Check if Github detected a merge conflict for a PR

Github uses libgit2 to compute merges without requiring a worktree [0].
In some cases this can lead to Github detecting a merge conflict while
for Zuul the PR merges fine.

To prevent such changes from entering dependent pipelines and e.g. cause
a gate reset, we'll also check if Github detected any merge conflicts.

[0] https://github.blog/2022-10-03-highlights-from-git-2-38/

Change-Id: I22275f24c903a8548bb0ef6c32a2e15ba9eadac8

8 files changed, 37 insertions, 4 deletions

diff --git a/tests/base.py b/tests/base.py
index 897e443d3..239b243a5 100644
--- a/tests/base.py
+++ b/tests/base.py
@@ -2340,7 +2340,7 @@ class FakeGithubPullRequest(object):
     def __init__(self, github, number, project, branch,
                  subject, upstream_root, files=None, number_of_commits=1,
                  writers=[], body=None, body_text=None, draft=False,
-                 base_sha=None):
+                 mergeable=True, base_sha=None):
         """Creates a new PR with several commits.
         Sends an event about opened PR.
 
@@ -2356,6 +2356,7 @@ class FakeGithubPullRequest(object):
         self.body = body
         self.body_text = body_text
         self.draft = draft
+        self.mergeable = mergeable
         self.number_of_commits = 0
         self.upstream_root = upstream_root
         # Dictionary of FakeFile -> content
@@ -2911,12 +2912,12 @@ class FakeGithubConnection(githubconnection.GithubConnection):
 
     def openFakePullRequest(self, project, branch, subject, files=[],
                             body=None, body_text=None, draft=False,
-                            base_sha=None):
+                            mergeable=True, base_sha=None):
         self.pr_number += 1
         pull_request = FakeGithubPullRequest(
             self, self.pr_number, project, branch, subject, self.upstream_root,
             files=files, body=body, body_text=body_text, draft=draft,
-            base_sha=base_sha)
+            mergeable=mergeable, base_sha=base_sha)
         self.pull_requests[self.pr_number] = pull_request
         return pull_request
 
diff --git a/tests/fake_graphql.py b/tests/fake_graphql.py
index 6c7567910..9a3f07060 100644
--- a/tests/fake_graphql.py
+++ b/tests/fake_graphql.py
@@ -181,10 +181,14 @@ class FakeCommit(ObjectType):
 class FakePullRequest(ObjectType):
     isDraft = Boolean()
     reviewDecision = String()
+    mergeable = String()
 
     def resolve_isDraft(parent, info):
         return parent.draft
 
+    def resolve_mergeable(parent, info):
+        return "MERGEABLE" if parent.mergeable else "CONFLICTING"
+
     def resolve_reviewDecision(parent, info):
         if hasattr(info.context, 'version') and info.context.version:
             if info.context.version < (2, 21, 0):
diff --git a/tests/fakegithub.py b/tests/fakegithub.py
index 47cefd08d..4255d5022 100644
--- a/tests/fakegithub.py
+++ b/tests/fakegithub.py
@@ -547,7 +547,7 @@ class FakePull(object):
                 'login': 'octocat'
             },
             'draft': pr.draft,
-            'mergeable': True,
+            'mergeable': pr.mergeable,
             'state': pr.state,
             'head': {
                 'sha': pr.head_sha,
diff --git a/tests/unit/test_github_driver.py b/tests/unit/test_github_driver.py
index 90d69a7bc..47cc6c624 100644
--- a/tests/unit/test_github_driver.py
+++ b/tests/unit/test_github_driver.py
@@ -785,6 +785,18 @@ class TestGithubDriver(ZuulTestCase):
         self.assertFalse(A.is_merged)
         self.assertHistory([])
 
+    @simple_layout('layouts/dependent-github.yaml', driver='github')
+    def test_non_mergeable_pr(self):
+        # pipeline merges the pull request on success
+        A = self.fake_github.openFakePullRequest('org/project', 'master',
+                                                 'PR title', mergeable=False)
+        self.fake_github.emitEvent(A.addLabel('merge'))
+        self.waitUntilSettled()
+
+        # A non-mergeable pull request must not enter gate
+        self.assertFalse(A.is_merged)
+        self.assertHistory([])
+
     @simple_layout('layouts/reporting-multiple-github.yaml', driver='github')
     def test_reporting_multiple_github(self):
         project = 'org/project1'
diff --git a/zuul/driver/github/githubconnection.py b/zuul/driver/github/githubconnection.py
index 1e3af1e78..15a41a3b8 100644
--- a/zuul/driver/github/githubconnection.py
+++ b/zuul/driver/github/githubconnection.py
@@ -1692,6 +1692,8 @@ class GithubConnection(ZKChangeCacheMixin, ZKBranchCacheMixin, BaseConnection):
 
         change.contexts = self._get_contexts(canmerge_data)
         change.draft = canmerge_data.get('isDraft', False)
+        change.mergeable = (canmerge_data.get('mergeable', 'MERGEABLE').lower()
+                            in ('mergeable', 'unknown'))
         change.review_decision = canmerge_data['reviewDecision']
         change.required_contexts = set(
             canmerge_data['requiredStatusCheckContexts']
@@ -1860,6 +1862,11 @@ class GithubConnection(ZKChangeCacheMixin, ZKBranchCacheMixin, BaseConnection):
             log.debug('Change %s can not merge because it is a draft', change)
             return False
 
+        if not change.mergeable:
+            log.debug('Change %s can not merge because Github detected a '
+                      'merge conflict', change)
+            return False
+
         missing_status_checks = self._getMissingStatusChecks(
             change, allow_needs)
         if missing_status_checks:
diff --git a/zuul/driver/github/githubmodel.py b/zuul/driver/github/githubmodel.py
index 256333234..30610cf4e 100644
--- a/zuul/driver/github/githubmodel.py
+++ b/zuul/driver/github/githubmodel.py
@@ -38,6 +38,7 @@ class PullRequest(Change):
         self.files = []
         self.labels = []
         self.draft = None
+        self.mergeable = True
         self.review_decision = None
         self.required_contexts = set()
         self.contexts = set()
@@ -74,6 +75,7 @@ class PullRequest(Change):
             "reviews": list(self.reviews),
             "labels": self.labels,
             "draft": self.draft,
+            "mergeable": self.mergeable,
             "review_decision": self.review_decision,
             "required_contexts": list(self.required_contexts),
             "contexts": list(self.contexts),
@@ -90,6 +92,7 @@ class PullRequest(Change):
         self.reviews = data.get("reviews", [])
         self.labels = data.get("labels", [])
         self.draft = data.get("draft")
+        self.mergeable = data.get("mergeable", True)
         self.review_decision = data.get("review_decision")
         self.required_contexts = set(data.get("required_contexts", []))
         self.contexts = set(tuple(c) for c in data.get("contexts", []))
diff --git a/zuul/driver/github/graphql/__init__.py b/zuul/driver/github/graphql/__init__.py
index f0d17f5f0..babf780ee 100644
--- a/zuul/driver/github/graphql/__init__.py
+++ b/zuul/driver/github/graphql/__init__.py
@@ -116,6 +116,11 @@ class GraphQLClient:
         pull_request = nested_get(repository, 'pullRequest')
         result['isDraft'] = nested_get(pull_request, 'isDraft', default=False)
 
+        # Check if Github detected a merge conflict. Possible enum values
+        # are CONFLICTING, MERGEABLE and UNKNOWN.
+        result['mergeable'] = nested_get(pull_request, 'mergeable',
+                                         default='MERGEABLE')
+
         # Get review decision. This is supported since GHE 2.21. Default to
         # None to signal if the field is not present.
         result['reviewDecision'] = nested_get(
diff --git a/zuul/driver/github/graphql/canmerge.graphql b/zuul/driver/github/graphql/canmerge.graphql
index 7b7e862fd..fdd72c083 100644
--- a/zuul/driver/github/graphql/canmerge.graphql
+++ b/zuul/driver/github/graphql/canmerge.graphql
@@ -20,6 +20,7 @@ query canMergeData(
     }
     pullRequest(number: $pull) {
       isDraft
+      mergeable
       reviewDecision
     }
     object(expression: $head_sha) {