Git backend: Improve heuristic checking if GITLAB_MR_PULL_PATTERN is needed

This commit is an incremental improvement toward having full support
for self-hosted GitLab instance.

It ensures that the expected sources are fetched when a build is triggered
after a merge request event is processed in the context of a GitLab integration
webhook associated with a self-hosted GitLab instance.

Author: jcfr

readthedocs/vcs_support/backends/git.py

@@ -10,6 +10,7 @@
 
 from readthedocs.builds.constants import EXTERNAL
 from readthedocs.config import ALL
+from readthedocs.integrations.models import Integration
 from readthedocs.projects.constants import (
     GITHUB_BRAND,
     GITHUB_PR_PULL_PATTERN,
@@ -150,6 +151,28 @@ def use_shallow_clone(self):
         from readthedocs.projects.models import Feature
         return not self.project.has_feature(Feature.DONT_SHALLOW_CLONE)
 
+    @staticmethod
+    def _is_trigger_integration_gitlab(project):
+        # If there is only one integration associated with the project, we
+        # can unambiguously conclude that the external build was triggered by
+        # a merge request.
+        #
+        # https://github.com/readthedocs/readthedocs.org/issues/9464
+
+        # To avoid error like the following, the value corresponding to
+        # Integration.GITLAB_WEBHOOK is hardcoded.
+        #
+        #   ImportError: cannot import name 'Project' from partially initialized module
+        #   'readthedocs.projects.models' (most likely due to a circular import)
+        _gitlab_webhook = 'gitlab_webhook'  # Integration.GITLAB_WEBHOOK
+
+        if (
+            len(project.integrations) == 1
+            and project.integrations[0].integration_type == _gitlab_webhook
+        ):
+            return True
+        return False
+
     def fetch(self):
         # --force lets us checkout branches that are not fast-forwarded
         # https://github.com/readthedocs/readthedocs.org/issues/6097
@@ -166,10 +189,11 @@ def fetch(self):
                     GITHUB_PR_PULL_PATTERN.format(id=self.verbose_name)
                 )
 
-            if self.project.git_provider_name == GITLAB_BRAND:
-                cmd.append(
-                    GITLAB_MR_PULL_PATTERN.format(id=self.verbose_name)
-                )
+            if (
+                self.project.git_provider_name == GITLAB_BRAND
+                or self._is_trigger_integration_gitlab(self.project)
+            ):
+                cmd.append(GITLAB_MR_PULL_PATTERN.format(id=self.verbose_name))
 
         code, stdout, stderr = self.run(*cmd)
         return code, stdout, stderr