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

8 files changed, 206 insertions, 124 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 8be91cd..978576a 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,42 @@
+2020-10-30  Pedro Alves  <pedro@palves.net>
+
+	* 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.
+
 2020-10-29  Simon Marchi  <simon.marchi@polymtl.ca>
 
 	* gdbarch.sh (displaced_step_hw_singlestep): Adjust
diff --git a/gdb/blockframe.c b/gdb/blockframe.c
index 80b7695..80537e0 100644
--- a/gdb/blockframe.c
+++ b/gdb/blockframe.c
@@ -464,14 +464,10 @@ find_gnu_ifunc_target_type (CORE_ADDR resolver_funaddr)
 struct frame_info *
 block_innermost_frame (const struct block *block)
 {
-  struct frame_info *frame;
-
   if (block == NULL)
     return NULL;
 
-  frame = get_selected_frame_if_set ();
-  if (frame == NULL)
-    frame = get_current_frame ();
+  frame_info *frame = get_selected_frame ();
   while (frame != NULL)
     {
       const struct block *frame_block = get_frame_block (frame, NULL);
diff --git a/gdb/frame.c b/gdb/frame.c
index 15168fb..bb835e2 100644
--- a/gdb/frame.c
+++ b/gdb/frame.c
@@ -317,17 +317,15 @@ frame_stash_invalidate (void)
 /* See frame.h  */
 scoped_restore_selected_frame::scoped_restore_selected_frame ()
 {
-  m_fid = get_frame_id (get_selected_frame (NULL));
+  m_lang = current_language->la_language;
+  save_selected_frame (&m_fid, &m_level);
 }
 
 /* See frame.h  */
 scoped_restore_selected_frame::~scoped_restore_selected_frame ()
 {
-  frame_info *frame = frame_find_by_id (m_fid);
-  if (frame == NULL)
-    warning (_("Unable to restore previously selected frame."));
-  else
-    select_frame (frame);
+  restore_selected_frame (m_fid, m_level);
+  set_language (m_lang);
 }
 
 /* Flag to control debugging.  */
@@ -1685,10 +1683,63 @@ get_current_frame (void)
 }
 
 /* The "selected" stack frame is used by default for local and arg
-   access.  May be zero, for no selected frame.  */
-
+   access.
+
+   The "single source of truth" for the selected frame is the
+   SELECTED_FRAME_ID / SELECTED_FRAME_LEVEL pair.
+
+   Frame IDs can be saved/restored across reinitializing the frame
+   cache, while frame_info pointers can't (frame_info objects are
+   invalidated).  If we know the corresponding frame_info object, it
+   is cached in SELECTED_FRAME.
+
+   If SELECTED_FRAME_ID / SELECTED_FRAME_LEVEL are null_frame_id / -1,
+   and the target has stack and is stopped, the selected frame is the
+   current (innermost) frame.  This means that SELECTED_FRAME_LEVEL is
+   never 0 and SELECTED_FRAME_ID is never the ID of the innermost
+   frame.
+
+   If SELECTED_FRAME_ID / SELECTED_FRAME_LEVEL are null_frame_id / -1,
+   and the target has no stack or is executing, then there's no
+   selected frame.  */
+static frame_id selected_frame_id = null_frame_id;
+static int selected_frame_level = -1;
+
+/* The cached frame_info object pointing to the selected frame.
+   Looked up on demand by get_selected_frame.  */
 static struct frame_info *selected_frame;
 
+/* See frame.h.  */
+
+void
+save_selected_frame (frame_id *frame_id, int *frame_level)
+  noexcept
+{
+  *frame_id = selected_frame_id;
+  *frame_level = selected_frame_level;
+}
+
+/* See frame.h.  */
+
+void
+restore_selected_frame (frame_id frame_id, int frame_level)
+  noexcept
+{
+  /* save_selected_frame never returns level == 0, so we shouldn't see
+     it here either.  */
+  gdb_assert (frame_level != 0);
+
+  /* FRAME_ID can be null_frame_id only IFF frame_level is -1.  */
+  gdb_assert ((frame_level == -1 && !frame_id_p (frame_id))
+	      || (frame_level != -1 && frame_id_p (frame_id)));
+
+  selected_frame_id = frame_id;
+  selected_frame_level = frame_level;
+
+  /* Will be looked up later by get_selected_frame.  */
+  selected_frame = nullptr;
+}
+
 bool
 has_stack_frames ()
 {
@@ -1716,9 +1767,7 @@ has_stack_frames ()
   return true;
 }
 
-/* Return the selected frame.  Always non-NULL (unless there isn't an
-   inferior sufficient for creating a frame) in which case an error is
-   thrown.  */
+/* See frame.h.  */
 
 struct frame_info *
 get_selected_frame (const char *message)
@@ -1727,24 +1776,14 @@ get_selected_frame (const char *message)
     {
       if (message != NULL && !has_stack_frames ())
 	error (("%s"), message);
-      /* Hey!  Don't trust this.  It should really be re-finding the
-	 last selected frame of the currently selected thread.  This,
-	 though, is better than nothing.  */
-      select_frame (get_current_frame ());
+
+      lookup_selected_frame (selected_frame_id, selected_frame_level);
     }
   /* There is always a frame.  */
   gdb_assert (selected_frame != NULL);
   return selected_frame;
 }
 
-/* If there is a selected frame, return it.  Otherwise, return NULL.  */
-
-struct frame_info *
-get_selected_frame_if_set (void)
-{
-  return selected_frame;
-}
-
 /* This is a variant of get_selected_frame() which can be called when
    the inferior does not have a frame; in that case it will return
    NULL instead of calling error().  */
@@ -1757,12 +1796,42 @@ deprecated_safe_get_selected_frame (void)
   return get_selected_frame (NULL);
 }
 
-/* Select frame FI (or NULL - to invalidate the current frame).  */
+/* Select frame FI (or NULL - to invalidate the selected frame).  */
 
 void
 select_frame (struct frame_info *fi)
 {
   selected_frame = fi;
+  selected_frame_level = frame_relative_level (fi);
+  if (selected_frame_level == 0)
+    {
+      /* Treat the current frame especially -- we want to always
+	 save/restore it without warning, even if the frame ID changes
+	 (see lookup_selected_frame).  E.g.:
+
+	  // The current frame is selected, the target had just stopped.
+	  {
+	    scoped_restore_selected_frame restore_frame;
+	    some_operation_that_changes_the_stack ();
+	  }
+	  // scoped_restore_selected_frame's dtor runs, but the
+	  // original frame_id can't be found.  No matter whether it
+	  // is found or not, we still end up with the now-current
+	  // frame selected.  Warning in lookup_selected_frame in this
+	  // case seems pointless.
+
+	 Also get_frame_id may access the target's registers/memory,
+	 and thus skipping get_frame_id optimizes the common case.
+
+	 Saving the selected frame this way makes get_selected_frame
+	 and restore_current_frame return/re-select whatever frame is
+	 the innermost (current) then.  */
+      selected_frame_level = -1;
+      selected_frame_id = null_frame_id;
+    }
+  else
+    selected_frame_id = get_frame_id (fi);
+
   /* NOTE: cagney/2002-05-04: FI can be NULL.  This occurs when the
      frame is being invalidated.  */
 
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 *);
diff --git a/gdb/gdbthread.h b/gdb/gdbthread.h
index ab5771f..da20dd4 100644
--- a/gdb/gdbthread.h
+++ b/gdb/gdbthread.h
@@ -673,6 +673,10 @@ private:
   frame_id m_selected_frame_id;
   int m_selected_frame_level;
   bool m_was_stopped;
+  /* 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;
 };
 
 /* Returns a pointer into the thread_info corresponding to
diff --git a/gdb/infrun.c b/gdb/infrun.c
index e8c8fc0..b007af0 100644
--- a/gdb/infrun.c
+++ b/gdb/infrun.c
@@ -9003,8 +9003,10 @@ struct infcall_control_state
   enum stop_stack_kind stop_stack_dummy = STOP_NONE;
   int stopped_by_random_signal = 0;
 
-  /* ID if the selected frame when the inferior function call was made.  */
+  /* ID and level of the selected frame when the inferior function
+     call was made.  */
   struct frame_id selected_frame_id {};
+  int selected_frame_level = -1;
 };
 
 /* Save all of the information associated with the inferior<==>gdb
@@ -9033,27 +9035,12 @@ save_infcall_control_state ()
   inf_status->stop_stack_dummy = stop_stack_dummy;
   inf_status->stopped_by_random_signal = stopped_by_random_signal;
 
-  inf_status->selected_frame_id = get_frame_id (get_selected_frame (NULL));
+  save_selected_frame (&inf_status->selected_frame_id,
+		       &inf_status->selected_frame_level);
 
   return inf_status;
 }
 
-static void
-restore_selected_frame (const frame_id &fid)
-{
-  frame_info *frame = frame_find_by_id (fid);
-
-  /* If inf_status->selected_frame_id is NULL, there was no previously
-     selected frame.  */
-  if (frame == NULL)
-    {
-      warning (_("Unable to restore previously selected frame."));
-      return;
-    }
-
-  select_frame (frame);
-}
-
 /* Restore inferior session state to INF_STATUS.  */
 
 void
@@ -9081,21 +9068,8 @@ restore_infcall_control_state (struct infcall_control_state *inf_status)
 
   if (target_has_stack ())
     {
-      /* The point of the try/catch is that if the stack is clobbered,
-         walking the stack might encounter a garbage pointer and
-         error() trying to dereference it.  */
-      try
-	{
-	  restore_selected_frame (inf_status->selected_frame_id);
-	}
-      catch (const gdb_exception_error &ex)
-	{
-	  exception_fprintf (gdb_stderr, ex,
-			     "Unable to restore previously selected frame:\n");
-	  /* Error in restoring the selected frame.  Select the
-	     innermost frame.  */
-	  select_frame (get_current_frame ());
-	}
+      restore_selected_frame (inf_status->selected_frame_id,
+			      inf_status->selected_frame_level);
     }
 
   delete inf_status;
diff --git a/gdb/stack.c b/gdb/stack.c
index 4ba5500..83c3366 100644
--- a/gdb/stack.c
+++ b/gdb/stack.c
@@ -1842,9 +1842,9 @@ trailing_outermost_frame (int count)
 static void
 select_frame_command_core (struct frame_info *fi, bool ignored)
 {
-  struct frame_info *prev_frame = get_selected_frame_if_set ();
+  frame_info *prev_frame = get_selected_frame ();
   select_frame (fi);
-  if (get_selected_frame_if_set () != prev_frame)
+  if (get_selected_frame () != prev_frame)
     gdb::observers::user_selected_context_changed.notify (USER_SELECTED_FRAME);
 }
 
@@ -1863,10 +1863,9 @@ select_frame_for_mi (struct frame_info *fi)
 static void
 frame_command_core (struct frame_info *fi, bool ignored)
 {
-  struct frame_info *prev_frame = get_selected_frame_if_set ();
-
+  frame_info *prev_frame = get_selected_frame ();
   select_frame (fi);
-  if (get_selected_frame_if_set () != prev_frame)
+  if (get_selected_frame () != prev_frame)
     gdb::observers::user_selected_context_changed.notify (USER_SELECTED_FRAME);
   else
     print_selected_thread_frame (current_uiout, USER_SELECTED_FRAME);
diff --git a/gdb/thread.c b/gdb/thread.c
index d832443..193f9d4 100644
--- a/gdb/thread.c
+++ b/gdb/thread.c
@@ -1325,20 +1325,26 @@ 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 current frame.  In either case, select 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
@@ -1409,64 +1415,28 @@ 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 ();
 }
 
 scoped_restore_current_thread::scoped_restore_current_thread ()
 {
   m_inf = inferior_ref::new_reference (current_inferior ());
 
+  m_lang = current_language->la_language;
+
   if (inferior_ptid != null_ptid)
     {
       m_thread = thread_info_ref::new_reference (inferior_thread ());
 
-      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)
-	    throw;
-	}
+      save_selected_frame (&m_selected_frame_id, &m_selected_frame_level);
     }
 }