[gerrit_util] Cache resolved Authenticator as a class variable.

I noticed that the Authenticator is resolved maybe 5 or 6 times per
git-cl invocation. This should lead to more consistent behavior and
will likely be a bit faster, especially for SSOAuthenticator and
LuciAuthAuthenticator which involve subprocess invocations.

R=ayatane@chromium.org

Bug: 336351842
Change-Id: Id6c2873a6960a171305560acb98afe2c4f397295
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5589865
Auto-Submit: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Yiwei Zhang <yiwzhang@google.com>
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>

gerrit_util.py

@@ -174,6 +174,10 @@ ssoHelper = SSOHelper()
 class Authenticator(object):
     """Base authenticator class for authenticator implementations to subclass."""
 
+    # Cached Authenticator subclass instance, resolved via get().
+    _resolved: Optional[Authenticator] = None
+    _resolved_lock = threading.Lock()
+
     def authenticate(self, conn: HttpConn):
         """Adds authentication information to the HttpConn."""
         raise NotImplementedError()
@@ -193,20 +197,24 @@ class Authenticator(object):
         environment."""
         raise NotImplementedError()
 
-    @staticmethod
-    def get():
+    @classmethod
+    def get(cls):
         """Returns: (Authenticator) The identified Authenticator to use.
 
         Probes the local system and its environment and identifies the
         Authenticator instance to use.
+
+        The resolved Authenticator instance is cached as a class variable.
         """
+        with cls._resolved_lock:
+            if ret := cls._resolved:
+                return ret
+
             use_new_auth = newauth.Enabled()
 
             # Allow skipping SSOAuthenticator for local testing purposes.
             skip_sso = newauth.SkipSSO()
 
-        authenticators: List[Type[Authenticator]]
-
             if use_new_auth:
                 LOGGER.debug('Authenticator.get: using new auth stack.')
                 authenticators = [
@@ -229,11 +237,14 @@ class Authenticator(object):
                 if candidate.is_applicable():
                     LOGGER.debug('Authenticator.get: Selected %s.',
                                  candidate.__name__)
-                return candidate()
+                    ret = candidate()
+                    cls._resolved = ret
+                    return ret
 
             auth_names = ', '.join(a.__name__ for a in authenticators)
             raise ValueError(
-            f"Could not find suitable authenticator, tried: [{auth_names}].")
+                f"Could not find suitable authenticator, tried: [{auth_names}]."
+            )
 
 
 class SSOAuthenticator(Authenticator):

tests/git_cl_test.py

@@ -669,6 +669,7 @@ class TestGitCl(unittest.TestCase):
         # It's important to reset settings to not have inter-tests interference.
         git_cl.settings = git_cl.Settings()
         self.addCleanup(mock.patch.stopall)
+        gerrit_util.Authenticator._resolved = None
 
     def tearDown(self):
         try:
@@ -3623,6 +3624,7 @@ class ChangelistTest(unittest.TestCase):
         self.mockGit = GitMocks()
         mock.patch('scm.GIT.GetConfig', self.mockGit.GetConfig).start()
         mock.patch('scm.GIT.SetConfig', self.mockGit.SetConfig).start()
+        gerrit_util.Authenticator._resolved = None
 
     def testRunHook(self):
         expected_results = {