesn't try to resume them
+	   automatically.  */
 	if (thread_is_in_step_over_chain (tp))
 	  thread_step_over_chain_remove (tp);
-      }
 
-  iterate_over_threads (infrun_thread_stop_requested_callback, &ptid);
+	/* If the thread is stopped, but the user/frontend doesn't
+	   know about that yet, queue a pending event, as if the
+	   thread had just stopped now.  Unless the thread already had
+	   a pending event.  */
+	if (!tp->suspend.waitstatus_pending_p)
+	  {
+	    tp->suspend.waitstatus_pending_p = 1;
+	    tp->suspend.waitstatus.kind = TARGET_WAITKIND_STOPPED;
+	    tp->suspend.waitstatus.value.sig = GDB_SIGNAL_0;
+	  }
+
+	/* Clear the inline-frame state, since we're re-processing the
+	   stop.  */
+	clear_inline_frame_state (tp->ptid);
+
+	/* If this thread was paused because some other thread was
+	   doing an inline-step over, let that finish first.  Once
+	   that happens, we'll restart all threads and consume pending
+	   stop events then.  */
+	if (step_over_info_valid_p ())
+	  continue;
+
+	/* Otherwise we can process the (new) pending event now.  Set
+	   it so this pending event is considered by
+	   do_target_wait.  */
+	tp->resumed = 1;
+      }
 }
 
 static void
@@ -4248,6 +4228,23 @@ stepped_in_from (struct frame_info *frame, struct frame_id step_frame_id)
   return 0;
 }
 
+/* If the event thread has the stop requested flag set, pretend it
+   stopped for a GDB_SIGNAL_0 (i.e., as if it stopped due to
+   target_stop).  */
+
+static bool
+handle_stop_requested (struct execution_control_state *ecs)
+{
+  if (ecs->event_thread->stop_requested)
+    {
+      ecs->ws.kind = TARGET_WAITKIND_STOPPED;
+      ecs->ws.value.sig = GDB_SIGNAL_0;
+      handle_signal_stop (ecs);
+      return true;
+    }
+  return false;
+}
+
 /* Auxiliary function that handles syscall entry/return events.
    It returns 1 if the inferior should keep going (and GDB
    should ignore the event), or 0 if the event deserves to be
@@ -4277,6 +4274,9 @@ handle_syscall_event (struct execution_control_state *ecs)
 	= bpstat_stop_status (get_regcache_aspace (regcache),
 			      stop_pc, ecs->ptid, &ecs->ws);
 
+      if (handle_stop_requested (ecs))
+	return 0;
+
       if (bpstat_causes_stop (ecs->event_thread->control.stop_bpstat))
 	{
 	  /* Catchpoint hit.  */
@@ -4284,6 +4284,9 @@ handle_syscall_event (struct execution_control_state *ecs)
 	}
     }
 
+  if (handle_stop_requested (ecs))
+    return 0;
+
   /* If no catchpoint triggered for this, then keep going.  */
   keep_going (ecs);
   return 1;
@@ -4974,6 +4977,9 @@ handle_inferior_event_1 (struct execution_control_state *ecs)
 	    = bpstat_stop_status (get_regcache_aspace (regcache),
 				  stop_pc, ecs->ptid, &ecs->ws);
 
+	  if (handle_stop_requested (ecs))
+	    return;
+
 	  if (bpstat_causes_stop (ecs->event_thread->control.stop_bpstat))
 	    {
 	      /* A catchpoint triggered.  */
@@ -5028,6 +5034,8 @@ handle_inferior_event_1 (struct execution_control_state *ecs)
     case TARGET_WAITKIND_SPURIOUS:
       if (debug_infrun)
         fprintf_unfiltered (gdb_stdlog, "infrun: TARGET_WAITKIND_SPURIOUS\n");
+      if (handle_stop_requested (ecs))
+	return;
       if (!ptid_equal (ecs->ptid, inferior_ptid))
 	context_switch (ecs->ptid);
       resume (GDB_SIGNAL_0);
@@ -5037,6 +5045,8 @@ handle_inferior_event_1 (struct execution_control_state *ecs)
     case TARGET_WAITKIND_THREAD_CREATED:
       if (debug_infrun)
         fprintf_unfiltered (gdb_stdlog, "infrun: TARGET_WAITKIND_THREAD_CREATED\n");
+      if (handle_stop_requested (ecs))
+	return;
       if (!ptid_equal (ecs->ptid, inferior_ptid))
 	context_switch (ecs->ptid);
       if (!switch_back_to_stepped_thread (ecs))
@@ -5221,6 +5231,9 @@ Cannot fill $_exitsignal with the correct signal number.\n"));
 	= bpstat_stop_status (get_regcache_aspace (get_current_regcache ()),
 			      stop_pc, ecs->ptid, &ecs->ws);
 
+      if (handle_stop_requested (ecs))
+	return;
+
       /* If no catchpoint triggered for this, then keep going.  Note
 	 that we're interested in knowing the bpstat actually causes a
 	 stop, not just if it may explain the signal.  Software
@@ -5295,6 +5308,10 @@ Cannot fill $_exitsignal with the correct signal number.\n"));
 
       current_inferior ()->waiting_for_vfork_done = 0;
       current_inferior ()->pspace->breakpoints_not_allowed = 0;
+
+      if (handle_stop_requested (ecs))
+	return;
+
       /* This also takes care of reinserting breakpoints in the
 	 previously locked inferior.  */
       keep_going (ecs);
@@ -5331,6 +5348,9 @@ Cannot fill $_exitsignal with the correct signal number.\n"));
       xfree (ecs->ws.value.execd_pathname);
       ecs->ws.value.execd_pathname = NULL;
 
+      if (handle_stop_requested (ecs))
+	return;
+
       /* If no catchpoint triggered for this, then keep going.  */
       if (!bpstat_causes_stop (ecs->event_thread->control.stop_bpstat))
 	{
@@ -5368,7 +5388,6 @@ Cannot fill $_exitsignal with the correct signal number.\n"));
     case TARGET_WAITKIND_STOPPED:
       if (debug_infrun)
         fprintf_unfiltered (gdb_stdlog, "infrun: TARGET_WAITKIND_STOPPED\n");
-      ecs->event_thread->suspend.stop_signal = ecs->ws.value.sig;
       handle_signal_stop (ecs);
       return;
 
@@ -5385,6 +5404,10 @@ Cannot fill $_exitsignal with the correct signal number.\n"));
 
       delete_just_stopped_threads_single_step_breakpoints ();
       stop_pc = regcache_read_pc (get_thread_regcache (inferior_ptid));
+
+      if (handle_stop_requested (ecs))
+	return;
+
       observer_notify_no_history ();
       stop_waiting (ecs);
       return;
@@ -5474,6 +5497,8 @@ restart_threads (struct thread_info *event_thread)
 	  continue;
 	}
 
+      gdb_assert (!tp->stop_requested);
+
       /* If some thread needs to start a step-over at this point, it
 	 should still be in the step-over queue, and thus skipped
 	 above.  */
@@ -5543,8 +5568,7 @@ finish_step_over (struct execution_control_state *ecs)
 	 back an event.  */
       gdb_assert (ecs->event_thread->control.trap_expected);
 
-      if (ecs->event_thread->suspend.stop_signal == GDB_SIGNAL_TRAP)
-	clear_step_over_info ();
+      clear_step_over_info ();
     }
 
   if (!target_is_non_stop_p ())
@@ -5656,6 +5680,8 @@ handle_signal_stop (struct execution_control_state *ecs)
 
   gdb_assert (ecs->ws.kind == TARGET_WAITKIND_STOPPED);
 
+  ecs->event_thread->suspend.stop_signal = ecs->ws.value.sig;
+
   /* Do we need to clean up the state of a thread that has
      completed a displaced single-step?  (Doing so usually affects
      the PC, so do it here, before we set stop_pc.)  */
@@ -6045,6 +6071,15 @@ handle_signal_stop (struct execution_control_state *ecs)
   if (random_signal)
     random_signal = !stopped_by_watchpoint;
 
+  /* Always stop if the user explicitly requested this thread to
+     remain stopped.  */
+  if (ecs->event_thread->stop_requested)
+    {
+      random_signal = 1;
+      if (debug_infrun)
+	fprintf_unfiltered (gdb_stdlog, "infrun: user-requested stop\n");
+    }
+
   /* For the program's own signals, act according to
      the signal handling tables.  */
 
@@ -6091,8 +6126,6 @@ handle_signal_stop (struct execution_control_state *ecs)
 	  && ecs->event_thread->control.trap_expected
 	  && ecs->event_thread->control.step_resume_breakpoint == NULL)
 	{
-	  int was_in_line;
-
 	  /* We were just starting a new sequence, attempting to
 	     single-step off of a breakpoint and expecting a SIGTRAP.
 	     Instead this signal arrives.  This signal will take us out
@@ -6108,34 +6141,11 @@ handle_signal_stop (struct execution_control_state *ecs)
                                 "infrun: signal arrived while stepping over "
                                 "breakpoint\n");
 
-	  was_in_line = step_over_info_valid_p ();
-	  clear_step_over_info ();
 	  insert_hp_step_resume_breakpoint_at_frame (frame);
 	  ecs->event_thread->step_after_step_resume_breakpoint = 1;
 	  /* Reset trap_expected to ensure breakpoints are re-inserted.  */
 	  ecs->event_thread->control.trap_expected = 0;
 
-	  if (target_is_non_stop_p ())
-	    {
-	      /* Either "set non-stop" is "on", or the target is
-		 always in non-stop mode.  In this case, we have a bit
-		 more work to do.  Resume the current thread, and if
-		 we had paused all threads, restart them while the
-		 signal handler runs.  */
-	      keep_going (ecs);
-
-	      if (was_in_line)
-		{
-		  restart_threads (ecs->event_thread);
-		}
-	      else if (debug_infrun)
-		{
-		  fprintf_unfiltered (gdb_stdlog,
-				      "infrun: no need to restart threads\n");
-		}
-	      return;
-	    }
-
 	  /* If we were nexting/stepping some other thread, switch to
 	     it, so that we don't continue it, losing control.  */
 	  if (!switch_back_to_stepped_thread (ecs))
@@ -7676,8 +7686,6 @@ stop_waiting (struct execution_control_state *ecs)
   if (debug_infrun)
     fprintf_unfiltered (gdb_stdlog, "infrun: stop_waiting\n");
 
-  clear_step_over_info ();
-
   /* Let callers know we don't want to wait for the inferior anymore.  */
   ecs->wait_some_more = 0;