tide: fallback to individual PR merge in batch failures

Author: Prucek

pkg/tide/github.go

@@ -287,8 +287,9 @@ func (gi *GitHubProvider) mergePRs(sp subpool, prs []CodeReviewCommon, dontUpdat
 			return gi.ghc.Merge(sp.org, sp.repo, pr.Number, ghMergeDetails)
 		})
 		if err != nil {
-			// These are user errors, shouldn't be printed as tide errors
 			log.WithError(err).Debug("Merge failed.")
+			failed = append(failed, pr.Number)
+			errs = append(errs, err)
 		} else {
 			log.Info("Merged.")
 			merged = append(merged, pr)

pkg/tide/tide.go

@@ -1439,7 +1439,15 @@ func (c *syncController) takeAction(sp subpool, batchPending, successes, pending
 	// Merge the batch!
 	if len(batchMerges) > 0 {
 		merged, err = c.provider.mergePRs(sp, batchMerges, c.statusUpdate.dontUpdateStatus)
-		return MergeBatch, batchMerges, err
+		// If the entire batch failed to merge (e.g., due to conflicts), fall through
+		// to attempt merging individual PRs to break the tie.
+		if len(merged) > 0 || err == nil {
+			return MergeBatch, batchMerges, err
+		}
+		if ok, pr := pickHighestPriorityPR(sp.log, batchMerges, sp.cc, c.isPassingTests, c.config().Tide.Priority); ok {
+			merged, err = c.provider.mergePRs(sp, []CodeReviewCommon{pr}, c.statusUpdate.dontUpdateStatus)
+			return Merge, []CodeReviewCommon{pr}, err
+		}
 	}
 	// Do not merge PRs while waiting for a batch to complete. We don't want to
 	// invalidate the old batch result.

pkg/tide/tide_test.go

@@ -1959,6 +1959,17 @@ func testTakeAction(clients localgit.Clients, t *testing.T) {
 			action:           Trigger,
 			enableScheduling: true,
 		},
+		{
+			name:        "batch merge fails, falls back to individual PR merge (issue #474)",
+			batchMerges: []int{0, 1},
+			mergeErrs: map[int]error{
+				0: github.UnmergablePRError("merge conflict"),
+				1: github.UnmergablePRError("merge conflict"),
+			},
+			merged:    0,
+			triggered: 0,
+			action:    Merge,
+		},
 	}
 
 	for _, tc := range testcases {