[PATCH v2 3/9] gdb: replace inferior::waiting_for_vfork_done with inferior::thread_waiting_for_vfork_done

[Simon Marchi <simon.marchi@polymtl.ca>]

The inferior::waiting_for_vfork_done flag indicates that some thread in
that inferior is waiting for a vfork-done event.  Subsequent patches
will need to know which thread is waiting for that event.

I think there is a latent buglet in that waiting_for_vfork_done is
currently not reset on inferior exec or exit.  I could imagine that if a
thread in the parent process calls exec or exit while another thread of
the parent process is waiting for its vfork child to exec or exit, we
could end up with inferior::waiting_for_vfork_done without a thread
actually waiting for a vfork-done event anymore.  And since that flag is
checked in resume_1, things could misbehave there.

Since the new field points to a thread_info object, and those are
destroyed on exec or exit, it could be worse now since we could try to
access freed memory, if thread_waiting_for_vfork_done were to point to a
stale thread_info.  To avoid this, clear the field in
infrun_inferior_exit and infrun_inferior_execd.

Change-Id: I31b847278613a49ba03fc4915f74d9ceb228fdce
---
 gdb/inferior.h |  8 ++++----
 gdb/infrun.c   | 14 +++++++++-----
 2 files changed, 13 insertions(+), 9 deletions(-)

diff --git a/gdb/inferior.h b/gdb/inferior.h
index ec0fb6e8b16c..7b82703f1470 100644
--- a/gdb/inferior.h
+++ b/gdb/inferior.h
@@ -531,10 +531,10 @@ class inferior : public refcounted_object,
      exits or execs.  */
   bool pending_detach = false;
 
-  /* True if this inferior is a vfork parent waiting for a vfork child
-     not under our control to be done with the shared memory region,
-     either by exiting or execing.  */
-  bool waiting_for_vfork_done = false;
+  /* If non-nullptr, points to a thread that called vfork and is now waiting
+     for a vfork child not under our control to be done with the shared memory
+     region, either by exiting or execing.  */
+  thread_info *thread_waiting_for_vfork_done = nullptr;
 
   /* True if we're in the process of detaching from this inferior.  */
   bool detaching = false;
diff --git a/gdb/infrun.c b/gdb/infrun.c
index 46e14d720cce..40b4bdd73130 100644
--- a/gdb/infrun.c
+++ b/gdb/infrun.c
@@ -507,7 +507,8 @@ holding the child stopped.  Try \"set detach-on-fork\" or \
 	     insert breakpoints, so that we can debug it.  A
 	     subsequent child exec or exit is enough to know when does
 	     the child stops using the parent's address space.  */
-	  parent_inf->waiting_for_vfork_done = detach_fork;
+	  parent_inf->thread_waiting_for_vfork_done
+	    = detach_fork ? inferior_thread () : nullptr;
 	  parent_inf->pspace->breakpoints_not_allowed = detach_fork;
 	}
     }
@@ -639,7 +640,7 @@ holding the child stopped.  Try \"set detach-on-fork\" or \
 	  child_inf->pending_detach = 0;
 	  parent_inf->vfork_child = child_inf;
 	  parent_inf->pending_detach = detach_fork;
-	  parent_inf->waiting_for_vfork_done = 0;
+	  parent_inf->thread_waiting_for_vfork_done = nullptr;
 	}
       else if (detach_fork)
 	{
@@ -1484,6 +1485,7 @@ static void
 infrun_inferior_exit (struct inferior *inf)
 {
   inf->displaced_step_state.reset ();
+  inf->thread_waiting_for_vfork_done = nullptr;
 }
 
 static void
@@ -1502,6 +1504,8 @@ infrun_inferior_execd (inferior *inf)
      one in progress at the time of the exec, it must have been the exec'ing
      thread.  */
   clear_step_over_info ();
+
+  inf->thread_waiting_for_vfork_done = nullptr;
 }
 
 /* If ON, and the architecture supports it, GDB will use displaced
@@ -2254,7 +2258,7 @@ resume_1 (enum gdb_signal sig)
   /* Depends on stepped_breakpoint.  */
   step = currently_stepping (tp);
 
-  if (current_inferior ()->waiting_for_vfork_done)
+  if (current_inferior ()->thread_waiting_for_vfork_done != nullptr)
     {
       /* Don't try to single-step a vfork parent that is waiting for
 	 the child to get out of the shared memory region (by exec'ing
@@ -2374,7 +2378,7 @@ resume_1 (enum gdb_signal sig)
       && use_displaced_stepping (tp)
       && !step_over_info_valid_p ()
       && sig == GDB_SIGNAL_0
-      && !current_inferior ()->waiting_for_vfork_done)
+      && current_inferior ()->thread_waiting_for_vfork_done == nullptr)
     {
       displaced_step_prepare_status prepare_status
 	= displaced_step_prepare (tp);
@@ -5618,7 +5622,7 @@ handle_inferior_event (struct execution_control_state *ecs)
 
       context_switch (ecs);
 
-      current_inferior ()->waiting_for_vfork_done = 0;
+      current_inferior ()->thread_waiting_for_vfork_done = nullptr;
       current_inferior ()->pspace->breakpoints_not_allowed = 0;
 
       if (handle_stop_requested (ecs))
-- 
2.34.1