api docs: Load the OpenAPI file only when needed.

We found out in #9953 that, appparently, loading the OpenAPI file was
taking abut a 5% of the Zulip server startup time.

Since in many cases (especially in development) having the file loaded
won't be necessary at all, we read it on the first time data from the
OpenAPI spec is needed.

Tweaked by tabbott to add a test.

zerver/lib/openapi.py

@@ -23,7 +23,7 @@ EXCLUDE_PROPERTIES = {
 class OpenAPISpec():
     def __init__(self, path: str) -> None:
         self.path = path
-        self.reload()
+        self.last_update = None  # type: Optional[float]
 
     def reload(self) -> None:
         self.last_update = os.path.getmtime(self.path)

zerver/tests/test_openapi.py

@@ -1,5 +1,6 @@
 # -*- coding: utf-8 -*-
 
+import mock
 from typing import Dict, Any
 
 import zerver.lib.openapi as openapi
@@ -132,3 +133,8 @@ class OpenAPIToolsTest(ZulipTestCase):
         # Check that the file has been reloaded by verifying that the last
         # update date isn't zero anymore
         self.assertNotEqual(openapi_spec.last_update, 0)
+
+        # Now verify calling it again doesn't call reload
+        with mock.patch('zerver.lib.openapi.openapi_spec.reload') as mock_reload:
+            get_openapi_fixture(TEST_ENDPOINT, TEST_METHOD)
+            self.assertFalse(mock_reload.called)