fix(pull): recover PRs whose recorded merge commit never reached the base repo (#39831) (#39840)

Backport #39831 by @lunny

When pushing the merge commit to the base repo fails (e.g.
non-fast-forward), the merge intent recorded by
https://github.com/go-gitea/gitea/pull/39442 is left behind.
`merged_commit_id` then points to a commit that does not exist in the
base repo, and `git branch --contains` fails with `no such commit`, so
the PR can never be checked or merged again.

- Clear the recorded merge intent if the push fails and the commit is
not on the base branch
- `IsCommitInBranch` now returns false for a missing commit, so
already-stuck PRs recover on the next check

Fixes: https://github.com/go-gitea/gitea/issues/39824
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>

services/pull/merge.go

@@ -279,6 +279,18 @@ func recordMergeIntent(ctx context.Context, pr *issues_model.PullRequest, doer *
 	})
 }
 
+// clearMergeIntent reverts recordMergeIntent after the merged commit failed to reach the base repo
+func clearMergeIntent(ctx context.Context, pr *issues_model.PullRequest) error {
+	return db.WithTx(ctx, func(ctx context.Context) error {
+		pr.MergedUnix, pr.MergedCommitID, pr.MergerID = 0, "", 0
+		if _, err := db.GetEngine(ctx).NoAutoTime().ID(pr.ID).Cols("merged_unix", "merged_commit_id", "merger_id").Update(pr); err != nil {
+			return err
+		}
+		_, err := db.GetEngine(ctx).Where("pull_id=?", pr.ID).Cols("merged_commit_id").Update(&pull_model.AutoMerge{})
+		return err
+	})
+}
+
 func hasPullRequestCommitBeenMerged(ctx context.Context, pr *issues_model.PullRequest) (bool, error) {
 	if pr.MergedCommitID == "" {
 		return false, nil
@@ -286,6 +298,7 @@ func hasPullRequestCommitBeenMerged(ctx context.Context, pr *issues_model.PullRe
 	if err := pr.LoadBaseRepo(ctx); err != nil {
 		return false, err
 	}
+	// the recorded commit is missing from the base repo if the push never succeeded, then it is not merged
 	return git.IsCommitInBranch(ctx, pr.BaseRepo, pr.MergedCommitID, pr.BaseBranch)
 }
 
@@ -340,6 +353,12 @@ func Merge(outerCtx context.Context, prID int64, doer *user_model.User, mergeSty
 				},
 			})
 			if err != nil {
+				// clear the intent only if the commit verifiably didn't reach the base branch
+				if merged, checkErr := hasPullRequestCommitBeenMerged(ctx, pr); checkErr == nil && !merged && pr.MergedCommitID != "" {
+					if clearErr := clearMergeIntent(ctx, pr); clearErr != nil {
+						log.Error("Unable to clear merge intent of %-v: %v", pr, clearErr)
+					}
+				}
 				return err
 			}
 		}