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

3 files changed, 18 insertions, 5 deletions

diff --git a/src/mongo/s/commands/cluster_is_master_cmd.cpp b/src/mongo/s/commands/cluster_is_master_cmd.cpp
index 5e864ec3532..68ffe4281f0 100644
--- a/src/mongo/s/commands/cluster_is_master_cmd.cpp
+++ b/src/mongo/s/commands/cluster_is_master_cmd.cpp
@@ -102,6 +102,11 @@ public:
 
         waitInIsMaster.pauseWhileSet(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) {
@@ -170,7 +175,7 @@ public:
         auto mongosIsMasterResponse =
             mongosTopCoord->awaitIsMasterResponse(opCtx, clientTopologyVersion, deadline);
 
-        mongosIsMasterResponse->appendToBuilder(&result);
+        mongosIsMasterResponse->appendToBuilder(&result, useLegacyResponseFields);
         // The isMaster response always includes a topologyVersion.
         auto currentMongosTopologyVersion = mongosIsMasterResponse->getTopologyVersion();
 
diff --git a/src/mongo/s/mongos_is_master_response.cpp b/src/mongo/s/mongos_is_master_response.cpp
index 7a58489c9bb..11b0145f73c 100644
--- a/src/mongo/s/mongos_is_master_response.cpp
+++ b/src/mongo/s/mongos_is_master_response.cpp
@@ -39,8 +39,13 @@ MongosIsMasterResponse::MongosIsMasterResponse(TopologyVersion topologyVersion)
     _msg = "isdbgrid";
 }
 
-void MongosIsMasterResponse::appendToBuilder(BSONObjBuilder* builder) const {
-    builder->append(kIsMasterFieldName, _isMaster);
+void MongosIsMasterResponse::appendToBuilder(BSONObjBuilder* builder,
+                                             bool useLegacyResponseFields) const {
+    if (useLegacyResponseFields) {
+        builder->append(kIsMasterFieldName, _isMaster);
+    } else {
+        builder->append(kIsWritablePrimaryFieldName, _isMaster);
+    }
     builder->append(kMsgFieldName, _msg);
 
     BSONObjBuilder topologyVersionBuilder(builder->subobjStart(kTopologyVersionFieldName));
diff --git a/src/mongo/s/mongos_is_master_response.h b/src/mongo/s/mongos_is_master_response.h
index aa9d523a845..00413126377 100644
--- a/src/mongo/s/mongos_is_master_response.h
+++ b/src/mongo/s/mongos_is_master_response.h
@@ -47,6 +47,7 @@ class MongosIsMasterResponse {
 public:
     static constexpr StringData kTopologyVersionFieldName = "topologyVersion"_sd;
     static constexpr StringData kIsMasterFieldName = "ismaster"_sd;
+    static constexpr StringData kIsWritablePrimaryFieldName = "isWritablePrimary"_sd;
     static constexpr StringData kMsgFieldName = "msg"_sd;
 
     /**
@@ -55,9 +56,11 @@ public:
     MongosIsMasterResponse(TopologyVersion topologyVersion);
 
     /**
-     * Appends MongosIsMasterResponse fields to "builder".
+     * Appends MongosIsMasterResponse fields to "builder". When true, "useLegacyResponseFields"
+     * indicates that we are responding to an isMaster command and not a hello command. Attach
+     * the legacy "ismaster" field if true, and the "isWritablePrimary" field otherwise.
      */
-    void appendToBuilder(BSONObjBuilder* builder) const;
+    void appendToBuilder(BSONObjBuilder* builder, bool useLegacyResponseFields) const;
 
     TopologyVersion getTopologyVersion() const {
         return _topologyVersion;