Fix PR backtrace/15558

This PR is about an assertion failure in GDB that can be triggered by
setting "backtrace limit" to a value that causes GDB to stop unwinding
after an inline frame.  In this case, an assertion in
inline_frame_this_id will trigger:

  /* We need a valid frame ID, so we need to be based on a valid
     frame.  (...).  */
  gdb_assert (frame_id_p (*this_id));

Looking at the function:

 static void
 inline_frame_this_id (struct frame_info *this_frame,
		       void **this_cache,
		       struct frame_id *this_id)
 {
   struct symbol *func;

   /* In order to have a stable frame ID for a given inline function,
      we must get the stack / special addresses from the underlying
      real frame's this_id method.  So we must call get_prev_frame.
      Because we are inlined into some function, there must be previous
      frames, so this is safe - as long as we're careful not to
      create any cycles.  */
   *this_id = get_frame_id (get_prev_frame (this_frame));

we see we're computing the frame id for the inline frame.  If this is
an inline frame, which is a virtual frame constructed based on debug
info, on top of a real stack frame, we should _always_ be able to find
where the frame was inlined into, as that ultimately just means
peeling off the virtual frames on top of the real stack frame.  If
there ultimately was no prev (real) stack frame, then we wouldn't have
been able to construct the inline frame either, by design.  That's
what the assertion catches.

So we have an inline frame, we should _always_ be able to compute its
ID, even if that means bypassing the user backtrace limits to get at
the real stack frame's info.  The problem is that inline_frame_id
calls get_prev_frame, and that takes user backtrace limits into
account.  Code that wants to bypass the limits calls get_prev_frame_1
instead.

Note how get_prev_frame_1 already skips all checks for inline frames:

   /* If we are unwinding from an inline frame, all of the below tests
      were already performed when we unwound from the next non-inline
      frame.  We must skip them, since we can not get THIS_FRAME's ID
      until we have unwound all the way down to the previous non-inline
      frame.  */
   if (get_frame_type (this_frame) == INLINE_FRAME)
     return get_prev_frame_if_no_cycle (this_frame);

And note how the related frame_unwind_caller_id function also uses
get_prev_frame_1:

 struct frame_id
 frame_unwind_caller_id (struct frame_info *next_frame)
 {
   struct frame_info *this_frame;

   /* Use get_prev_frame_1, and not get_prev_frame.  The latter will truncate
      the frame chain, leading to this function unintentionally
      returning a null_frame_id (e.g., when a caller requests the frame
      ID of "main()"s caller.  */

   next_frame = skip_artificial_frames (next_frame);
   this_frame = get_prev_frame_1 (next_frame);
   if (this_frame)
     return get_frame_id (skip_artificial_frames (this_frame));
   else
     return null_frame_id;
 }

get_prev_frame_1 is currently static in frame.c.  As a _1 suffix is
not a good name for an extern function, I've renamed it.

Tested on x86-64 Fedora 17.

gdb/
2014-04-18  Pedro alves  <palves@redhat.com>
	    Tom Tromey  <tromey@redhat.com>

	PR backtrace/15558
	* frame.c (get_prev_frame_1): Rename to ...
	(get_prev_frame_always): ... this, and make extern.  Adjust.
	(skip_artificial_frames): Use get_prev_frame_always.
	(frame_unwind_caller_id, frame_pop, get_prev_frame)
	(get_frame_unwind_stop_reason): Adjust to rename.
	* frame.h (get_prev_frame_always): Declare.
	* inline-frame.c: Include frame.h.
	(inline_frame_this_id): Use get_prev_frame_always.

gdb/testsuite/
2014-04-18  Tom Tromey  <palves@redhat.com>
	    Pedro alves  <tromey@redhat.com>

	PR backtrace/15558
	* gdb.opt/inline-bt.exp: Test backtracing from an inline function
	with a backtrace limit.
	* gdb.python/py-frame-inline.exp: Test running to an inline
	function with a backtrace limit, and printing the newest frame.
	* gdb.python/py-frame-inline.c (main): Call f.

2 files changed, 17 insertions, 1 deletions

diff --git a/gdb/testsuite/gdb.python/py-frame-inline.c b/gdb/testsuite/gdb.python/py-frame-inline.c
index a3669bc..f08e84b 100644
--- a/gdb/testsuite/gdb.python/py-frame-inline.c
+++ b/gdb/testsuite/gdb.python/py-frame-inline.c
@@ -39,5 +39,7 @@ g (void)
 int
 main (void)
 {
-  return g ();
+  int x = g ();
+  x += f ();
+  return x;
 }
diff --git a/gdb/testsuite/gdb.python/py-frame-inline.exp b/gdb/testsuite/gdb.python/py-frame-inline.exp
index f5cf33e..8851d87 100644
--- a/gdb/testsuite/gdb.python/py-frame-inline.exp
+++ b/gdb/testsuite/gdb.python/py-frame-inline.exp
@@ -37,3 +37,17 @@ gdb_test "info frame" "inlined into frame 1\r\n.*"
 gdb_test "up" "#1  g .*"
 
 gdb_test "python print (gdb.selected_frame().read_var('l'))" "\r\n42"
+
+# A regression test for having a backtrace limit that forces unwinding
+# to stop after an inline frame.  GDB needs to compute the frame_id of
+# the inline frame, which requires unwinding past all the inline
+# frames to the real stack frame, even if that means bypassing the
+# user visible backtrace limit.  See PR backtrace/15558.
+#
+# Set the limit, and run to an inline function.  It's important that
+# the frame cache is flushed somehow after setting the limit, to force
+# frame id recomputation.
+gdb_test_no_output "set backtrace limit 1"
+gdb_continue_to_breakpoint "Block break here."
+
+gdb_test "python print (gdb.newest_frame())" ".*"