SERVER-49988 Rename response fields if hello command is sent on mongos

(cherry picked from commit 8d495ce686890719e96c66ee6f3d1ddbf7704e78)

1 files changed, 11 insertions, 1 deletions

diff --git a/src/mongo/s/commands/cluster_is_master_cmd.cpp b/src/mongo/s/commands/cluster_is_master_cmd.cpp
index ed3f8181d4c..490605fbefe 100644
--- a/src/mongo/s/commands/cluster_is_master_cmd.cpp
+++ b/src/mongo/s/commands/cluster_is_master_cmd.cpp
@@ -83,6 +83,12 @@ public:
              const BSONObj& cmdObj,
              BSONObjBuilder& result) override {
         CommandHelpers::handleMarkKillOnClientDisconnect(opCtx);
+
+        // Parse the command name, which should be one of the following: hello, isMaster, or
+        // ismaster. If the command is "hello", we must attach an "isWritablePrimary" response field
+        // instead of "ismaster".
+        bool useLegacyResponseFields = (cmdObj.firstElementFieldNameStringData() != kHelloString);
+
         auto& clientMetadataIsMasterState = ClientMetadataIsMasterState::get(opCtx->getClient());
         bool seenIsMaster = clientMetadataIsMasterState.hasSeenIsMaster();
         if (!seenIsMaster) {
@@ -112,7 +118,11 @@ public:
                 opCtx->getClient(), std::move(swParseClientMetadata.getValue()));
         }
 
-        result.appendBool("ismaster", true);
+        if (useLegacyResponseFields) {
+            result.appendBool("ismaster", true);
+        } else {
+            result.appendBool("isWritablePrimary", true);
+        }
         result.append("msg", "isdbgrid");
         result.appendNumber("maxBsonObjectSize", BSONObjMaxUserSize);
         result.appendNumber("maxMessageSizeBytes", MaxMessageSizeBytes);