GODRIVER-2209 Omit readPreference on BatchCursor commands (#1346)

Author: prestonvasquez

x/mongo/driver/batch_cursor.go

@@ -305,6 +305,12 @@ func (bc *BatchCursor) KillCursor(ctx context.Context) error {
 		Legacy:         LegacyKillCursors,
 		CommandMonitor: bc.cmdMonitor,
 		ServerAPI:      bc.serverAPI,
+
+		// No read preference is passed to the killCursor command,
+		// resulting in the default read preference: "primaryPreferred".
+		// Since this could be confusing, and there is no requirement
+		// to use a read preference here, we omit it.
+		omitReadPreference: true,
 	}.Execute(ctx)
 }
 
@@ -398,6 +404,12 @@ func (bc *BatchCursor) getMore(ctx context.Context) {
 		CommandMonitor: bc.cmdMonitor,
 		Crypt:          bc.crypt,
 		ServerAPI:      bc.serverAPI,
+
+		// No read preference is passed to the getMore command,
+		// resulting in the default read preference: "primaryPreferred".
+		// Since this could be confusing, and there is no requirement
+		// to use a read preference here, we omit it.
+		omitReadPreference: true,
 	}.Execute(ctx)
 
 	// Once the cursor has been drained, we can unpin the connection if one is currently pinned.

x/mongo/driver/operation.go

@@ -308,6 +308,12 @@ type Operation struct {
 
 	// cmdName is only set when serializing OP_MSG and is used internally in readWireMessage.
 	cmdName string
+
+	// omitReadPreference is a boolean that indicates whether to omit the
+	// read preference from the command. This omition includes the case
+	// where a default read preference is used when the operation
+	// ReadPreference is not specified.
+	omitReadPreference bool
 }
 
 // shouldEncrypt returns true if this operation should automatically be encrypted.
@@ -1515,6 +1521,10 @@ func (op Operation) getReadPrefBasedOnTransaction() (*readpref.ReadPref, error)
 }
 
 func (op Operation) createReadPref(desc description.SelectedServer, isOpQuery bool) (bsoncore.Document, error) {
+	if op.omitReadPreference {
+		return nil, nil
+	}
+
 	// TODO(GODRIVER-2231): Instead of checking if isOutputAggregate and desc.Server.WireVersion.Max < 13, somehow check
 	// TODO if supplied readPreference was "overwritten" with primary in description.selectForReplicaSet.
 	if desc.Server.Kind == description.Standalone || (isOpQuery && desc.Server.Kind != description.Mongos) ||