Make BatchCursor.Next return false if first batch is empty

GODRIVER-1160

Change-Id: I467bb96f8d9ec820f4f0814e939ced4e4d450c28

Author: mattChiaravalloti

mongo/change_stream.go

@@ -474,7 +474,7 @@ func (cs *ChangeStream) loopNext(ctx context.Context) {
 
 // Returns true if the underlying cursor's batch is empty
 func (cs *ChangeStream) emptyBatch() bool {
-	return len(cs.cursor.Batch().Data) == 5 // empty BSON array
+	return cs.cursor.Batch().Empty()
 }
 
 // StreamType represents the type of a change stream.

x/bsonx/bsoncore/document_sequence.go

@@ -74,6 +74,22 @@ func (ds *DocumentSequence) DocumentCount() int {
 	}
 }
 
+// Empty returns true if the sequence is empty. It always returns true for unknown sequence styles.
+func (ds *DocumentSequence) Empty() bool {
+	if ds == nil {
+		return true
+	}
+
+	switch ds.Style {
+	case SequenceStyle:
+		return len(ds.Data) == 0
+	case ArrayStyle:
+		return len(ds.Data) <= 5
+	default:
+		return true
+	}
+}
+
 //ResetIterator resets the iteration point for the Next method to the beginning of the document
 //sequence.
 func (ds *DocumentSequence) ResetIterator() {

x/bsonx/bsoncore/document_sequence_test.go

@@ -349,6 +349,29 @@ func TestDocumentSequence(t *testing.T) {
 			})
 		}
 	})
+	t.Run("Empty", func(t *testing.T) {
+		testCases := []struct {
+			name    string
+			ds      *DocumentSequence
+			isEmpty bool
+		}{
+			{"ArrayStyle/is empty/nil", nil, true},
+			{"ArrayStyle/is empty/0", &DocumentSequence{Style: ArrayStyle, Data: []byte{0x05, 0x00, 0x00, 0x00, 0x00}}, true},
+			{"ArrayStyle/is not empty/non-0", &DocumentSequence{Style: ArrayStyle, Data: genArrayStyle(10)}, false},
+			{"SequenceStyle/is empty/nil", nil, true},
+			{"SequenceStyle/is empty/0", &DocumentSequence{Style: SequenceStyle, Data: []byte{}}, true},
+			{"SequenceStyle/is not empty/non-0", &DocumentSequence{Style: SequenceStyle, Data: genSequenceStyle(10)}, false},
+		}
+
+		for _, tc := range testCases {
+			t.Run(tc.name, func(t *testing.T) {
+				isEmpty := tc.ds.Empty()
+				if isEmpty != tc.isEmpty {
+					t.Errorf("Unexpected Empty result. got %v; want %v", isEmpty, tc.isEmpty)
+				}
+			})
+		}
+	})
 	t.Run("ResetIterator", func(t *testing.T) {
 		ds := &DocumentSequence{Pos: 1234567890}
 		want := 0
@@ -368,6 +391,11 @@ func TestDocumentSequence(t *testing.T) {
 			var ds *DocumentSequence
 			_ = ds.DocumentCount()
 		})
+		t.Run("Empty", func(t *testing.T) {
+			defer capturePanic()
+			var ds *DocumentSequence
+			_ = ds.Empty()
+		})
 		t.Run("ResetIterator", func(t *testing.T) {
 			defer capturePanic()
 			var ds *DocumentSequence

x/mongo/driver/batch_cursor.go

@@ -166,7 +166,7 @@ func (bc *BatchCursor) Next(ctx context.Context) bool {
 
 	if bc.firstBatch {
 		bc.firstBatch = false
-		return true
+		return !bc.currentBatch.Empty()
 	}
 
 	if bc.id == 0 || bc.server == nil {
@@ -175,14 +175,7 @@ func (bc *BatchCursor) Next(ctx context.Context) bool {
 
 	bc.getMore(ctx)
 
-	switch bc.currentBatch.Style {
-	case bsoncore.SequenceStyle:
-		return len(bc.currentBatch.Data) > 0
-	case bsoncore.ArrayStyle:
-		return len(bc.currentBatch.Data) > 5
-	default:
-		return false
-	}
+	return !bc.currentBatch.Empty()
 }
 
 // Batch will return a DocumentSequence for the current batch of documents. The returned