Fix gdb.trace/mi-traceframe-changed.exp on s390.

The test fails on s390 with:

  -trace-find frame-number 0^M
  &"PC not available\n"^M
  ^done,found="1",tracepoint="1",traceframe="0",frame={level="-1",addr="<unavailable>",func="??",args=[]}^M
  (gdb) ^M
  FAIL: gdb.trace/mi-traceframe-changed.exp: tfile: -trace-find frame-number 0

tfile knows to infer the PC from the tracepoint's address if the PC
wasn't collected (tfile_fetch_registers) but, that only works on
targets whose PC register is a raw register, and on s390, the PC
register is a pseudo register.

But even if GDB doesn't know how to infer the value of PC, saying the
current frame is level -1 is a bug:

  ^done,found="1",tracepoint="1",traceframe="0",frame={level="-1",addr="<unavailable>",func="??",args=[]}^M
                                                       ^^^^^^^^^

'-1' is the level of the sentinel frame, which should never be visible.

This is caused by the s390's heuristic unwinder accepting the frame
(the fallback heuristic unwinders _always_ accept the frame), but then
the unwind->this_id method throws that "PC not available\n" error.

IOW, the s390's heuristic unwinder was never adjusted to handle
unavailable register values gracefully, which can happen with e.g., a
trimmed core file too.

This is just the minimal necessary for
<unavailable> frames, which at least gets us:

  (gdb) tfind
  Found trace frame 0, tracepoint 1
  #0  <unavailable> in ?? ()

That is, frame #0 instead of -1.

We could get better info out of "info frame" (this patch makes us show
"outermost"), but this change would still be necessary.

gdb/
2014-01-16  Pedro Alves  <palves@redhat.com>

	* s390-linux-tdep.c (s390_frame_unwind_cache): Swallow
	NOT_AVAILABLE_ERROR errors while parsing the prologue or reading
	the backchain.

2 files changed, 17 insertions, 4 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index f614f1b..6eef81a 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,9 @@
+2014-01-16  Pedro Alves  <palves@redhat.com>
+
+	* s390-linux-tdep.c (s390_frame_unwind_cache): Swallow
+	NOT_AVAILABLE_ERROR errors while parsing the prologue or reading
+	the backchain.
+
 2014-01-16  Doug Evans  <dje@google.com>
 
 	* dwarf2read.c (open_and_init_dwp_file): Fix typo in comment.
diff --git a/gdb/s390-linux-tdep.c b/gdb/s390-linux-tdep.c
index b75c86a..8b71e78 100644
--- a/gdb/s390-linux-tdep.c
+++ b/gdb/s390-linux-tdep.c
@@ -2039,7 +2039,9 @@ static struct s390_unwind_cache *
 s390_frame_unwind_cache (struct frame_info *this_frame,
 			 void **this_prologue_cache)
 {
+  volatile struct gdb_exception ex;
   struct s390_unwind_cache *info;
+
   if (*this_prologue_cache)
     return *this_prologue_cache;
 
@@ -2050,10 +2052,15 @@ s390_frame_unwind_cache (struct frame_info *this_frame,
   info->frame_base = -1;
   info->local_base = -1;
 
-  /* Try to use prologue analysis to fill the unwind cache.
-     If this fails, fall back to reading the stack backchain.  */
-  if (!s390_prologue_frame_unwind_cache (this_frame, info))
-    s390_backchain_frame_unwind_cache (this_frame, info);
+  TRY_CATCH (ex, RETURN_MASK_ERROR)
+    {
+      /* Try to use prologue analysis to fill the unwind cache.
+	 If this fails, fall back to reading the stack backchain.  */
+      if (!s390_prologue_frame_unwind_cache (this_frame, info))
+	s390_backchain_frame_unwind_cache (this_frame, info);
+    }
+  if (ex.reason < 0 && ex.error != NOT_AVAILABLE_ERROR)
+    throw_exception (ex);
 
   return info;
 }