Add test

storage/gcp/gcp_test.go

@@ -63,7 +63,7 @@ func TestSpannerSequencerAssignEntries(t *testing.T) {
 	close := newSpannerDB(t)
 	defer close()
 
-	seq, err := newSpannerSequencer(ctx, "projects/p/instances/i/databases/d")
+	seq, err := newSpannerSequencer(ctx, "projects/p/instances/i/databases/d", 1000)
 	if err != nil {
 		t.Fatalf("newSpannerSequencer: %v", err)
 	}
@@ -86,12 +86,67 @@ func TestSpannerSequencerAssignEntries(t *testing.T) {
 	}
 }
 
+func TestSpannerSequencerPushback(t *testing.T) {
+	ctx := context.Background()
+
+	for _, test := range []struct {
+		name           string
+		threshold      uint64
+		initialEntries int
+		wantPushback   bool
+	}{
+		{
+			name:           "no pushback: num < threshold",
+			threshold:      10,
+			initialEntries: 5,
+		},
+		{
+			name:           "no pushback: num = threshold",
+			threshold:      10,
+			initialEntries: 10,
+		},
+		{
+			name:           "pushback: initial > threshold",
+			threshold:      10,
+			initialEntries: 15,
+			wantPushback:   true,
+		},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			close := newSpannerDB(t)
+			defer close()
+
+			seq, err := newSpannerSequencer(ctx, "projects/p/instances/i/databases/d", test.threshold)
+			if err != nil {
+				t.Fatalf("newSpannerSequencer: %v", err)
+			}
+			// Set up the test scenario with the configured number of initial outstanding entries
+			entries := []*tessera.Entry{}
+			for i := 0; i < test.initialEntries; i++ {
+				entries = append(entries, tessera.NewEntry([]byte(fmt.Sprintf("initial item %d", i))))
+			}
+			if err := seq.assignEntries(ctx, entries); err != nil {
+				t.Fatalf("initial assignEntries: %v", err)
+			}
+
+			// Now perform the test with a single additional entry to check for pushback
+			entries = []*tessera.Entry{tessera.NewEntry([]byte("additional"))}
+			err = seq.assignEntries(ctx, entries)
+			if gotPushback := errors.Is(err, tessera.ErrPushback); gotPushback != test.wantPushback {
+				t.Fatalf("assignEntries: got pushback %t (%v), want pushback: %t", gotPushback, err, test.wantPushback)
+			} else if !gotPushback && err != nil {
+				t.Fatalf("assignEntries: %v", err)
+			}
+		})
+	}
+}
+
 func TestSpannerSequencerRoundTrip(t *testing.T) {
 	ctx := context.Background()
 	close := newSpannerDB(t)
 	defer close()
 
-	s, err := newSpannerSequencer(ctx, "projects/p/instances/i/databases/d")
+	s, err := newSpannerSequencer(ctx, "projects/p/instances/i/databases/d", 1000)
 	if err != nil {
 		t.Fatalf("newSpannerSequencer: %v", err)
 	}