Fix id reporting bug with _ids containing arrays (#823)

JAVA-4403

Author: rozza

driver-core/src/main/com/mongodb/internal/connection/IdHoldingBsonWriter.java

@@ -55,6 +55,7 @@ public class IdHoldingBsonWriter extends LevelCountingBsonWriter {
     private BasicOutputBuffer outputBuffer;
     private String currentFieldName;
     private BsonValue id;
+    private boolean idFieldIsAnArray = false;
 
     public IdHoldingBsonWriter(final BsonWriter bsonWriter) {
         super(bsonWriter);
@@ -105,6 +106,7 @@ public void writeEndDocument() {
     public void writeStartArray() {
         if (isWritingId()) {
             if (getIdBsonWriter().getCurrentLevel() == -1) {
+                idFieldIsAnArray = true;
                 getIdBsonWriter().writeStartDocument();
                 getIdBsonWriter().writeName(ID_FIELD_NAME);
             }
@@ -129,7 +131,7 @@ public void writeStartArray(final String name) {
     public void writeEndArray() {
         if (isWritingId()) {
             getIdBsonWriter().writeEndArray();
-            if (getIdBsonWriter().getCurrentLevel() == 0) {
+            if (getIdBsonWriter().getCurrentLevel() == 0 && idFieldIsAnArray) {
                 getIdBsonWriter().writeEndDocument();
                 id = new RawBsonDocument(getBytes()).get(ID_FIELD_NAME);
             }

driver-core/src/test/unit/com/mongodb/internal/connection/IdHoldingBsonWriterSpecification.groovy

@@ -88,6 +88,33 @@ class IdHoldingBsonWriterSpecification extends Specification {
         encodedDocument == document
     }
 
+    def 'serialize _id documents containing arrays'() {
+        def bsonBinaryWriter = new BsonBinaryWriter(new BasicOutputBuffer())
+        def idTrackingBsonWriter = new IdHoldingBsonWriter(bsonBinaryWriter)
+        BsonDocument document = BsonDocument.parse(json)
+
+        when:
+        new BsonDocumentCodec().encode(idTrackingBsonWriter, document, EncoderContext.builder()
+                .isEncodingCollectibleDocument(true).build())
+        def encodedDocument = getEncodedDocument(bsonBinaryWriter.getBsonOutput())
+
+        then:
+        encodedDocument == document
+
+        where:
+        json << ['{"_id": {"a": []}, "b": 123}',
+                 '{"_id": {"a": [1, 2]}, "b": 123}',
+                 '{"_id": {"a": [[[[1]]]]}, "b": 123}',
+                 '{"_id": {"a": [{"a": [1, 2]}]}, "b": 123}',
+                 '{"_id": {"a": {"a": [1, 2]}}, "b": 123}',
+                 '{"_id": {"a": [1, 2], "b": [123]}}',
+                 '{"_id": [], "b": 123}',
+                 '{"_id": [1, 2], "b": 123}',
+                 '{"_id": [[1], [[2]]], "b": 123}',
+                 '{"_id": [{"a": 1}], "b": 123}',
+                 '{"_id": [{"a": [{"b": 123}]}]}']
+    }
+
     private static BsonDocument getEncodedDocument(BsonOutput buffer) {
         new BsonDocumentCodec().decode(new BsonBinaryReader(buffer.getByteBuffers().get(0).asNIO()),
                 DecoderContext.builder().build())