SERVER-33465 forward-port handling upgrade/downgrade states when sending setFCV to new shard in addShard

1 files changed, 22 insertions, 11 deletions

diff --git a/src/mongo/db/s/config/sharding_catalog_manager_shard_operations.cpp b/src/mongo/db/s/config/sharding_catalog_manager_shard_operations.cpp
index ec37ae9e83e..79489af4c9e 100644
--- a/src/mongo/db/s/config/sharding_catalog_manager_shard_operations.cpp
+++ b/src/mongo/db/s/config/sharding_catalog_manager_shard_operations.cpp
@@ -659,18 +659,29 @@ StatusWith<std::string> ShardingCatalogManager::addShard(
         return batchResponseStatus;
     }
 
-    // The featureCompatibilityVersion should be the same throughout the cluster. We don't
-    // explicitly send writeConcern majority to the added shard, because a 3.4 mongod will reject
-    // it (setFCV did not support writeConcern until 3.6), and a 3.6 mongod will still default to
-    // majority writeConcern.
+
+    // Since addShard runs under the fcvLock, it is guaranteed the fcv state won't change, but it's
+    // possible an earlier setFCV failed partway, so we handle all possible fcv states. Note, if
+    // the state is upgrading (downgrading), a user cannot switch to downgrading (upgrading) without
+    // first finishing the upgrade (downgrade).
     //
-    // TODO SERVER-32045: propagate the user's writeConcern
-    auto versionResponse = _runCommandForAddShard(
-        opCtx,
-        targeter.get(),
-        "admin",
-        BSON(FeatureCompatibilityVersion::kCommandName << FeatureCompatibilityVersion::toString(
-                 serverGlobalParams.featureCompatibility.getVersion())));
+    // Note, we don't explicitly send writeConcern majority to the added shard, because a 3.4 mongod
+    // will reject it (setFCV did not support writeConcern until 3.6), and a 3.6 mongod will still
+    // default to majority writeConcern.
+    // TODO SERVER-32045: propagate the user's writeConcern.
+    BSONObj setFCVCmd;
+    switch (serverGlobalParams.featureCompatibility.getVersion()) {
+        case ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo40:
+        case ServerGlobalParams::FeatureCompatibility::Version::kUpgradingTo40:
+            setFCVCmd = BSON(FeatureCompatibilityVersion::kCommandName
+                             << FeatureCompatibilityVersionCommandParser::kVersion40);
+            break;
+        default:
+            setFCVCmd = BSON(FeatureCompatibilityVersion::kCommandName
+                             << FeatureCompatibilityVersionCommandParser::kVersion36);
+            break;
+    }
+    auto versionResponse = _runCommandForAddShard(opCtx, targeter.get(), "admin", setFCVCmd);
     if (!versionResponse.isOK()) {
         return versionResponse.getStatus();
     }