SERVER-54918 Hide the `fcvLock` behind a proper FCV set/check interface

1 files changed, 14 insertions, 21 deletions

diff --git a/src/mongo/db/commands/feature_compatibility_version.h b/src/mongo/db/commands/feature_compatibility_version.h
index 8c4af173521..41563a7f42c 100644
--- a/src/mongo/db/commands/feature_compatibility_version.h
+++ b/src/mongo/db/commands/feature_compatibility_version.h
@@ -38,21 +38,9 @@
 
 namespace mongo {
 
-class BSONObj;
-class OperationContext;
-
 class FeatureCompatibilityVersion {
 public:
     /**
-     * Should be taken in shared mode by any operations that should not run while
-     * setFeatureCompatibilityVersion is running.
-     *
-     * setFCV takes this lock in exclusive mode so that it both does not run with the shared mode
-     * operations and does not run with itself.
-     */
-    static Lock::ResourceMutex fcvLock;
-
-    /**
      * Reads the featureCompatibilityVersion (FCV) document in admin.system.version and initializes
      * the FCV global state. Returns an error if the FCV document exists and is invalid. Does not
      * return an error if it is missing. This should be checked after startup with
@@ -106,6 +94,13 @@ public:
      * current featureCompatibilityVersion value.
      */
     static void updateMinWireVersion();
+
+    /**
+     * Returns a scoped object, which holds the 'fcvLock' in exclusive mode for the given scope. It
+     * must only be used by the setFeatureCompatibilityVersion command in order to serialise with
+     * concurrent 'FixedFCVRegions'.
+     */
+    static Lock::ExclusiveLock enterFCVChangeRegion(OperationContext* opCtx);
 };
 
 /**
@@ -113,19 +108,17 @@ public:
  */
 class FixedFCVRegion {
 public:
-    explicit FixedFCVRegion(OperationContext* opCtx) {
-        invariant(!opCtx->lockState()->isLocked());
-        _lk.emplace(opCtx->lockState(), FeatureCompatibilityVersion::fcvLock);
-    }
+    explicit FixedFCVRegion(OperationContext* opCtx);
+    ~FixedFCVRegion();
 
-    ~FixedFCVRegion() = default;
+    bool operator==(const ServerGlobalParams::FeatureCompatibility::Version& other) const;
+    bool operator!=(const ServerGlobalParams::FeatureCompatibility::Version& other) const;
 
-    void release() {
-        _lk.reset();
-    }
+    const ServerGlobalParams::FeatureCompatibility& operator*() const;
+    const ServerGlobalParams::FeatureCompatibility* operator->() const;
 
 private:
-    boost::optional<Lock::SharedLock> _lk;
+    Lock::SharedLock _lk;
 };
 
 }  // namespace mongo