telemetry: clean up Python registry objects with a module capsule

subprojects/robotpy-telemetry/telemetry/src/main.cpp

@@ -1,7 +1,17 @@
 #include "rpy/MockTelemetryBackendFunctions.h"
 #include "semiwrap_init.telemetry._telemetry.hpp"
+#include "wpi/telemetry/TelemetryRegistry.hpp"
 
 SEMIWRAP_PYBIND11_MODULE(m) {
   initWrapper(m);
   wpi::telemetry::python::InitializeMockBackendValueTypes(m);
+
+  static int unused;  // the capsule needs something to reference
+  py::capsule cleanup(&unused, [](void*) {
+    // Release Python callbacks, backends, and cached entries during module
+    // teardown, not from C++ static destructors after Python has finalized.
+    wpi::telemetry::TelemetryRegistry::SetReportWarning(nullptr);
+    wpi::telemetry::TelemetryRegistry::Reset();
+  });
+  m.add_object("_cleanup", cleanup);
 }

subprojects/robotpy-telemetry/tests/test_shutdown.py

@@ -0,0 +1,46 @@
+import subprocess
+import sys
+import textwrap
+
+import pytest
+
+
+@pytest.mark.parametrize(
+    "setup",
+    [
+        'telemetry.TelemetryRegistry.register_backend("/test", Backend())',
+        'telemetry.TelemetryRegistry.register_backend("/test", Backend()); '
+        'telemetry.log("/test/value", 1)',
+        'telemetry.TelemetryRegistry.register_backend("/test", '
+        "telemetry.MultiTelemetryBackend([Backend()])); "
+        'telemetry.log("/test/value", 1)',
+        "telemetry.TelemetryRegistry.set_report_warning(lambda path, msg: None)",
+    ],
+    ids=["backend", "cached-entry", "multi-backend", "warning-callback"],
+)
+def test_python_registry_objects_are_safe_at_shutdown(setup):
+    # A subprocess is necessary: normal test cleanup masks registry objects
+    # being destroyed by C++ static destructors after Python has finalized.
+    code = textwrap.dedent("""
+        import telemetry
+
+        class Entry(telemetry.TelemetryEntry):
+            def log_int64(self, value, timestamp):
+                pass
+
+        class Backend(telemetry.TelemetryBackend):
+            def get_entry(self, path):
+                return Entry()
+        """)
+    code += "\n" + setup
+
+    result = subprocess.run(
+        [sys.executable, "-c", code],
+        capture_output=True,
+        text=True,
+        timeout=30,
+    )
+
+    assert result.returncode == 0, result.stdout + result.stderr
+    # Cleanup must not emit ignored exceptions during interpreter shutdown.
+    assert result.stderr == ""