A check ask may ask the merge alone, so the checker can name every group member that does not merge in one verdict

A group's check named only its first member in conflict; each hidden one cost a check round (issue 1921's kind; no open issue covers it).

go/buildstep/askrun.go

@@ -170,8 +170,8 @@ func (s AskSteps) check(ctx context.Context, job delivery.CheckJob) (any, error)
 	}
 	defer s.release(f.Workspace)
 	answer := delivery.CheckAnswer{Fetch: r}
-	if len(f.Conflicts) > 0 {
-		return answer, nil // the asker reads the conflict from the fetch's answer: a verdict, not an error
+	if len(f.Conflicts) > 0 || job.MergeOnly {
+		return answer, nil // the asker reads the merge or the conflict from the fetch's answer: a verdict, not an error
 	}
 	if s.Test == nil {
 		return nil, Refuse("this build module tests nothing")

go/buildstep/askrun_test.go

@@ -235,7 +235,7 @@ test {"level":"unit","runner":"workspace","workspace":"/w"}`},
 }
 
 // A Node check tests each module directory, the one whose tests changed first; a conflict ends the ask with the fetch
-// alone, a verdict for the asker.
+// alone, a verdict for the asker; so does a merge-only ask of a head that merges.
 func TestRunAskChecksNodeModulesAndConflicts(t *testing.T) {
 	r := &recorder{fetch: map[string]any{"workspace": "/w", "modules": []string{"a", "b"}, "changed": []string{"b/test/x.test.ts"}},
 		tests: map[string]bool{"a": true, "b": true}}
@@ -252,6 +252,12 @@ func TestRunAskChecksNodeModulesAndConflicts(t *testing.T) {
 	if err != nil || len(r.calls) != 1 || len(out.(delivery.CheckAnswer).Levels) != 0 {
 		t.Fatalf("a conflict: %v %v", err, r.calls)
 	}
+	// A merge-only ask answers the fetch of a head that merges and runs no level.
+	r = &recorder{fetch: map[string]any{"workspace": "/w", "merge": "m1"}, tests: map[string]bool{"a": true}}
+	out, err = RunAsk(context.Background(), r.steps(), ask(delivery.CheckJob{Levels: []string{delivery.LevelUnit}, MergeOnly: true, Fetch: map[string]any{"repository": "novox/x", "commit": runCommit}}))
+	if err != nil || len(r.calls) != 1 || len(out.(delivery.CheckAnswer).Levels) != 0 || !strings.Contains(string(out.(delivery.CheckAnswer).Fetch), `"m1"`) {
+		t.Fatalf("merge only: %v %v %v", err, r.calls, out)
+	}
 }
 
 // Issue 1921: a failure in the packages whose tests changed does not stop the level: the other packages still run, so two

go/delivery/stepasks.go

@@ -345,6 +345,11 @@ type CheckJob struct {
 	// has to list them. The build step hands them to its test tool as the argument NodeNamesArg, and the build module
 	// gives them to the tests in NodeNamesEnv.
 	NodeNames []string `json:"node-names,omitempty"`
+	// MergeOnly says the build module answers the fetch alone, its merge or the files in conflict, and runs no level: the
+	// checker asks it of a delivery group's members after one member's check failed, so one verdict names every member
+	// that does not merge into its trunk instead of the first. Levels stay named, so a build module from before it (which
+	// ignores it) still takes the ask; the checker reads only the fetch of such an answer.
+	MergeOnly bool `json:"merge-only,omitempty"`
 }
 
 // NodeNamesArg is the test tool's argument that carries a check's node names, and NodeNamesEnv the environment