test(browser): keep the scratch on disk when /dev/shm is short, delete in place after cleanup (#384)

The largest scratch copy is about 8 MB; Docker's default /dev/shm is 64 MB
and shared with the browsers. A reset that empties a folder after the
server's cleanup closed its trash deletes in place instead of starting a
thread nobody stops.

tests/browser/fast_reset.py

@@ -36,6 +36,11 @@ SEED_ON_LIVE_ENV = "PM_TEST_SEED_ON_LIVE"
 SCRATCH_SCHEMA = "reset_seed"
 # Linux's shared-memory tmpfs (present in the Playwright image and on runners).
 MEMORY_ROOT = Path("/dev/shm")
+# The largest fixtures' scratch copy is about 8 MB (database plus a WAL that
+# autocheckpoints near 4 MB). Docker's default /dev/shm is 64 MB, shared with
+# the browsers and the other workers; with less free than this, the scratch
+# copy goes on disk instead.
+MEMORY_FREE_BYTES = 32 * 1024 * 1024
 
 
 def _quote(name: str) -> str:
@@ -225,7 +230,12 @@ class ScratchSeeder:
         kernel is writing back the pages they rewrite.
         """
         root = self._memory_root
-        if root is not None and root.is_dir() and os.access(root, os.W_OK):
+        if (
+            root is not None
+            and root.is_dir()
+            and os.access(root, os.W_OK)
+            and shutil.disk_usage(root).free >= MEMORY_FREE_BYTES
+        ):
             return Path(tempfile.mkdtemp(prefix="pm-reset-", dir=root))
         self._fallback.mkdir(parents=True, exist_ok=True)
         return self._fallback

tests/browser/folder_trash.py

@@ -46,6 +46,9 @@ class _Trash:
 
 
 _trashes: dict[Path, _Trash] = {}
+# Trash folders whose server has cleaned up: a reset still running then
+# deletes in place instead of starting a thread nobody would stop.
+_closed: set[Path] = set()
 _lock = threading.Lock()
 
 
@@ -56,15 +59,20 @@ def replace_folder(folder: Path) -> None:
         directory = folder.parent / TRASH_NAME
         with _lock:
             trash = _trashes.get(directory)
-            if trash is None:
+            if trash is None and directory not in _closed:
                 trash = _trashes[directory] = _Trash(directory)
-            trash.discard(folder)
+            if trash is not None:
+                trash.discard(folder)
+        if folder.exists():
+            shutil.rmtree(folder)
     folder.mkdir()
 
 
 def close_trash(parent: Path) -> None:
     """Stop deleting under ``parent``'s trash and remove it (blocking)."""
+    directory = Path(parent) / TRASH_NAME
     with _lock:
-        trash = _trashes.pop(Path(parent) / TRASH_NAME, None)
+        _closed.add(directory)
+        trash = _trashes.pop(directory, None)
     if trash is not None:
         trash.close()

tests/test_browser_fast_reset.py

@@ -255,6 +255,21 @@ class TestScratchSeeder(unittest.TestCase):
         seeder.close()
         self.assertEqual(list(memory.iterdir()), [])
 
+    def test_a_full_memory_root_puts_the_scratch_on_disk(self):
+        memory = self.root / "memory"
+        memory.mkdir()
+        seeder = ScratchSeeder(self.api, self.root / "fallback", memory_root=memory)
+        self.addCleanup(seeder.close)
+        full = mock.patch.object(
+            fast_reset.shutil,
+            "disk_usage",
+            return_value=types.SimpleNamespace(free=fast_reset.MEMORY_FREE_BYTES - 1),
+        )
+        with full:
+            seeder.run(reseed)
+        self.assertEqual(list(memory.iterdir()), [])
+        self.assertTrue((self.root / "fallback" / "seed.db").exists())
+
     def test_close_releases_the_scratch_files(self):
         self.seeder.run(reseed)
         self.seeder.close()

tests/test_browser_folder_trash.py

@@ -48,5 +48,20 @@ class TestFolderTrash(unittest.TestCase):
         close_trash(self.root)  # idempotent
 
 
+class TestFolderTrashAfterClose(unittest.TestCase):
+    def test_a_replacement_after_close_deletes_in_place_without_a_thread(self):
+        with tempfile.TemporaryDirectory() as tmp:
+            root = Path(tmp)
+            close_trash(root)
+            folder = root / "output"
+            folder.mkdir()
+            (folder / "late.png").write_bytes(b"png")
+            replace_folder(folder)
+            self.assertEqual(list(folder.iterdir()), [])
+            self.assertFalse((root / TRASH_NAME).exists())
+            names = [t.name for t in threading.enumerate()]
+            self.assertNotIn("pm-reset-trash", names)
+
+
 if __name__ == "__main__":
     unittest.main()