[gerrit_util] Add dogfoodable luci-auth Authenticator (reland)

Inspired by https://chromium-review.googlesource.com/c/5577744.

This implementation allows toggling the entire new-auth-stack with
the git configuration:

    [depot-tools]
        useNewAuthStack = 1

Additionally, you can set:

    [depot-tools]
        newAuthSkipSSO = 1

To intentionally skip SSOAuthenticator for now while doing local
evaluation of these auth methods.

This CL was uploaded without gitcookies using the new luci-auth
Authenticator.

Subsequent CLs will adjust creds-check and EnsureAuthenticated to
work correctly with the new auth stack.

Originally landed in
https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5590324

Reverted because of the preceding CL
https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5586799

Bug: 336351842, 336652327
Change-Id: I72df6689ec1ea5429fecc86e5752366fcda7465a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5627835
Reviewed-by: Scott Lee <ddoman@chromium.org>
Reviewed-by: Yiwei Zhang <yiwzhang@google.com>
Commit-Queue: Allen Li <ayatane@chromium.org>

auth.py

@@ -107,7 +107,7 @@ class Authenticator(object):
             return self._access_token
 
         # Nope, still expired. Needs user interaction.
-        logging.error('Failed to create access token')
+        logging.debug('Failed to create access token')
         raise LoginRequiredError(self._scopes)
 
     def get_id_token(self):
@@ -127,7 +127,7 @@ class Authenticator(object):
             return self._id_token
 
         # Nope, still expired. Needs user interaction.
-        logging.error('Failed to create id token')
+        logging.debug('Failed to create id token')
         raise LoginRequiredError()
 
     def authorize(self, http, use_id_token=False):

gerrit_util.py

@@ -177,24 +177,42 @@ class Authenticator(object):
         Probes the local system and its environment and identifies the
         Authenticator instance to use.
         """
-        authenticators: List[Type[Authenticator]] = [
-            SSOAuthenticator,
-
-            # LUCI Context takes priority since it's normally present only on bots,
-            # which then must use it.
-            LuciContextAuthenticator,
-
-            # TODO(crbug.com/1059384): Automatically detect when running on
-            # cloudtop, and use CookiesAuthenticator instead.
-            GceAuthenticator,
-            CookiesAuthenticator,
-        ]
+        use_new_auth = scm.GIT.GetConfig(os.getcwd(),
+                                         'depot-tools.usenewauthstack') == '1'
+
+        # Allow skipping SSOAuthenticator for local testing purposes.
+        skip_sso = scm.GIT.GetConfig(os.getcwd(),
+                                     'depot-tools.newauthskipsso') == '1'
+
+        authenticators: List[Type[Authenticator]]
+
+        if use_new_auth:
+            LOGGER.debug('Authenticator.get: using new auth stack.')
+            authenticators = [
+                SSOAuthenticator,
+                LuciContextAuthenticator,
+                GceAuthenticator,
+                LuciAuthAuthenticator,
+            ]
+            if skip_sso:
+                LOGGER.debug('Authenticator.get: skipping SSOAuthenticator.')
+                authenticators = authenticators[1:]
+        else:
+            authenticators = [
+                LuciContextAuthenticator,
+                GceAuthenticator,
+                CookiesAuthenticator,
+            ]
+
         for candidate in authenticators:
             if candidate.is_applicable():
+                LOGGER.debug('Authenticator.get: Selected %s.',
+                             candidate.__name__)
                 return candidate()
 
+        auth_names = ', '.join(a.__name__ for a in authenticators)
         raise ValueError(
-            f"Could not find suitable authenticator, tried: {authenticators}")
+            f"Could not find suitable authenticator, tried: [{auth_names}].")
 
 
 class SSOAuthenticator(Authenticator):
@@ -234,14 +252,7 @@ class SSOAuthenticator(Authenticator):
     def is_applicable(cls) -> bool:
         """If the git-remote-sso binary is in $PATH, we consider this
         authenticator to be applicable."""
-        if scm.GIT.GetConfig(os.getcwd(), 'depot-tools.usenewauthstack') != '1':
-            LOGGER.debug('SSOAuthenticator: skipping due to missing opt-in.')
-            return False
-
-        pth = cls._resolve_sso_binary_path()
-        if pth:
-            LOGGER.debug('SSOAuthenticator: enabled %r.', pth)
-        return bool(pth)
+        return bool(cls._resolve_sso_binary_path())
 
     @classmethod
     def _parse_config(cls, config: str) -> SSOInfo:
@@ -636,6 +647,18 @@ class LuciContextAuthenticator(Authenticator):
         return ''
 
 
+class LuciAuthAuthenticator(LuciContextAuthenticator):
+    """Authenticator implementation that uses `luci-auth` credentials.
+
+    This is the same as LuciContextAuthenticator, except that it is for local
+    non-google.com developer credentials.
+    """
+
+    @staticmethod
+    def is_applicable():
+        return True
+
+
 class ReqParams(TypedDict):
     uri: str
     method: str