SERVER-35651: Don't downgrade data files when a 4.2 binary running as an arbiter is shut down.

To downgrade binaries for an arbiter, the user must delete the dbpath.

1 files changed, 4 insertions, 1 deletions

diff --git a/src/mongo/db/storage/wiredtiger/wiredtiger_kv_engine.cpp b/src/mongo/db/storage/wiredtiger/wiredtiger_kv_engine.cpp
index 3353b835cd9..a1595730c5b 100644
--- a/src/mongo/db/storage/wiredtiger/wiredtiger_kv_engine.cpp
+++ b/src/mongo/db/storage/wiredtiger/wiredtiger_kv_engine.cpp
@@ -124,7 +124,10 @@ bool WiredTigerFileVersion::shouldDowngrade(bool readOnly,
     const auto replCoord = repl::ReplicationCoordinator::get(getGlobalServiceContext());
     const auto memberState = replCoord->getMemberState();
     if (memberState.arbiter()) {
-        return true;
+        // SERVER-35361: Arbiters will no longer downgrade their data files. To downgrade
+        // binaries, the user must delete the dbpath. It's not particularly expensive for a
+        // replica set to re-initialize an arbiter that comes online.
+        return false;
     }
 
     if (!serverGlobalParams.featureCompatibility.isVersionInitialized()) {