SERVER-31209 Persist targetVersion in FCV document to indicate an upgrade/downgrade in progress.

1 files changed, 11 insertions, 3 deletions

diff --git a/src/mongo/db/server_options.h b/src/mongo/db/server_options.h
index a82d20acc79..23f8bddff4f 100644
--- a/src/mongo/db/server_options.h
+++ b/src/mongo/db/server_options.h
@@ -158,14 +158,22 @@ struct ServerGlobalParams {
              * 3.4 node can participate in a cluster whose feature compatibility version is 3.6.
              */
             k36,
+            /**
+             * This is only used for targetVersion to indicate that no upgrade is in progress.
+             */
+            kUnset
         };
 
         // Read-only parameter featureCompatibilityVersion.
         AtomicWord<Version> version{Version::k34};
 
-        // Read-only global isSchemaVersion36. This determines whether to give Collections UUIDs
-        // upon creation.
-        AtomicWord<bool> isSchemaVersion36{false};
+        // If set, an upgrade or downgrade is in progress to the set version.
+        AtomicWord<Version> targetVersion{Version::kUnset};
+
+        // This determines whether to give Collections UUIDs upon creation.
+        bool isSchemaVersion36() {
+            return (version.load() == Version::k36 || targetVersion.load() == Version::k36);
+        }
 
         // Feature validation differs depending on the role of a mongod in a replica set or
         // master/slave configuration. Masters/primaries can accept user-initiated writes and