REP-5201 Fix change stream iteration. (#36)

PR #30 added persistence of the change stream’s resume token but mishandled some of the refactoring by making every change stream iteration loop read all events.

This restores the intended logic: check for context cancellation or the end of the change stream after each event.

Author: FGasper

internal/verifier/change_stream.go

@@ -100,45 +100,66 @@ func (verifier *Verifier) iterateChangeStream(ctx context.Context, cs *mongo.Cha
 		return err
 	}
 
+	readOneChangeEvent := func() (bool, error) {
+		gotEvent := cs.TryNext(ctx)
+		if gotEvent {
+			if err := cs.Decode(&changeEvent); err != nil {
+				return false, errors.Wrap(err, "failed to decode change event")
+			}
+			err := verifier.HandleChangeStreamEvent(ctx, &changeEvent)
+			if err != nil {
+				return false, errors.Wrap(err, "failed to handle change event")
+			}
+		}
+
+		return gotEvent, errors.Wrap(cs.Err(), "change stream iteration failed")
+	}
+
 	for {
 		var err error
+		var changeStreamEnded bool
 
-		for cs.TryNext(ctx) {
-			if err = cs.Decode(&changeEvent); err != nil {
-				err = errors.Wrap(err, "failed to decode change event")
-				break
+		select {
+
+		// If the context is canceled, return immmediately.
+		case <-ctx.Done():
+			return
+
+		// If the changeStreamEnderChan has a message, the user has indicated that
+		// source writes are ended. This means we should exit rather than continue
+		// reading the change stream since there should be no more events.
+		case <-verifier.changeStreamEnderChan:
+			var gotEvent bool
+
+			changeStreamEnded = true
+
+			// Read all change events until the source reports no events.
+			// (i.e., the `getMore` call returns empty)
+			for {
+				gotEvent, err = readOneChangeEvent()
+
+				if !gotEvent || err != nil {
+					break
+				}
 			}
-			err = verifier.HandleChangeStreamEvent(ctx, &changeEvent)
+
 			if err != nil {
-				err = errors.Wrap(err, "failed to handle change event")
 				break
 			}
-		}
 
-		if cs.Err() != nil {
-			err = errors.Wrap(
-				cs.Err(),
-				"change stream iteration failed",
-			)
+		default:
+			_, err = readOneChangeEvent()
 		}
 
 		if err == nil {
 			err = persistResumeTokenIfNeeded()
 		}
 
-		if err != nil {
-			if !errors.Is(err, context.Canceled) {
-				verifier.changeStreamErrChan <- err
-			}
-
-			return
+		if err != nil && !errors.Is(err, context.Canceled) {
+			verifier.changeStreamErrChan <- err
 		}
 
-		select {
-		// If the changeStreamEnderChan has a message, the user has indicated that
-		// source writes are ended. This means we should exit rather than continue
-		// reading the change stream since there should be no more events.
-		case <-verifier.changeStreamEnderChan:
+		if changeStreamEnded {
 			verifier.mux.Lock()
 			verifier.changeStreamRunning = false
 			if verifier.lastChangeEventTime != nil {
@@ -151,7 +172,6 @@ func (verifier *Verifier) iterateChangeStream(ctx context.Context, cs *mongo.Cha
 			// since the changeStream is exhausted, we now return
 			verifier.logger.Debug().Msg("Change stream is done")
 			return
-		default:
 		}
 	}
 }