infcmd, btrace: fix crash in 'finish' for tailcall-only frames

Patch 7eb895307f Skip unwritable frames in command "finish"
skips non-writable frames in addition to tailcall frames.

If skip_tailcall_frames already returns NULL, skip_unwritable_frames
will be called with a NULL frame and crash in get_frame_arch.  This is
caught by gdb.btrace/tailcall-only.exp.

Further, if we ever end up with a mixture of tailcall and non-writable
frames, we may not skip all of them, as intended.

Loop over skip_tailcall_frames and skip_unwritable_frames as long as at least
one of them makes progress.

gdb/
	* infcmd.c (skip_finish_frames): New.
	(finish_command): Call skip_finish_frames.

gdb/ChangeLog

@@ -1,3 +1,8 @@
+2016-06-01  Markus Metzger  <markus.t.metzger@intel.com>
+
+	* infcmd.c (skip_finish_frames): New.
+	(finish_command): Call skip_finish_frames.
+
 2016-06-01  Yao Qi  <yao.qi@linaro.org>
 
 	PR remote/19998

gdb/infcmd.c

@@ -1927,6 +1927,30 @@ finish_forward (struct finish_command_fsm *sm, struct frame_info *frame)
   proceed ((CORE_ADDR) -1, GDB_SIGNAL_DEFAULT);
 }
 
+/* Skip frames for "finish".  */
+
+static struct frame_info *
+skip_finish_frames (struct frame_info *frame)
+{
+  struct frame_info *start;
+
+  do
+    {
+      start = frame;
+
+      frame = skip_tailcall_frames (frame);
+      if (frame == NULL)
+	break;
+
+      frame = skip_unwritable_frames (frame);
+      if (frame == NULL)
+	break;
+    }
+  while (start != frame);
+
+  return frame;
+}
+
 /* "finish": Set a temporary breakpoint at the place the selected
    frame will return to, then continue.  */
 
@@ -2025,11 +2049,7 @@ finish_command (char *arg, int from_tty)
     finish_backward (sm);
   else
     {
-      /* Ignore TAILCALL_FRAME type frames, they were executed already before
-	 entering THISFRAME.  */
-      frame = skip_tailcall_frames (frame);
-
-      frame = skip_unwritable_frames (frame);
+      frame = skip_finish_frames (frame);
 
       if (frame == NULL)
 	error (_("Cannot find the caller frame."));