Capture a stack when tests/ui wedges, instead of killing the run blind

tests/ui/conftest.py

@@ -0,0 +1,68 @@
+"""Optional hang diagnostics for the UI test suite.
+
+Issue #39: tests/ui occasionally hangs -- no output, no crash, no progress
+-- rather than failing outright, and nobody has ever captured a stack from
+one. MATTELOOP_TEST_HANG_DUMP_SECONDS arms faulthandler.dump_traceback_later()
+around every test item, the same technique matteloop.logs already uses for
+native crashes in the shipped app (see _enable_fault_reports there). It
+mirrors pytest's own built-in faulthandler_timeout plugin
+(_pytest/faulthandler.py): open the target file once for the whole session
+rather than per test -- closing it while a dump from the previous test's
+timer could still be in flight reproducibly segfaulted the interpreter here
+-- and switch it by an environment variable rather than an ini option so it
+can be turned on for a single measurement run without touching
+pyproject.toml. Writing to a plain file instead of the duped stderr fd
+means the dump survives however a caller redirects the subprocess's own
+stdout/stderr.
+
+Off by default: the normal suite is unaffected by this file.
+"""
+
+from __future__ import annotations
+
+import faulthandler
+import os
+from collections.abc import Generator
+from pathlib import Path
+from typing import TextIO
+
+import pytest
+
+_SECONDS_ENV = "MATTELOOP_TEST_HANG_DUMP_SECONDS"
+_FILE_ENV = "MATTELOOP_TEST_HANG_DUMP_FILE"
+_DEFAULT_FILE = "tests-ui-hang-dump.log"
+
+_dump_file: TextIO | None = None
+
+
+def _configured_seconds() -> float | None:
+    raw = os.environ.get(_SECONDS_ENV, "").strip()
+    return float(raw) if raw else None
+
+
+def pytest_configure(config: pytest.Config) -> None:
+    global _dump_file
+    if _configured_seconds() is None:
+        return
+    dump_path = Path(os.environ.get(_FILE_ENV, _DEFAULT_FILE))
+    _dump_file = dump_path.open("a", buffering=1, encoding="utf-8")
+
+
+def pytest_unconfigure(config: pytest.Config) -> None:
+    global _dump_file
+    if _dump_file is not None:
+        _dump_file.close()
+        _dump_file = None
+
+
+@pytest.hookimpl(wrapper=True, trylast=True)
+def pytest_runtest_protocol(item: pytest.Item) -> Generator[None, object, object]:
+    seconds = _configured_seconds()
+    if seconds is None or _dump_file is None:
+        return (yield)
+    _dump_file.write(f"\n--- armed for {item.nodeid} ({seconds}s) ---\n")
+    faulthandler.dump_traceback_later(seconds, file=_dump_file, exit=False)
+    try:
+        return (yield)
+    finally:
+        faulthandler.cancel_dump_traceback_later()