FD-leak regression test for Datasette.close()

Creates and disposes 50 Datasette instances in a loop and asserts that
the number of open file descriptors and live threads does not grow,
exercising the full close() path end to end.

Refs #2692

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

pyproject.toml

@@ -80,6 +80,7 @@ dev = [
     "myst-parser",
     "sphinx-markdown-builder",
     "ruamel.yaml",
+    "psutil>=5.9",
 ]
 
 [project.optional-dependencies]

tests/test_fd_leak.py

@@ -0,0 +1,56 @@
+"""
+Regression test for https://github.com/simonw/datasette/issues/2692 —
+confirm that creating and closing Datasette instances in a loop does not
+leak open file descriptors.
+
+Each Datasette() with is_temp_disk internal DB opens a temp file and a
+write thread with its own SQLite connection. Without Datasette.close()
+nothing unwinds this state, and a large pytest run exhausts the process
+FD limit.
+"""
+
+import asyncio
+import threading
+
+import pytest
+
+try:
+    import psutil
+except ImportError:  # pragma: no cover
+    psutil = None
+
+from datasette.app import Datasette
+
+
+def _count_open_files():
+    return len(psutil.Process().open_files())
+
+
+def _count_threads():
+    return threading.active_count()
+
+
+@pytest.mark.skipif(psutil is None, reason="psutil not installed")
+def test_close_releases_file_descriptors():
+    # Warm-up so Python/library caches don't skew the baseline
+    ds = Datasette(memory=True)
+    asyncio.run(ds.invoke_startup())
+    ds.close()
+
+    baseline_fds = _count_open_files()
+    baseline_threads = _count_threads()
+
+    for _ in range(50):
+        ds = Datasette(memory=True)
+        asyncio.run(ds.invoke_startup())
+        ds.close()
+
+    after_fds = _count_open_files()
+    after_threads = _count_threads()
+
+    assert (
+        after_fds - baseline_fds <= 2
+    ), f"Leaked FDs: baseline={baseline_fds}, after=50 iterations={after_fds}"
+    assert (
+        after_threads - baseline_threads <= 2
+    ), f"Leaked threads: baseline={baseline_threads}, after={after_threads}"