Fix panic when resume token load failed with a non-ErrNoDocuments error. (#178)

This also includes an additional (unrelated) error-wrapping in the retryer.

Author: FGasper

internal/verifier/change_reader.go

@@ -207,11 +207,15 @@ func (rc *ChangeReaderCommon) loadResumeToken(ctx context.Context) (option.Optio
 		bson.D{{"_id", rc.resumeTokenDocID()}},
 	).Raw()
 
-	if errors.Is(err, mongo.ErrNoDocuments) {
-		return option.None[bson.Raw](), nil
+	if err != nil {
+		if errors.Is(err, mongo.ErrNoDocuments) {
+			err = nil
+		}
+
+		return option.None[bson.Raw](), err
 	}
 
-	return option.Some(token), err
+	return option.Some(token), nil
 }
 
 func (rc *ChangeReaderCommon) updateLag(sess *mongo.Session, token bson.Raw) {

internal/verifier/change_stream.go

@@ -317,7 +317,7 @@ func (csr *ChangeStreamReader) iterateChangeStream(
 				err = csr.readAndHandleOneChangeEventBatch(ctx, ri, cs, sess)
 
 				if err != nil {
-					return err
+					return errors.Wrap(err, "finishing change stream after writes-off")
 				}
 			}