Gate overlap test fetches on a barrier instead of a 50ms window

internal/virtual/engine_test.go

@@ -492,9 +492,10 @@ func TestFetchFilesErrorCancelsRest(t *testing.T) {
 	}
 }
 
-// overlapEngine serves fixed member index paths, holding each index file fetch
-// until all are in flight or a short wait expires, and records the peak.
-func overlapEngine(index map[string][]byte, indexFiles int, peak *atomic.Int32) *Engine {
+// overlapEngine serves fixed member index paths and records the peak number of
+// index file fetches in flight. Each fetch blocks until want are in flight (or
+// fails after 5s); with want 1 it also holds briefly so a stray overlap shows.
+func overlapEngine(index map[string][]byte, want int32, peak *atomic.Int32) *Engine {
 	e := fakeEngine(nil)
 	var inFlight atomic.Int32
 	e.memberPath = func(_ context.Context, _ models.Remote, path string) ([]byte, error) {
@@ -509,9 +510,15 @@ func overlapEngine(index map[string][]byte, indexFiles int, peak *atomic.Int32)
 		defer inFlight.Add(-1)
 		for p := peak.Load(); n > p && !peak.CompareAndSwap(p, n); p = peak.Load() {
 		}
-		for deadline := time.Now().Add(50 * time.Millisecond); peak.Load() < int32(indexFiles) && time.Now().Before(deadline); {
+		for deadline := time.Now().Add(5 * time.Second); peak.Load() < want; {
+			if time.Now().After(deadline) {
+				return nil, fmt.Errorf("%s: only %d of %d fetches in flight", path, peak.Load(), want)
+			}
 			time.Sleep(time.Millisecond)
 		}
+		if want == 1 {
+			time.Sleep(10 * time.Millisecond)
+		}
 		return body, nil
 	}
 	return e
@@ -527,7 +534,7 @@ func TestFetchRPMMemberHonoursParallelFetch(t *testing.T) {
 	}
 	for parallel, want := range map[bool]int32{false: 1, true: 3} {
 		var peak atomic.Int32
-		m, err := overlapEngine(index, 3, &peak).fetchRPMMember(context.Background(), "a", parallel)
+		m, err := overlapEngine(index, want, &peak).fetchRPMMember(context.Background(), "a", parallel)
 		if err != nil {
 			t.Fatal(err)
 		}
@@ -552,7 +559,7 @@ func TestFetchDebMemberHonoursParallelFetch(t *testing.T) {
 	}
 	for parallel, want := range map[bool]int32{false: 1, true: 3} {
 		var peak atomic.Int32
-		m, err := overlapEngine(index, 3, &peak).fetchDebMember(context.Background(), "a", "dists/s/", parallel)
+		m, err := overlapEngine(index, want, &peak).fetchDebMember(context.Background(), "a", "dists/s/", parallel)
 		if err != nil {
 			t.Fatal(err)
 		}