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.

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