[PATCH 5/8] [AArch64] Teach stub unwinder to terminate gracefully

[Pierre Langlois <pierre.langlois@arm.com>]

The stub unwinder is used on AArch64 if the target's memory is not
readable at the current PC.  If we purposely kill the inferior before
examining the trace then we get the following issue:

~~~
...
(gdb) trace f
Tracepoint 3 at 0x7fb7fc28c0
(gdb) tstart
(gdb) continue
...
(gdb) tstop
(gdb) tsave /tmp/trace
(gdb) kill
...
(gdb) target tfile /tmp/trace
...
(gdb) tfind
Register 31 is not available.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Found trace frame 0, tracepoint 3
#-1 0x0000007fb7fc28c0 in f () ...
^^^
~~~

This patch teaches the stub unwinder to report to the core frame code
with UNWIND_UNAVAILABLE when either the stack pointer of the return
address are unavailable to read from the target.

The following test cases now pass when enabling tracepoints:

PASS: gdb.trace/pending.exp: trace works: tfind test frame 0
PASS: gdb.trace/pending.exp: trace resolved_in_trace: tfind test frame 0
PASS: gdb.trace/pending.exp: trace installed_in_trace: tfind test frame 0
PASS: gdb.trace/mi-tracepoint-changed.exp: pending resolved: -trace-find frame-number 0

gdb/ChangeLog:

	* aarch64-tdep.c (aarch64_make_stub_cache): Set available_p and
	swallow NOT_AVAILABLE_ERROR.
	(aarch64_stub_this_id): Call frame_id_build_unavailable_stack if
	available_p is not set.
	(aarch64_stub_frame_unwind_stop_reason): New function.
	(aarch64_stub_unwind): Install it.
---
 gdb/aarch64-tdep.c | 36 +++++++++++++++++++++++++++++++-----
 1 file changed, 31 insertions(+), 5 deletions(-)

diff --git a/gdb/aarch64-tdep.c b/gdb/aarch64-tdep.c
index 87a6d61..57976b7 100644
--- a/gdb/aarch64-tdep.c
+++ b/gdb/aarch64-tdep.c
@@ -1109,13 +1109,36 @@ aarch64_make_stub_cache (struct frame_info *this_frame, void **this_cache)
   cache->saved_regs = trad_frame_alloc_saved_regs (this_frame);
   *this_cache = cache;
 
-  cache->prev_sp
-    = get_frame_register_unsigned (this_frame, AARCH64_SP_REGNUM);
-  cache->prev_pc = get_frame_pc (this_frame);
+  TRY
+    {
+      cache->prev_sp
+	= get_frame_register_unsigned (this_frame, AARCH64_SP_REGNUM);
+      cache->prev_pc = get_frame_pc (this_frame);
+      cache->available_p = 1;
+    }
+  CATCH (ex, RETURN_MASK_ERROR)
+    {
+      if (ex.error != NOT_AVAILABLE_ERROR)
+	throw_exception (ex);
+    }
+  END_CATCH
 
   return cache;
 }
 
+static enum unwind_stop_reason
+aarch64_stub_frame_unwind_stop_reason (struct frame_info *this_frame,
+				       void **this_cache)
+{
+  struct aarch64_prologue_cache *cache
+    = aarch64_make_stub_cache (this_frame, this_cache);
+
+  if (!cache->available_p)
+    return UNWIND_UNAVAILABLE;
+
+  return UNWIND_NO_REASON;
+}
+
 /* Our frame ID for a stub frame is the current SP and LR.  */
 
 static void
@@ -1125,7 +1148,10 @@ aarch64_stub_this_id (struct frame_info *this_frame,
   struct aarch64_prologue_cache *cache
     = aarch64_make_stub_cache (this_frame, this_cache);
 
-  *this_id = frame_id_build (cache->prev_sp, cache->prev_pc);
+  if (cache->available_p)
+    *this_id = frame_id_build (cache->prev_sp, cache->prev_pc);
+  else
+    *this_id = frame_id_build_unavailable_stack (cache->prev_pc);
 }
 
 /* Implement the "sniffer" frame_unwind method.  */
@@ -1152,7 +1178,7 @@ aarch64_stub_unwind_sniffer (const struct frame_unwind *self,
 struct frame_unwind aarch64_stub_unwind =
 {
   NORMAL_FRAME,
-  default_frame_unwind_stop_reason,
+  aarch64_stub_frame_unwind_stop_reason,
   aarch64_stub_this_id,
   aarch64_prologue_prev_register,
   NULL,
-- 
2.1.0