Make "bt N" print correct number of frames when using a frame filter

PR python/16497 notes that using "bt" with a positive argument prints
the wrong number of frames when a frame filter is in use.  Also, in this
case, the non-frame-filter path will print a message about "More stack
frames" when there are more; but this is not done in the frame-filter
case.

The first problem is that backtrace_command_1 passes the wrong value
to apply_ext_lang_frame_filter -- that function takes the final
frame's number as an argument, but backtrace_command_1 passes the
count, which is off by one.

The solution to the second problem is to have the C stack-printing
code stop at the correct number of frames and then print the message.

Tested using the buildbot.

ChangeLog
2018-02-26  Tom Tromey  <tom@tromey.com>

	PR python/16497:
	* stack.c (backtrace_command_1): Set PRINT_MORE_FRAMES flag.  Fix
	off-by-one in py_end computation.
	* python/py-framefilter.c (gdbpy_apply_frame_filter): Handle
	PRINT_MORE_FRAMES.
	* extension.h (enum frame_filter_flags) <PRINT_MORE_FRAMES>: New
	constant.

2018-02-26  Tom Tromey  <tom@tromey.com>

	PR python/16497:
	* gdb.python/py-framefilter.exp: Update test.

1 files changed, 25 insertions, 0 deletions

diff --git a/gdb/python/py-framefilter.c b/gdb/python/py-framefilter.c
index e887849c..3a3fb97 100644
--- a/gdb/python/py-framefilter.c
+++ b/gdb/python/py-framefilter.c
@@ -1355,6 +1355,18 @@ gdbpy_apply_frame_filter (const struct extension_language_defn *extlang,
 
   gdbpy_enter enter_py (gdbarch, current_language);
 
+  /* When we're limiting the number of frames, be careful to request
+     one extra frame, so that we can print a message if there are more
+     frames.  */
+  int frame_countdown = -1;
+  if ((flags & PRINT_MORE_FRAMES) != 0 && frame_low >= 0 && frame_high >= 0)
+    {
+      ++frame_high;
+      /* This has an extra +1 because it is checked before a frame is
+	 printed.  */
+      frame_countdown = frame_high - frame_low + 1;
+    }
+
   gdbpy_ref<> iterable (bootstrap_python_frame_filters (frame, frame_low,
 							frame_high));
 
@@ -1402,6 +1414,19 @@ gdbpy_apply_frame_filter (const struct extension_language_defn *extlang,
 	  break;
 	}
 
+      if (frame_countdown != -1)
+	{
+	  gdb_assert ((flags & PRINT_MORE_FRAMES) != 0);
+	  --frame_countdown;
+	  if (frame_countdown == 0)
+	    {
+	      /* We've printed all the frames we were asked to
+		 print, but more frames existed.  */
+	      printf_filtered (_("(More stack frames follow...)\n"));
+	      break;
+	    }
+	}
+
       success = py_print_frame (item.get (), flags, args_type, out, 0,
 				levels_printed.get ());