Use WAL with SQLite cache, fix close

mypy/main.py

@@ -189,14 +189,20 @@ def main(
     list([res])  # noqa: C410
 
 
+class BuildResultThunk:
+    # We pass this around so that we avoid freeing memory, which is slow
+    def __init__(self, build_result: build.BuildResult | None) -> None:
+        self._result = build_result
+
+
 def run_build(
     sources: list[BuildSource],
     options: Options,
     fscache: FileSystemCache,
     t0: float,
     stdout: TextIO,
     stderr: TextIO,
-) -> tuple[build.BuildResult | None, list[str], bool]:
+) -> tuple[BuildResultThunk | None, list[str], bool]:
     formatter = util.FancyFormatter(
         stdout, stderr, options.hide_error_codes, hide_success=bool(options.output)
     )
@@ -227,8 +233,12 @@ def flush_errors(filename: str | None, new_messages: list[str], serious: bool) -
         blockers = True
         if not e.use_stdout:
             serious = True
+
+    if res:
+        res.manager.metastore.close()
+
     maybe_write_junit_xml(time.time() - t0, serious, messages, messages_by_file, options)
-    return res, messages, blockers
+    return BuildResultThunk(res), messages, blockers
 
 
 def show_messages(

mypy/metastore.py

@@ -163,6 +163,14 @@ def connect_db(db_file: str, sync_off: bool = False) -> sqlite3.Connection:
         # but without this flag, commits are *very* slow, especially when using HDDs,
         # see https://www.sqlite.org/faq.html#q19 for details.
         db.execute("PRAGMA synchronous=OFF")
+
+    mode = db.execute("PRAGMA journal_mode").fetchone()[0]
+    if mode.lower() != "wal":
+        try:
+            db.execute("PRAGMA journal_mode=WAL")
+        except sqlite3.OperationalError:
+            pass
+
     db.executescript(SCHEMA)
     return db
 
@@ -172,8 +180,8 @@ def __init__(self, cache_dir_prefix: str, sync_off: bool = False) -> None:
         # We check startswith instead of equality because the version
         # will have already been appended by the time the cache dir is
         # passed here.
+        self.db = None
         if cache_dir_prefix.startswith(os.devnull):
-            self.db = None
             return
 
         os.makedirs(cache_dir_prefix, exist_ok=True)