gdb/python: add PENDING_FRAMEPY_REQUIRE_VALID macro in py-unwind.c

This commit copies the pattern that is present in many other py-*.c
files: having a single macro to check that the Python object is still
valid.

This cleans up the code a little throughout the py-unwind.c file.

Some of the exception messages will change slightly with this commit,
though the type of the exceptions is still ValueError in all cases.

I started writing some tests for this change and immediately ran into
a problem: GDB would crash.  It turns out that the PendingFrame
objects are not being marked as invalid!

In pyuw_sniffer where the pending frames are created, we make use of a
scoped_restore to invalidate the pending frame objects.  However, this
only restores the pending_frame_object::frame_info field to its
previous value -- and it turns out we never actually give this field
an initial value, it's left undefined.

So, when the scoped_restore (called invalidate_frame) performs its
cleanup, it actually restores the frame_info field to an undefined
value.  If this undefined value is not nullptr then any future
accesses to the PendingFrame object result in undefined behaviour and
most likely, a crash.

As part of this commit I now initialize the frame_info field, which
ensures all the new tests now pass.

Reviewed-By: Tom Tromey <tom@tromey.com>

2 files changed, 26 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.python/py-unwind.exp b/gdb/testsuite/gdb.python/py-unwind.exp
index 337e5dc..3e214ee 100644
--- a/gdb/testsuite/gdb.python/py-unwind.exp
+++ b/gdb/testsuite/gdb.python/py-unwind.exp
@@ -142,3 +142,22 @@ gdb_test "python obj = simple_unwinder(True)" \
     [multi_line \
 	 "TypeError: incorrect type for name: <class 'bool'>" \
 	 "Error while executing Python code\\."]
+
+# Now register the simple_unwinder with a valid name, and use the
+# unwinder to capture a PendingFrame object.
+gdb_test_no_output "python obj = simple_unwinder(\"simple\")"
+gdb_test_no_output "python gdb.unwinder.register_unwinder(None, obj)"
+check_for_broken_backtrace "backtrace to capture a PendingFrame object"
+
+# Call methods on the captured gdb.PendingFrame and check we see the
+# expected error.
+gdb_test_no_output "python pf = captured_pending_frame"
+foreach cmd {"pf.read_register(\"pc\")" \
+		 "pf.create_unwind_info(None)" \
+		 "pf.architecture()" \
+		 "pf.level()"} {
+    gdb_test "python $cmd" \
+	[multi_line \
+	     "ValueError: gdb\\.PendingFrame is invalid\\." \
+	     "Error while executing Python code\\."]
+}
diff --git a/gdb/testsuite/gdb.python/py-unwind.py b/gdb/testsuite/gdb.python/py-unwind.py
index edd2e30..b30e843 100644
--- a/gdb/testsuite/gdb.python/py-unwind.py
+++ b/gdb/testsuite/gdb.python/py-unwind.py
@@ -133,12 +133,19 @@ class TestUnwinder(Unwinder):
 global_test_unwinder = TestUnwinder()
 gdb.unwinder.register_unwinder(None, global_test_unwinder, True)
 
+# This is filled in by the simple_unwinder class.
+captured_pending_frame = None
+
 
 class simple_unwinder(Unwinder):
     def __init__(self, name):
         super().__init__(name)
 
     def __call__(self, pending_frame):
+        global captured_pending_frame
+
+        if captured_pending_frame is None:
+            captured_pending_frame = pending_frame
         return None