Make scoped_restore_current_thread's cdtors exception free (RFC)

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.

There's a restore_selected_frame function in infrun.c that I think can
be replaced by the new one in frame.c.

Also done in this patch is make the get_selected_frame's parameter be
optional, so that we don't have to pass down nullptr explicitly all
over the place.

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): Make 'message' parameter optional.
	(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.
	* infrun.c (struct infcall_control_state) <selected_frame_level>:
	New field.
	(save_infcall_control_state): Use save_selected_frame.
	(restore_selected_frame): Delete.
	(restore_infcall_control_state): Use restore_selected_frame.
	* 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.

Change-Id: I73fd1cfc40d8513c28e5596383b7ecd8bcfe700f

1 files changed, 41 insertions, 10 deletions

diff --git a/gdb/frame.h b/gdb/frame.h
index 3ceb7b3..f3ec784 100644
--- a/gdb/frame.h
+++ b/gdb/frame.h
@@ -186,8 +186,14 @@ public:
 
 private:
 
-  /* The ID of the previously selected frame.  */
+  /* The ID and level of the previously selected frame.  */
   struct frame_id m_fid;
+  int m_level;
+
+  /* Save/restore the language as well, because selecting a frame
+     changes the current language to the frame's language if "set
+     language auto".  */
+  enum language m_lang;
 };
 
 /* Methods for constructing and comparing Frame IDs.  */
@@ -316,24 +322,49 @@ extern bool has_stack_frames ();
    modifies the target invalidating the frame cache).  */
 extern void reinit_frame_cache (void);
 
-/* 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,
+/* Return the selected frame.  Always returns non-NULL.  If there
+   isn't an inferior sufficient for creating a frame, an error is
+   thrown.  When MESSAGE is non-NULL, use it for the error message,
    otherwise 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);
-
-/* If there is a selected frame, return it.  Otherwise, return NULL.  */
-extern struct frame_info *get_selected_frame_if_set (void);
+extern struct frame_info *get_selected_frame (const char *message = nullptr);
 
-/* Select a specific frame.  NULL, apparently implies re-select the
-   inner most frame.  */
+/* Select a specific frame.  NULL implies re-select the inner most
+   frame.  */
 extern void select_frame (struct frame_info *);
 
+/* Save the frame ID and frame level of the selected frame in FRAME_ID
+   and FRAME_LEVEL, to be restored later with restore_selected_frame.
+
+   This is preferred over getting the same info out of
+   get_selected_frame directly because this function does not create
+   the selected-frame's frame_info object if it hasn't been created
+   yet, and thus is more efficient and doesn't throw.  */
+extern void save_selected_frame (frame_id *frame_id, int *frame_level)
+  noexcept;
+
+/* Restore selected frame as saved with save_selected_frame.
+
+   Does not try to find the corresponding frame_info object.  Instead
+   the next call to get_selected_frame will look it up and cache the
+   result.
+
+   This function does not throw.  It is designed to be safe to called
+   from the destructors of RAII types.  */
+extern void restore_selected_frame (frame_id frame_id, int frame_level)
+  noexcept;
+
+/* Lookup the frame_info object for the selected frame FRAME_ID /
+   FRAME_LEVEL and cache the result.
+
+   If FRAME_LEVEL > 0 and the originally selected frame isn't found,
+   warn and select the innermost (current) frame.  */
+extern void lookup_selected_frame (frame_id frame_id, int frame_level);
+
 /* Given a FRAME, return the next (more inner, younger) or previous
    (more outer, older) frame.  */
 extern struct frame_info *get_prev_frame (struct frame_info *);