returning the error when doing a nack in the message

protocol/pubsub/v2/message.go

@@ -120,13 +120,14 @@ func (m *Message) GetExtension(name string) interface{} {
 }
 
 // Finish marks the message to be forgotten.
-// If err is nil, the underlying Psubsub message will be acked;
-// otherwise nacked.
+// If err is nil, the underlying Pubsub message will be acked;
+// otherwise nacked and return the error.
 func (m *Message) Finish(err error) error {
 	if err != nil {
 		m.internal.Nack()
-	} else {
-		m.internal.Ack()
+		return err
 	}
+
+	m.internal.Ack()
 	return nil
 }

protocol/pubsub/v2/message_test.go

@@ -7,6 +7,7 @@ package pubsub
 
 import (
 	"context"
+	"fmt"
 	"testing"
 
 	"cloud.google.com/go/pubsub"
@@ -45,3 +46,42 @@ func TestReadStructured(t *testing.T) {
 		})
 	}
 }
+
+func TestFinish(t *testing.T) {
+	tests := []struct {
+		name    string
+		pm      *pubsub.Message
+		err     error
+		wantErr bool
+	}{
+		{
+			name: "return error",
+			pm: &pubsub.Message{
+				ID: "testid",
+			},
+			err:     fmt.Errorf("error"),
+			wantErr: true,
+		},
+		{
+			name: "no errors",
+			pm: &pubsub.Message{
+				ID: "testid",
+			},
+			wantErr: false,
+		},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			msg := NewMessage(tc.pm)
+			err := msg.Finish(tc.err)
+			if tc.wantErr {
+				if err != tc.err {
+					t.Errorf("Error mismatch. got: %v, want: %v", err, tc.err)
+				}
+			}
+			if !tc.wantErr && err != nil {
+				t.Errorf("Should not error but got: %v", err)
+			}
+		})
+	}
+}