Fix frame cache miss handling in RemoteUnwinder to return complete st…

…acks When the frame walker stopped at a cached frame boundary but the cache lookup failed to find the continuation (e.g., when a new unwinder instance encounters stale last_profiled_frame values from a previous profiler), the code would silently return an incomplete stack. Now when a cache lookup misses, the walker continues from that point to collect the remaining frames, ensuring complete stack traces are always returned.
python · pablogsal · Dec 6, 2025 · Dec 1, 2025 · Dec 1, 2025 · Dec 1, 2025
commit d2aa8a45dcee02ecf122c89f82b4d99f79999d3a
diff --git a/Lib/test/test_external_inspection.py b/Lib/test/test_external_inspection.py
@@ -2559,6 +2559,59 @@ def get_thread_frames(target_funcs):
             # No cross-contamination
             self.assertNotIn("blech1", t2_blech)
 
+    @skip_if_not_supported
+    @unittest.skipIf(
+        sys.platform == "linux" and not PROCESS_VM_READV_SUPPORTED,
+        "Test only runs on Linux with process_vm_readv support",
+    )
+    def test_new_unwinder_with_stale_last_profiled_frame(self):
+        """Test that a new unwinder returns complete stack when cache lookup misses."""
+        script_body = """\
+            def level4():
+                sock.sendall(b"sync1")
+                sock.recv(16)
+                sock.sendall(b"sync2")
+                sock.recv(16)
+
+            def level3():
+                level4()
+
+            def level2():
+                level3()
+
+            def level1():
+                level2()
+
+            level1()
+            """
+
+        with self._target_process(script_body) as (p, client_socket, make_unwinder):
+            expected = {"level1", "level2", "level3", "level4"}
+
+            # First unwinder samples - this sets last_profiled_frame in target
+            unwinder1 = make_unwinder(cache_frames=True)
+            frames1 = self._sample_frames(client_socket, unwinder1, b"sync1", b"ack", expected)
+
+            # Create NEW unwinder (empty cache) and sample
+            # The target still has last_profiled_frame set from unwinder1
+            unwinder2 = make_unwinder(cache_frames=True)
+            frames2 = self._sample_frames(client_socket, unwinder2, b"sync2", b"done", expected)
+
+        self.assertIsNotNone(frames1)
+        self.assertIsNotNone(frames2)
+
+        funcs1 = [f.funcname for f in frames1]
+        funcs2 = [f.funcname for f in frames2]
+
+        # Both should have all levels
+        for level in ["level1", "level2", "level3", "level4"]:
+            self.assertIn(level, funcs1, f"{level} missing from first sample")
+            self.assertIn(level, funcs2, f"{level} missing from second sample")
+
+        # Should have same stack depth
+        self.assertEqual(len(frames1), len(frames2),
+                         "New unwinder should return complete stack despite stale last_profiled_frame")
+
 
 if __name__ == "__main__":
     unittest.main()
@@ -770,6 +770,14 @@ collect_frames_with_cache(
             Py_DECREF(frame_addresses);
             return -1;
         }
+        if (cache_result == 0) {
+            // Cache miss - continue walking from last_profiled_frame to get the rest
+            if (process_frame_chain(unwinder, last_profiled_frame, chunks, frame_info, gc_frame,
+                                    0, NULL, frame_addresses) < 0) {
+                Py_DECREF(frame_addresses);
+                return -1;
+            }
+        }
     }
 
     // Convert frame_addresses (list of PyLong) to C array for efficient cache storage