Fix crash when checking file in a read only directory

mypy/metastore.py

@@ -189,7 +189,12 @@ def __init__(
         if cache_dir_prefix.startswith(os.devnull):
             return
 
-        os.makedirs(cache_dir_prefix, exist_ok=True)
+        try:
+            os.makedirs(cache_dir_prefix, exist_ok=True)
+        except OSError:
+            # Prevent failing on read-only filesystem and such.
+            return
+
         if num_shards <= 1:
             self.dbs.append(
                 connect_db(os_path_join(cache_dir_prefix, "cache.db"), set_journal_mode)

mypy/test/testmetastore.py

@@ -0,0 +1,40 @@
+from __future__ import annotations
+
+import os
+import sys
+import tempfile
+import unittest
+
+from mypy.metastore import SqliteMetadataStore
+
+
+@unittest.skipIf(
+    sys.platform == "win32",
+    "POSIX chmod semantics: os.chmod(dir, 0o555) does not prevent writes on Windows",
+)
+class TestSqliteMetadataStore(unittest.TestCase):
+    def test_init_degrades_to_noop_when_cache_dir_not_creatable(self) -> None:
+        with tempfile.TemporaryDirectory() as parent:
+            os.chmod(parent, 0o555)
+
+            cache_dir = os.path.join(parent, "mypy_cache")
+
+            # Must not raise.
+            store = SqliteMetadataStore(cache_dir)
+
+            # Degraded to no-op state, matching the os.devnull short-circuit
+            # and FilesystemMetadataStore's behavior on read-only filesystems.
+            self.assertEqual(store.dbs, [])
+            self.assertFalse(store.write("foo.meta.json", b"{}"))
+            with self.assertRaises(FileNotFoundError):
+                store.read("foo.meta.json")
+            with self.assertRaises(FileNotFoundError):
+                store.getmtime("foo.meta.json")
+            self.assertEqual(list(store.list_all()), [])
+            # commit/close must be safe on an empty store
+            store.commit()
+            store.close()
+
+
+if __name__ == "__main__":
+    unittest.main()