SERVER-45282 Unify how mongos and mongod default RWC caches handle deleted defaults document

3 files changed, 29 insertions, 4 deletions

diff --git a/src/mongo/db/read_write_concern_defaults.cpp b/src/mongo/db/read_write_concern_defaults.cpp
index 4b77b993b1b..d43a2d2258d 100644
--- a/src/mongo/db/read_write_concern_defaults.cpp
+++ b/src/mongo/db/read_write_concern_defaults.cpp
@@ -54,7 +54,7 @@ public:
                           const boost::optional<BSONObj>& newDefaultsDoc)
         : _service(service), _rwcDefaults(rwcDefaults) {
         // Note this will throw if the document can't be parsed. In the case of a delete, there will
-        // be no new defaults document and the RWConcern will be default constructed, which  matches
+        // be no new defaults document and the RWConcern will be default constructed, which matches
         // the behavior when lookup discovers a non-existent defaults document.
         _rwc = newDefaultsDoc
             ? RWConcernDefault::parse(IDLParserErrorContext("RWDefaultsWriteObserver"),
@@ -173,9 +173,10 @@ void ReadWriteConcernDefaults::refreshIfNecessary(OperationContext* opCtx) {
         return;
     }
     auto currentDefaultsHandle = _defaults.acquire(opCtx, Type::kReadWriteConcernEntry);
-    if (!currentDefaultsHandle ||
+    if (!currentDefaultsHandle || !possibleNewDefaults->getEpoch() ||
         (possibleNewDefaults->getEpoch() > (**currentDefaultsHandle)->getEpoch())) {
-        // Use the new defaults if they have a higher epoch, or if there are currently no defaults.
+        // Use the new defaults if they have a higher epoch, if there are no defaults in the cache,
+        // or if the found defaults have no epoch, meaning there are no defaults in config.settings.
         log() << "refreshed RWC defaults to " << possibleNewDefaults->toBSON();
         setDefault(std::move(*possibleNewDefaults));
     }
diff --git a/src/mongo/db/read_write_concern_defaults.h b/src/mongo/db/read_write_concern_defaults.h
index 17b971a14b1..7c0b3f520d7 100644
--- a/src/mongo/db/read_write_concern_defaults.h
+++ b/src/mongo/db/read_write_concern_defaults.h
@@ -114,7 +114,7 @@ public:
 
     /**
      * Manually looks up the latest defaults, and if their epoch is more recent than the cached
-     * defaults, then update the cache with the new defaults.
+     * defaults or indicates there are no defaults, then update the cache with the new defaults.
      */
     void refreshIfNecessary(OperationContext* opCtx);
 
diff --git a/src/mongo/db/read_write_concern_defaults_test.cpp b/src/mongo/db/read_write_concern_defaults_test.cpp
index 56e98f0cd6c..13532a03325 100644
--- a/src/mongo/db/read_write_concern_defaults_test.cpp
+++ b/src/mongo/db/read_write_concern_defaults_test.cpp
@@ -480,5 +480,29 @@ TEST_F(ReadWriteConcernDefaultsTestWithClusterTime,
     ASSERT_LT(*oldDefaults.getLocalSetTime(), *newDefaults.getLocalSetTime());
 }
 
+TEST_F(ReadWriteConcernDefaultsTestWithClusterTime, TestRefreshDefaultsWithDeletedDefaults) {
+    RWConcernDefault origDefaults;
+    origDefaults.setEpoch(Timestamp(10, 20));
+    origDefaults.setSetTime(Date_t::fromMillisSinceEpoch(1234));
+    _lookupMock.setLookupCallReturnValue(std::move(origDefaults));
+
+    auto origCachedDefaults = _rwcd.getDefault(operationContext());
+    ASSERT_EQ(Timestamp(10, 20), *origCachedDefaults.getEpoch());
+    ASSERT_EQ(Date_t::fromMillisSinceEpoch(1234), *origCachedDefaults.getSetTime());
+
+    getClock()->reserveTicks(1);
+    getMockClockSource()->advance(Milliseconds(1));
+
+    _lookupMock.setLookupCallReturnValue(RWConcernDefault());
+
+    _rwcd.refreshIfNecessary(operationContext());
+
+    // The cache should now contain default constructed defaults.
+    auto newCachedDefaults = _rwcd.getDefault(operationContext());
+    ASSERT(!newCachedDefaults.getEpoch());
+    ASSERT(!newCachedDefaults.getSetTime());
+    ASSERT_LT(*origCachedDefaults.getLocalSetTime(), *newCachedDefaults.getLocalSetTime());
+}
+
 }  // namespace
 }  // namespace mongo