SERVER-57602 Don't acquire FCV lock on reconfig triggered by signalDrainComplete()

2 files changed, 9 insertions, 2 deletions

diff --git a/src/mongo/db/commands/feature_compatibility_version.cpp b/src/mongo/db/commands/feature_compatibility_version.cpp
index 69046789c52..70a125c59df 100644
--- a/src/mongo/db/commands/feature_compatibility_version.cpp
+++ b/src/mongo/db/commands/feature_compatibility_version.cpp
@@ -577,6 +577,7 @@ Status FeatureCompatibilityVersionParameter::setFromString(const std::string&) {
 FixedFCVRegion::FixedFCVRegion(OperationContext* opCtx)
     : _lk([&] {
           invariant(!opCtx->lockState()->isLocked());
+          invariant(!opCtx->lockState()->isRSTLLocked());
           return Lock::SharedLock(opCtx->lockState(), fcvLock);
       }()) {}
 
diff --git a/src/mongo/db/repl/replication_coordinator_impl.cpp b/src/mongo/db/repl/replication_coordinator_impl.cpp
index 84224b73ad3..bb96489547b 100644
--- a/src/mongo/db/repl/replication_coordinator_impl.cpp
+++ b/src/mongo/db/repl/replication_coordinator_impl.cpp
@@ -3425,7 +3425,13 @@ Status ReplicationCoordinatorImpl::_doReplSetReconfig(OperationContext* opCtx,
     // So, acquire FCV mutex lock in shared mode to block writers from modifying the fcv document
     // to make sure fcv is not changed between getNewConfig() and storing the new config
     // document locally.
-    boost::optional<FixedFCVRegion> fixedFcvRegion(opCtx);
+    // Since 'skipSafetyChecks' is only true when this reconfig is invoked as part of
+    // 'signalDrainComplete', we can skip taking the FCV lock here because:
+    // 1. 'signalDrainComplete' acquires the RSTL in X mode prior to this reconfig, which will block
+    //    all external writers. This is also important because we must not acquire the FCV lock
+    //    while holding the RSTL to avoid deadlocking.
+    // 2. We are not able to accept replicated writes as primary until we fully exit drain mode.
+    auto fixedFcvRegion = skipSafetyChecks ? nullptr : std::make_unique<FixedFCVRegion>(opCtx);
 
     // Call the callback to get the new config given the old one.
     auto newConfigStatus = getNewConfig(oldConfig, topCoordTerm);
@@ -3519,7 +3525,7 @@ Status ReplicationCoordinatorImpl::_doReplSetReconfig(OperationContext* opCtx,
     // 1) For fcv 4.4, addition of new voter nodes.
     // 2) For fcv 4.7+, only if the current config doesn't contain the 'newlyAdded' field but the
     // new config got mutated to append 'newlyAdded' field.
-    if (force || !needsFcvLock()) {
+    if (fixedFcvRegion && (force || !needsFcvLock())) {
         fixedFcvRegion.reset();
     }