Make scoped_restore_current_thread's cdtors exception free (RFC)users/palves/pr26199-busy-loop-target-events

If the remote target closes while we're reading registers/memory for
restoring the selected frame in scoped_restore_current_thread's dtor,
the corresponding TARGET_CLOSE_ERROR error is swallowed by the
scoped_restore_current_thread's dtor, because letting exceptions
escape from a dtor is bad.  It isn't great to lose that errors like
that, though.  I've been thinking about how to avoid it, and I came up
with this patch.

The idea here is to make scoped_restore_current_thread's dtor do as
little as possible, to avoid any work that might throw in the first
place.  And to do that, instead of having the dtor call
restore_selected_frame, which re-finds the previously selected frame,
just record the frame_id/level of the desired selected frame, and have
get_selected_frame find the frame the next time it is called.  In
effect, this implements most of Cagney's suggestion, here:

  /* On demand, create the selected frame and then return it.  If the
     selected frame can not be created, this function prints then throws
     an error.  When MESSAGE is non-NULL, use it for the error message,
     otherwize use a generic error message.  */
  /* FIXME: cagney/2002-11-28: At present, when there is no selected
     frame, this function always returns the current (inner most) frame.
     It should instead, when a thread has previously had its frame
     selected (but not resumed) and the frame cache invalidated, find
     and then return that thread's previously selected frame.  */
  extern struct frame_info *get_selected_frame (const char *message);

The only thing missing to fully implement that would be to make
reinit_frame_cache just clear selected_frame instead of calling
select_frame(NULL), and the call select_frame(NULL) explicitly in the
places where we really wanted reinit_frame_cache to go back to the
current frame too.  That can done separately, though, I'm not
proposing to do that in this patch.

Note that this patch renames restore_selected_frame to
lookup_selected_frame, and adds a new restore_selected_frame function
that doesn't throw, to be paired with the also-new save_selected_frame
function.

lookup_selected_frame should really move from thread.c to frame.c, but
I didn't do that here, just to avoid churn in the patch while it
collects comments.  I did make it extern and declared it in frame.h
already, preparing for the move.  I will do the move as a follow up
patch if people agree with this approach.

Incidentally, this patch alone would fix the crashes fixed by the
previous patches in the series, because with this,
scoped_restore_current_thread's constructor doesn't throw either.

gdb/ChangeLog:

	* blockframe.c (block_innermost_frame): Use get_selected_frame.
	* frame.c
	(scoped_restore_selected_frame::scoped_restore_selected_frame):
	Use save_selected_frame.  Save language as well.
	(scoped_restore_selected_frame::~scoped_restore_selected_frame):
	Use restore_selected_frame, and restore language as well.
	(selected_frame_id, selected_frame_level): New.
	(selected_frame): Update comments.
	(save_selected_frame, restore_selected_frame): New.
	(get_selected_frame): Use lookup_selected_frame.
	(get_selected_frame_if_set): Delete.
	(select_frame): Record selected_frame_level and selected_frame_id.
	* frame.h (scoped_restore_selected_frame) <m_level, m_lang>: New
	fields.
	(get_selected_frame_if_set): Delete declaration.
	(select_frame): Update comments.
	(save_selected_frame, restore_selected_frame)
	(lookup_selected_frame): Declare.
	* gdbthread.h (scoped_restore_current_thread) <m_lang>: New field.
	* stack.c (select_frame_command_core, frame_command_core): Use
	get_selected_frame.
	* thread.c (restore_selected_frame): Rename to ...
	(lookup_selected_frame): ... this and make extern.  Select the
	current frame if the frame level is -1.
	(scoped_restore_current_thread::restore): Also restore the
	language.
	(scoped_restore_current_thread::~scoped_restore_current_thread):
	Don't try/catch.
	(scoped_restore_current_thread::scoped_restore_current_thread):
	Save the language as well.  Use save_selected_frame.

1 files changed, 16 insertions, 51 deletions

diff --git a/gdb/thread.c b/gdb/thread.c
index a3c2be7..e748029 100644
--- a/gdb/thread.c
+++ b/gdb/thread.c
@@ -1325,20 +1325,25 @@ switch_to_thread (process_stratum_target *proc_target, ptid_t ptid)
   switch_to_thread (thr);
 }
 
-static void
-restore_selected_frame (struct frame_id a_frame_id, int frame_level)
+/* See frame.h.  */
+
+void
+lookup_selected_frame (struct frame_id a_frame_id, int frame_level)
 {
   struct frame_info *frame = NULL;
   int count;
 
-  /* This means there was no selected frame.  */
+  /* This either means there was no selected frame, or the selected
+     frame was the innermost (the current frame).  */
   if (frame_level == -1)
     {
-      select_frame (NULL);
+      select_frame (get_current_frame ());
       return;
     }
 
-  gdb_assert (frame_level >= 0);
+  /* select_frame never saves 0 in SELECTED_FRAME_LEVEL, so we
+     shouldn't see it here.  */
+  gdb_assert (frame_level > 0);
 
   /* Restore by level first, check if the frame id is the same as
      expected.  If that fails, try restoring by frame id.  If that
@@ -1409,22 +1414,14 @@ scoped_restore_current_thread::restore ()
       && target_has_stack
       && target_has_memory)
     restore_selected_frame (m_selected_frame_id, m_selected_frame_level);
+
+  set_language (m_lang);
 }
 
 scoped_restore_current_thread::~scoped_restore_current_thread ()
 {
   if (!m_dont_restore)
-    {
-      try
-	{
-	  restore ();
-	}
-      catch (const gdb_exception &ex)
-	{
-	  /* We're in a dtor, there's really nothing else we can do
-	     but swallow the exception.  */
-	}
-    }
+    restore ();
 
   if (m_thread != NULL)
     m_thread->decref ();
@@ -1436,47 +1433,15 @@ scoped_restore_current_thread::scoped_restore_current_thread ()
   m_inf = current_inferior ();
   m_inf->incref ();
 
+  m_lang = current_language->la_language;
+
   if (inferior_ptid != null_ptid)
     {
       m_thread = inferior_thread ();
       m_thread->incref ();
 
-      struct frame_info *frame;
-
       m_was_stopped = m_thread->state == THREAD_STOPPED;
-      if (m_was_stopped
-	  && target_has_registers
-	  && target_has_stack
-	  && target_has_memory)
-	{
-	  /* When processing internal events, there might not be a
-	     selected frame.  If we naively call get_selected_frame
-	     here, then we can end up reading debuginfo for the
-	     current frame, but we don't generally need the debuginfo
-	     at this point.  */
-	  frame = get_selected_frame_if_set ();
-	}
-      else
-	frame = NULL;
-
-      try
-	{
-	  m_selected_frame_id = get_frame_id (frame);
-	  m_selected_frame_level = frame_relative_level (frame);
-	}
-      catch (const gdb_exception_error &ex)
-	{
-	  m_selected_frame_id = null_frame_id;
-	  m_selected_frame_level = -1;
-
-	  /* Better let this propagate.  */
-	  if (ex.error == TARGET_CLOSE_ERROR)
-	    {
-	      m_thread->decref ();
-	      m_inf->decref ();
-	      throw;
-	    }
-	}
+      save_selected_frame (&m_selected_frame_id, &m_selected_frame_level);
     }
   else
     m_thread = NULL;