SERVER-57042 Don't store defaultWriteConcernSource on disk

3 files changed, 180 insertions, 91 deletions

diff --git a/src/mongo/db/read_write_concern_defaults.cpp b/src/mongo/db/read_write_concern_defaults.cpp
index debf69a43f6..2822f620688 100644
--- a/src/mongo/db/read_write_concern_defaults.cpp
+++ b/src/mongo/db/read_write_concern_defaults.cpp
@@ -110,11 +110,6 @@ RWConcernDefault ReadWriteConcernDefaults::generateNewCWRWCToBeSavedOnDisk(
             rc || wc);
 
     RWConcernDefault rwc;
-    const bool isDefaultWCMajorityFeatureFlagEnabled =
-        repl::feature_flags::gDefaultWCMajority.isEnabled(serverGlobalParams.featureCompatibility);
-    if (isDefaultWCMajorityFeatureFlagEnabled) {
-        rwc.setDefaultWriteConcernSource(DefaultWriteConcernSourceEnum::kImplicit);
-    }
 
     if (rc && !rc->isEmpty()) {
         checkSuitabilityAsDefault(*rc);
@@ -123,9 +118,6 @@ RWConcernDefault ReadWriteConcernDefaults::generateNewCWRWCToBeSavedOnDisk(
     if (wc && !wc->usedDefault) {
         checkSuitabilityAsDefault(*wc);
         rwc.setDefaultWriteConcern(wc);
-        if (isDefaultWCMajorityFeatureFlagEnabled) {
-            rwc.setDefaultWriteConcernSource(DefaultWriteConcernSourceEnum::kGlobal);
-        }
     }
 
     auto* const serviceContext = opCtx->getServiceContext();
@@ -139,10 +131,6 @@ RWConcernDefault ReadWriteConcernDefaults::generateNewCWRWCToBeSavedOnDisk(
     }
     if (!wc && current) {
         rwc.setDefaultWriteConcern(current->getDefaultWriteConcern());
-        if (isDefaultWCMajorityFeatureFlagEnabled && current->getDefaultWriteConcern() &&
-            !current->getDefaultWriteConcern().get().usedDefault) {
-            rwc.setDefaultWriteConcernSource(DefaultWriteConcernSourceEnum::kGlobal);
-        }
     }
     // If the setDefaultRWConcern command tries to unset the global default write concern when it
     // has already been set, throw an error.
@@ -162,20 +150,11 @@ RWConcernDefault ReadWriteConcernDefaults::generateNewCWRWCToBeSavedOnDisk(
 }
 
 bool ReadWriteConcernDefaults::isCWWCSet(OperationContext* opCtx) {
-    // TODO (SERVER-57042): Call getDefault() instead and return writeConcernSource == KGlobal.
-    auto cached = _getDefaultCWRWCFromDisk(opCtx);
-    if (!cached) {
-        return false;
-    }
-
-    auto wc = cached.get().getDefaultWriteConcern();
-    if (!wc) {
-        return false;
-    }
-
-    // CWWC should have "usedDefault" flag set to false, as this flag only set to true if the
-    // default constructed WC is used.
-    return !wc.get().usedDefault;
+    // This function is only valid if the gDefaultWCMajority feature flag is enabled. It only
+    // returns true if the gDefaultWCMajority feature flag is enabled, and the cluster-wide write
+    // concern has been set. It will return false otherwise.
+    auto rwcd = getDefault(opCtx);
+    return rwcd.getDefaultWriteConcernSource() == DefaultWriteConcernSourceEnum::kGlobal;
 }
 
 void ReadWriteConcernDefaults::observeDirectWriteToConfigSettings(OperationContext* opCtx,
@@ -269,30 +248,32 @@ ReadWriteConcernDefaults::RWConcernDefaultAndTime ReadWriteConcernDefaults::getD
         }
     }
 
-    const bool isDefaultWCMajorityFeatureFlagEnabled =
-        serverGlobalParams.featureCompatibility.isVersionInitialized() &&
-        repl::feature_flags::gDefaultWCMajority.isEnabled(serverGlobalParams.featureCompatibility);
-    if (isDefaultWCMajorityFeatureFlagEnabled && !cached.getDefaultWriteConcernSource()) {
-        cached.setDefaultWriteConcernSource(DefaultWriteConcernSourceEnum::kImplicit);
-    }
-
     // The implicit default write concern will be w:1 if the feature compatibility version is not
     // yet initialized. Similarly, if the config hasn't yet been loaded on the node, the default
     // will be w:1, since we have no way of calculating the implicit default. This means that after
     // we have loaded our config, nodes could change their implicit write concern default. This is
     // safe since we shouldn't be accepting writes that need a write concern before we have loaded
     // our config.
-    if (!isDefaultWCMajorityFeatureFlagEnabled || !_implicitDefaultWriteConcernMajority) {
-        return cached;
-    }
+    const bool isDefaultWCMajorityFeatureFlagEnabled =
+        serverGlobalParams.featureCompatibility.isVersionInitialized() &&
+        repl::feature_flags::gDefaultWCMajority.isEnabled(serverGlobalParams.featureCompatibility);
 
-    if ((!cached.getDefaultWriteConcern() || cached.getDefaultWriteConcern().get().usedDefault) &&
-        _implicitDefaultWriteConcernMajority.get()) {
-        cached.setDefaultWriteConcern(WriteConcernOptions(WriteConcernOptions::kMajority,
-                                                          WriteConcernOptions::SyncMode::UNSET,
-                                                          WriteConcernOptions::kNoTimeout));
-        if (isDefaultWCMajorityFeatureFlagEnabled) {
+    // This prevents overriding the default write concern and its source on mongos if it has
+    // already been set through the config server.
+    if (isDefaultWCMajorityFeatureFlagEnabled && !cached.getDefaultWriteConcernSource()) {
+        const bool isCWWCSet =
+            cached.getDefaultWriteConcern() && !cached.getDefaultWriteConcern().get().usedDefault;
+        if (isCWWCSet) {
+            cached.setDefaultWriteConcernSource(DefaultWriteConcernSourceEnum::kGlobal);
+        } else {
             cached.setDefaultWriteConcernSource(DefaultWriteConcernSourceEnum::kImplicit);
+            if (_implicitDefaultWriteConcernMajority &&
+                _implicitDefaultWriteConcernMajority.get()) {
+                cached.setDefaultWriteConcern(
+                    WriteConcernOptions(WriteConcernOptions::kMajority,
+                                        WriteConcernOptions::SyncMode::UNSET,
+                                        WriteConcernOptions::kNoTimeout));
+            }
         }
     }
 
diff --git a/src/mongo/db/read_write_concern_defaults.h b/src/mongo/db/read_write_concern_defaults.h
index cf061d9122d..ccb0b9cdef0 100644
--- a/src/mongo/db/read_write_concern_defaults.h
+++ b/src/mongo/db/read_write_concern_defaults.h
@@ -135,7 +135,8 @@ public:
                                                      const boost::optional<WriteConcern>& wc);
 
     /**
-     * Returns true if cluster-wide write concern is set and false otherwise.
+     * Returns true if cluster-wide write concern is set and the gDefaultWCMajority feature flag
+     * is enabled, and false otherwise.
      */
     bool isCWWCSet(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 1d28f29ab57..e4168dd107b 100644
--- a/src/mongo/db/read_write_concern_defaults_test.cpp
+++ b/src/mongo/db/read_write_concern_defaults_test.cpp
@@ -250,9 +250,31 @@ TEST_F(ReadWriteConcernDefaultsTest, TestGetDefaultWithUnsetCWRWCWithImplicitWCM
     ASSERT(defaults.getDefaultWriteConcernSource() == DefaultWriteConcernSourceEnum::kImplicit);
 }
 
-TEST_F(ReadWriteConcernDefaultsTest, TestGetDefaultWithSetCWRWCWithImplicitWCW1) {
+TEST_F(ReadWriteConcernDefaultsTest, TestGetDefaultWithCWRWCNotSetThenSetWithImplicitWCW1) {
     createDefaults(false /* isImplicitWCMajority */);
 
+    // _defaults.lookup() returning default constructed RWConcern not boost::none.
+    _lookupMock.setLookupCallReturnValue({});
+    ASSERT(!isCWWCSet());
+    auto oldDefaults = getDefault();
+    if (_isDefaultRCLocalEnabled) {
+        ASSERT(oldDefaults.getDefaultReadConcern()->getLevel() ==
+               repl::ReadConcernLevel::kLocalReadConcern);
+        ASSERT(oldDefaults.getDefaultReadConcernSource());
+        ASSERT(oldDefaults.getDefaultReadConcernSource() ==
+               DefaultReadConcernSourceEnum::kImplicit);
+    } else {
+        ASSERT(!oldDefaults.getDefaultReadConcernSource());
+    }
+    ASSERT(!oldDefaults.getDefaultWriteConcern());
+    ASSERT(!oldDefaults.getUpdateOpTime());
+    ASSERT(!oldDefaults.getUpdateWallClockTime());
+    ASSERT_GT(oldDefaults.localUpdateWallClockTime(), Date_t());
+    if (_isDefaultWCMajorityEnabled) {
+        ASSERT(oldDefaults.getDefaultWriteConcernSource() ==
+               DefaultWriteConcernSourceEnum::kImplicit);
+    }
+
     RWConcernDefault newDefaults;
     newDefaults.setDefaultReadConcern(
         repl::ReadConcernArgs(repl::ReadConcernLevel::kLocalReadConcern));
@@ -263,11 +285,71 @@ TEST_F(ReadWriteConcernDefaultsTest, TestGetDefaultWithSetCWRWCWithImplicitWCW1)
     newDefaults.setDefaultWriteConcern(wc);
     newDefaults.setUpdateOpTime(Timestamp(1, 2));
     newDefaults.setUpdateWallClockTime(Date_t::fromMillisSinceEpoch(1234));
+    _lookupMock.setLookupCallReturnValue(std::move(newDefaults));
+
+    ReadWriteConcernDefaults::get(getServiceContext()).invalidate();
     if (_isDefaultWCMajorityEnabled) {
-        newDefaults.setDefaultWriteConcernSource(DefaultWriteConcernSourceEnum::kGlobal);
+        ASSERT(isCWWCSet());
+    }
+    auto defaults = getDefault();
+    ASSERT(defaults.getDefaultReadConcern()->getLevel() ==
+           repl::ReadConcernLevel::kLocalReadConcern);
+    if (_isDefaultRCLocalEnabled) {
+        ASSERT(defaults.getDefaultReadConcernSource());
+        ASSERT(defaults.getDefaultReadConcernSource() == DefaultReadConcernSourceEnum::kGlobal);
+    } else {
+        ASSERT(!defaults.getDefaultReadConcernSource());
+    }
+    ASSERT_EQ(4, defaults.getDefaultWriteConcern()->wNumNodes);
+    ASSERT_EQ(Timestamp(1, 2), *defaults.getUpdateOpTime());
+    ASSERT_EQ(1234, defaults.getUpdateWallClockTime()->toMillisSinceEpoch());
+    ASSERT_GT(defaults.localUpdateWallClockTime(), Date_t());
+    if (_isDefaultWCMajorityEnabled) {
+        ASSERT(defaults.getDefaultWriteConcernSource());
+        ASSERT(defaults.getDefaultWriteConcernSource() == DefaultWriteConcernSourceEnum::kGlobal);
     }
+}
+
+TEST_F(ReadWriteConcernDefaultsTest, TestGetDefaultWithCWRWCNotSetThenSetWithImplicitWCMajority) {
+    if (!_isDefaultWCMajorityEnabled) {
+        return;
+    }
+
+    createDefaults(true /* isImplicitWCMajority */);
+
+    // _defaults.lookup() returning default constructed RWConcern not boost::none.
+    _lookupMock.setLookupCallReturnValue({});
+    ASSERT(!isCWWCSet());
+    auto oldDefaults = getDefault();
+    if (_isDefaultRCLocalEnabled) {
+        ASSERT(oldDefaults.getDefaultReadConcern()->getLevel() ==
+               repl::ReadConcernLevel::kLocalReadConcern);
+        ASSERT(oldDefaults.getDefaultReadConcernSource());
+        ASSERT(oldDefaults.getDefaultReadConcernSource() ==
+               DefaultReadConcernSourceEnum::kImplicit);
+    } else {
+        ASSERT(!oldDefaults.getDefaultReadConcernSource());
+    }
+    ASSERT(oldDefaults.getDefaultWriteConcern());
+    ASSERT_EQ(WriteConcernOptions::kMajority, oldDefaults.getDefaultWriteConcern().get().wMode);
+    ASSERT(!oldDefaults.getUpdateOpTime());
+    ASSERT(!oldDefaults.getUpdateWallClockTime());
+    ASSERT_GT(oldDefaults.localUpdateWallClockTime(), Date_t());
+    ASSERT(oldDefaults.getDefaultWriteConcernSource() == DefaultWriteConcernSourceEnum::kImplicit);
+
+    RWConcernDefault newDefaults;
+    newDefaults.setDefaultReadConcern(
+        repl::ReadConcernArgs(repl::ReadConcernLevel::kLocalReadConcern));
+    WriteConcernOptions wc;
+    wc.wNumNodes = 4;
+    wc.usedDefault = false;
+    wc.usedDefaultW = false;
+    newDefaults.setDefaultWriteConcern(wc);
+    newDefaults.setUpdateOpTime(Timestamp(1, 2));
+    newDefaults.setUpdateWallClockTime(Date_t::fromMillisSinceEpoch(1234));
     _lookupMock.setLookupCallReturnValue(std::move(newDefaults));
 
+    ReadWriteConcernDefaults::get(getServiceContext()).invalidate();
     ASSERT(isCWWCSet());
     auto defaults = getDefault();
     ASSERT(defaults.getDefaultReadConcern()->getLevel() ==
@@ -283,6 +365,41 @@ TEST_F(ReadWriteConcernDefaultsTest, TestGetDefaultWithSetCWRWCWithImplicitWCW1)
     ASSERT_EQ(Timestamp(1, 2), *defaults.getUpdateOpTime());
     ASSERT_EQ(1234, defaults.getUpdateWallClockTime()->toMillisSinceEpoch());
     ASSERT_GT(defaults.localUpdateWallClockTime(), Date_t());
+    ASSERT(defaults.getDefaultWriteConcernSource());
+    ASSERT(defaults.getDefaultWriteConcernSource() == DefaultWriteConcernSourceEnum::kGlobal);
+}
+
+TEST_F(ReadWriteConcernDefaultsTest, TestGetDefaultWithSetCWRWCWithImplicitWCW1) {
+    createDefaults(false /* isImplicitWCMajority */);
+
+    RWConcernDefault newDefaults;
+    newDefaults.setDefaultReadConcern(
+        repl::ReadConcernArgs(repl::ReadConcernLevel::kLocalReadConcern));
+    WriteConcernOptions wc;
+    wc.wNumNodes = 4;
+    wc.usedDefault = false;
+    wc.usedDefaultW = false;
+    newDefaults.setDefaultWriteConcern(wc);
+    newDefaults.setUpdateOpTime(Timestamp(1, 2));
+    newDefaults.setUpdateWallClockTime(Date_t::fromMillisSinceEpoch(1234));
+    _lookupMock.setLookupCallReturnValue(std::move(newDefaults));
+
+    if (_isDefaultWCMajorityEnabled) {
+        ASSERT(isCWWCSet());
+    }
+    auto defaults = getDefault();
+    ASSERT(defaults.getDefaultReadConcern()->getLevel() ==
+           repl::ReadConcernLevel::kLocalReadConcern);
+    if (_isDefaultRCLocalEnabled) {
+        ASSERT(defaults.getDefaultReadConcernSource());
+        ASSERT(defaults.getDefaultReadConcernSource() == DefaultReadConcernSourceEnum::kGlobal);
+    } else {
+        ASSERT(!defaults.getDefaultReadConcernSource());
+    }
+    ASSERT_EQ(4, defaults.getDefaultWriteConcern()->wNumNodes);
+    ASSERT_EQ(Timestamp(1, 2), *defaults.getUpdateOpTime());
+    ASSERT_EQ(1234, defaults.getUpdateWallClockTime()->toMillisSinceEpoch());
+    ASSERT_GT(defaults.localUpdateWallClockTime(), Date_t());
     if (_isDefaultWCMajorityEnabled) {
         ASSERT(defaults.getDefaultWriteConcernSource() == DefaultWriteConcernSourceEnum::kGlobal);
     }
@@ -305,9 +422,6 @@ TEST_F(ReadWriteConcernDefaultsTest, TestGetDefaultWithSetCWRWCWithImplicitWCMaj
     newDefaults.setDefaultWriteConcern(wc);
     newDefaults.setUpdateOpTime(Timestamp(1, 2));
     newDefaults.setUpdateWallClockTime(Date_t::fromMillisSinceEpoch(1234));
-    if (_isDefaultWCMajorityEnabled) {
-        newDefaults.setDefaultWriteConcernSource(DefaultWriteConcernSourceEnum::kGlobal);
-    }
     _lookupMock.setLookupCallReturnValue(std::move(newDefaults));
 
     ASSERT(isCWWCSet());
@@ -323,9 +437,7 @@ TEST_F(ReadWriteConcernDefaultsTest, TestGetDefaultWithSetCWRWCWithImplicitWCMaj
     ASSERT_EQ(Timestamp(1, 2), *defaults.getUpdateOpTime());
     ASSERT_EQ(1234, defaults.getUpdateWallClockTime()->toMillisSinceEpoch());
     ASSERT_GT(defaults.localUpdateWallClockTime(), Date_t());
-    if (_isDefaultWCMajorityEnabled) {
-        ASSERT(defaults.getDefaultWriteConcernSource() == DefaultWriteConcernSourceEnum::kGlobal);
-    }
+    ASSERT(defaults.getDefaultWriteConcernSource() == DefaultWriteConcernSourceEnum::kGlobal);
 }
 
 TEST_F(ReadWriteConcernDefaultsTest, TestGetDefaultWriteConcernSourceImplicitWithUsedDefaultWC) {
@@ -339,7 +451,6 @@ TEST_F(ReadWriteConcernDefaultsTest, TestGetDefaultWriteConcernSourceImplicitWit
     newDefaults.setDefaultWriteConcern(WriteConcernOptions());
     newDefaults.setUpdateOpTime(Timestamp(1, 2));
     newDefaults.setUpdateWallClockTime(Date_t::fromMillisSinceEpoch(1234));
-    newDefaults.setDefaultWriteConcernSource(DefaultWriteConcernSourceEnum::kGlobal);
 
     _lookupMock.setLookupCallReturnValue(std::move(newDefaults));
 
@@ -591,13 +702,12 @@ TEST_F(ReadWriteConcernDefaultsTest, TestInvalidate) {
     newDefaults2.setDefaultWriteConcern(wc);
     newDefaults2.setUpdateOpTime(Timestamp(3, 4));
     newDefaults2.setUpdateWallClockTime(Date_t::fromMillisSinceEpoch(5678));
-    if (_isDefaultWCMajorityEnabled) {
-        newDefaults2.setDefaultWriteConcernSource(DefaultWriteConcernSourceEnum::kGlobal);
-    }
     _lookupMock.setLookupCallReturnValue(std::move(newDefaults2));
 
     ReadWriteConcernDefaults::get(getServiceContext()).invalidate();
-    ASSERT(isCWWCSet());
+    if (_isDefaultWCMajorityEnabled) {
+        ASSERT(isCWWCSet());
+    }
     auto defaults2 = getDefault();
     ASSERT(defaults2.getDefaultReadConcern()->getLevel() ==
            repl::ReadConcernLevel::kAvailableReadConcern);
@@ -750,10 +860,8 @@ protected:
         ASSERT_EQ(4, defaults.getDefaultWriteConcern()->wNumNodes);
         ASSERT(defaults.getUpdateOpTime());
         ASSERT(defaults.getUpdateWallClockTime());
-        if (_isDefaultWCMajorityEnabled) {
-            ASSERT(defaults.getDefaultWriteConcernSource() ==
-                   DefaultWriteConcernSourceEnum::kGlobal);
-        }
+        // Default write concern source is not saved on disk.
+        ASSERT(!defaults.getDefaultWriteConcernSource());
 
         _lookupMock.setLookupCallReturnValue(std::move(defaults));
         auto oldDefaults = _rwcd.getDefault(operationContext());
@@ -859,15 +967,15 @@ TEST_F(ReadWriteConcernDefaultsTestWithClusterTime,
     ASSERT_EQ(5, defaults.getDefaultWriteConcern()->wNumNodes);
     ASSERT_LT(*oldDefaults.getUpdateOpTime(), *defaults.getUpdateOpTime());
     ASSERT_LT(*oldDefaults.getUpdateWallClockTime(), *defaults.getUpdateWallClockTime());
-    if (_isDefaultWCMajorityEnabled) {
-        ASSERT(defaults.getDefaultWriteConcernSource() == DefaultWriteConcernSourceEnum::kGlobal);
-    }
+    // Default write concern source is not saved on disk.
+    ASSERT(!defaults.getDefaultWriteConcernSource());
 
     _lookupMock.setLookupCallReturnValue(std::move(defaults));
     _rwcd.refreshIfNecessary(operationContext());
     auto newDefaults = _rwcd.getDefault(operationContext());
     ASSERT_LT(oldDefaults.localUpdateWallClockTime(), newDefaults.localUpdateWallClockTime());
     if (_isDefaultWCMajorityEnabled) {
+        // Default write concern source is calculated through 'getDefault'.
         ASSERT(newDefaults.getDefaultWriteConcernSource() ==
                DefaultWriteConcernSourceEnum::kGlobal);
     }
@@ -888,15 +996,15 @@ TEST_F(ReadWriteConcernDefaultsTestWithClusterTime,
               defaults.getDefaultWriteConcern()->wNumNodes);
     ASSERT_LT(*oldDefaults.getUpdateOpTime(), *defaults.getUpdateOpTime());
     ASSERT_LT(*oldDefaults.getUpdateWallClockTime(), *defaults.getUpdateWallClockTime());
-    if (_isDefaultWCMajorityEnabled) {
-        ASSERT(defaults.getDefaultWriteConcernSource() == DefaultWriteConcernSourceEnum::kGlobal);
-    }
+    // Default write concern source is not saved on disk.
+    ASSERT(!defaults.getDefaultWriteConcernSource());
 
     _lookupMock.setLookupCallReturnValue(std::move(defaults));
     _rwcd.refreshIfNecessary(operationContext());
     auto newDefaults = _rwcd.getDefault(operationContext());
     ASSERT_LT(oldDefaults.localUpdateWallClockTime(), newDefaults.localUpdateWallClockTime());
     if (_isDefaultWCMajorityEnabled) {
+        // Default write concern source is calculated through 'getDefault'.
         ASSERT(newDefaults.getDefaultWriteConcernSource() ==
                DefaultWriteConcernSourceEnum::kGlobal);
     }
@@ -911,14 +1019,14 @@ TEST_F(ReadWriteConcernDefaultsTestWithClusterTime,
     ASSERT(defaults.getDefaultReadConcern()->getLevel() ==
            repl::ReadConcernLevel::kMajorityReadConcern);
     ASSERT(!defaults.getDefaultWriteConcern());
-    if (_isDefaultWCMajorityEnabled) {
-        ASSERT(defaults.getDefaultWriteConcernSource() == DefaultWriteConcernSourceEnum::kImplicit);
-    }
+    // Default write concern source is not saved on disk.
+    ASSERT(!defaults.getDefaultWriteConcernSource());
 
     _lookupMock.setLookupCallReturnValue(std::move(defaults));
     _rwcd.refreshIfNecessary(operationContext());
     auto newDefaults = _rwcd.getDefault(operationContext());
     if (_isDefaultWCMajorityEnabled) {
+        // Default write concern source is calculated through 'getDefault'.
         ASSERT(newDefaults.getDefaultWriteConcernSource() ==
                DefaultWriteConcernSourceEnum::kImplicit);
     }
@@ -938,15 +1046,15 @@ TEST_F(ReadWriteConcernDefaultsTestWithClusterTime,
     ASSERT_EQ(5, defaults.getDefaultWriteConcern()->wNumNodes);
     ASSERT_LT(*oldDefaults.getUpdateOpTime(), *defaults.getUpdateOpTime());
     ASSERT_LT(*oldDefaults.getUpdateWallClockTime(), *defaults.getUpdateWallClockTime());
-    if (_isDefaultWCMajorityEnabled) {
-        ASSERT(defaults.getDefaultWriteConcernSource() == DefaultWriteConcernSourceEnum::kGlobal);
-    }
+    // Default write concern source is not saved on disk.
+    ASSERT(!defaults.getDefaultWriteConcernSource());
 
     _lookupMock.setLookupCallReturnValue(std::move(defaults));
     _rwcd.refreshIfNecessary(operationContext());
     auto newDefaults = _rwcd.getDefault(operationContext());
     ASSERT_LT(oldDefaults.localUpdateWallClockTime(), newDefaults.localUpdateWallClockTime());
     if (_isDefaultWCMajorityEnabled) {
+        // Default write concern source is calculated through 'getDefault'.
         ASSERT(newDefaults.getDefaultWriteConcernSource() ==
                DefaultWriteConcernSourceEnum::kGlobal);
     }
@@ -961,15 +1069,14 @@ TEST_F(ReadWriteConcernDefaultsTestWithClusterTime,
     ASSERT(oldDefaults.getDefaultReadConcern()->getLevel() ==
            repl::ReadConcernLevel::kMajorityReadConcern);
     ASSERT(!oldDefaults.getDefaultWriteConcern());
-    if (_isDefaultWCMajorityEnabled) {
-        ASSERT(oldDefaults.getDefaultWriteConcernSource() ==
-               DefaultWriteConcernSourceEnum::kImplicit);
-    }
+    // Default write concern source is not saved on disk.
+    ASSERT(!oldDefaults.getDefaultWriteConcernSource());
 
     _lookupMock.setLookupCallReturnValue(std::move(oldDefaults));
     _rwcd.refreshIfNecessary(operationContext());
     auto newDefaults = _rwcd.getDefault(operationContext());
     if (_isDefaultWCMajorityEnabled) {
+        // Default write concern source is calculated through 'getDefault'.
         ASSERT(newDefaults.getDefaultWriteConcernSource() ==
                DefaultWriteConcernSourceEnum::kImplicit);
     }
@@ -984,9 +1091,8 @@ TEST_F(ReadWriteConcernDefaultsTestWithClusterTime,
     ASSERT(newDefaults.getDefaultReadConcern()->getLevel() ==
            defaults.getDefaultReadConcern()->getLevel());
     ASSERT_EQ(5, defaults.getDefaultWriteConcern()->wNumNodes);
-    if (_isDefaultWCMajorityEnabled) {
-        ASSERT(defaults.getDefaultWriteConcernSource() == DefaultWriteConcernSourceEnum::kGlobal);
-    }
+    // Default write concern source is not saved on disk.
+    ASSERT(!defaults.getDefaultWriteConcernSource());
 
     _lookupMock.setLookupCallReturnValue(std::move(defaults));
     _rwcd.refreshIfNecessary(operationContext());
@@ -994,6 +1100,7 @@ TEST_F(ReadWriteConcernDefaultsTestWithClusterTime,
     ASSERT(newDefaults.getDefaultWriteConcern());
     ASSERT_EQ(5, newDefaults.getDefaultWriteConcern()->wNumNodes);
     if (_isDefaultWCMajorityEnabled) {
+        // Default write concern source is calculated through 'getDefault'.
         ASSERT(newDefaults.getDefaultWriteConcernSource() ==
                DefaultWriteConcernSourceEnum::kGlobal);
     }
@@ -1013,15 +1120,15 @@ TEST_F(ReadWriteConcernDefaultsTestWithClusterTime,
               defaults.getDefaultWriteConcern()->wNumNodes);
     ASSERT_LT(*oldDefaults.getUpdateOpTime(), *defaults.getUpdateOpTime());
     ASSERT_LT(*oldDefaults.getUpdateWallClockTime(), *defaults.getUpdateWallClockTime());
-    if (_isDefaultWCMajorityEnabled) {
-        ASSERT(defaults.getDefaultWriteConcernSource() == DefaultWriteConcernSourceEnum::kGlobal);
-    }
+    // Default write concern source is not saved on disk.
+    ASSERT(!defaults.getDefaultWriteConcernSource());
 
     _lookupMock.setLookupCallReturnValue(std::move(defaults));
     _rwcd.refreshIfNecessary(operationContext());
     auto newDefaults = _rwcd.getDefault(operationContext());
     ASSERT_LT(oldDefaults.localUpdateWallClockTime(), newDefaults.localUpdateWallClockTime());
     if (_isDefaultWCMajorityEnabled) {
+        // Default write concern source is calculated through 'getDefault'.
         ASSERT(newDefaults.getDefaultWriteConcernSource() ==
                DefaultWriteConcernSourceEnum::kGlobal);
     }
@@ -1073,14 +1180,14 @@ TEST_F(ReadWriteConcernDefaultsTestWithClusterTime,
         operationContext(), boost::none, WriteConcernOptions());
     ASSERT(!defaults.getDefaultReadConcern());
     ASSERT(!defaults.getDefaultWriteConcern());
-    if (_isDefaultWCMajorityEnabled) {
-        ASSERT(defaults.getDefaultWriteConcernSource() == DefaultWriteConcernSourceEnum::kImplicit);
-    }
+    // Default write concern source is not saved on disk.
+    ASSERT(!defaults.getDefaultWriteConcernSource());
 
     _lookupMock.setLookupCallReturnValue(std::move(defaults));
     _rwcd.refreshIfNecessary(operationContext());
     auto newDefaults = _rwcd.getDefault(operationContext());
     if (_isDefaultWCMajorityEnabled) {
+        // Default write concern source is calculated through 'getDefault'.
         ASSERT(newDefaults.getDefaultWriteConcernSource() ==
                DefaultWriteConcernSourceEnum::kImplicit);
     }
@@ -1140,15 +1247,15 @@ TEST_F(ReadWriteConcernDefaultsTestWithClusterTime,
     ASSERT(WriteConcernOptions::SyncMode::JOURNAL == defaults.getDefaultWriteConcern()->syncMode);
     ASSERT_LT(*oldDefaults.getUpdateOpTime(), *defaults.getUpdateOpTime());
     ASSERT_LT(*oldDefaults.getUpdateWallClockTime(), *defaults.getUpdateWallClockTime());
-    if (_isDefaultWCMajorityEnabled) {
-        ASSERT(defaults.getDefaultWriteConcernSource() == DefaultWriteConcernSourceEnum::kGlobal);
-    }
+    // Default write concern source is not saved on disk.
+    ASSERT(!defaults.getDefaultWriteConcernSource());
 
     _lookupMock.setLookupCallReturnValue(std::move(defaults));
     _rwcd.refreshIfNecessary(operationContext());
     auto newDefaults = _rwcd.getDefault(operationContext());
     ASSERT_LT(oldDefaults.localUpdateWallClockTime(), newDefaults.localUpdateWallClockTime());
     if (_isDefaultWCMajorityEnabled) {
+        // Default write concern source is calculated through 'getDefault'.
         ASSERT(newDefaults.getDefaultWriteConcernSource() ==
                DefaultWriteConcernSourceEnum::kGlobal);
     }
@@ -1169,15 +1276,15 @@ TEST_F(ReadWriteConcernDefaultsTestWithClusterTime,
     ASSERT(WriteConcernOptions::SyncMode::UNSET == defaults.getDefaultWriteConcern()->syncMode);
     ASSERT_LT(*oldDefaults.getUpdateOpTime(), *defaults.getUpdateOpTime());
     ASSERT_LT(*oldDefaults.getUpdateWallClockTime(), *defaults.getUpdateWallClockTime());
-    if (_isDefaultWCMajorityEnabled) {
-        ASSERT(defaults.getDefaultWriteConcernSource() == DefaultWriteConcernSourceEnum::kGlobal);
-    }
+    // Default write concern source is not saved on disk.
+    ASSERT(!defaults.getDefaultWriteConcernSource());
 
     _lookupMock.setLookupCallReturnValue(std::move(defaults));
     _rwcd.refreshIfNecessary(operationContext());
     auto newDefaults = _rwcd.getDefault(operationContext());
     ASSERT_LT(oldDefaults.localUpdateWallClockTime(), newDefaults.localUpdateWallClockTime());
     if (_isDefaultWCMajorityEnabled) {
+        // Default write concern source is calculated through 'getDefault'.
         ASSERT(newDefaults.getDefaultWriteConcernSource() ==
                DefaultWriteConcernSourceEnum::kGlobal);
     }