SERVER-59689 Prevent setting FCV < 5.3 in case of ongoing defragmentation

3 files changed, 49 insertions, 0 deletions

diff --git a/jstests/multiVersion/upgrade_downgrade_sharded_cluster.js b/jstests/multiVersion/upgrade_downgrade_sharded_cluster.js
index 390d53a664c..ccb27566ac0 100644
--- a/jstests/multiVersion/upgrade_downgrade_sharded_cluster.js
+++ b/jstests/multiVersion/upgrade_downgrade_sharded_cluster.js
@@ -172,6 +172,25 @@ function checkClusterAfterBinaryDowngrade(fcv, call1Ns, call2Ns, call3Ns) {
     testDisabledLongNameSupport(call3Ns);
 }
 
+// TODO SERVER-xyz review this check before v6.0 branches out
+function checkDowngradeDisallowedDuringDefragmentation(oldVersion) {
+    // Block defragmenter to be sure the `balancerShouldMergeChunks` field is not unset too early
+    st.forEachConfigServer((config) => {
+        assert.commandWorked(config.adminCommand(
+            {configureFailPoint: "beforeTransitioningDefragmentationPhase", mode: "alwaysOn"}));
+    });
+
+    // Pretend one collection is being defragmented to test that downgrade is not allowed
+    assert.commandWorked(
+        st.config.collections.updateOne({}, {$set: {balancerShouldMergeChunks: true}}));
+    var setFCVCmdResult = st.s.adminCommand({setFeatureCompatibilityVersion: oldVersion});
+    assert.commandFailedWithCode(setFCVCmdResult, ErrorCodes.CannotDowngrade);
+
+    // Rollback the change to allow downgrade
+    assert.commandWorked(st.config.collections.updateOne(
+        {balancerShouldMergeChunks: {$exists: true}}, {$unset: {balancerShouldMergeChunks: 1}}));
+}
+
 for (const oldVersion of [lastLTSFCV, lastContinuousFCV]) {
     //////////////////////////////
     // Setting and testing cluster using old binaries in default FCV mode
@@ -201,6 +220,7 @@ for (const oldVersion of [lastLTSFCV, lastContinuousFCV]) {
     // Setting and testing cluster using old binaries in old FCV mode
 
     jsTest.log('Downgrading FCV to ' + oldVersion);
+    checkDowngradeDisallowedDuringDefragmentation(oldVersion);
     assert.commandWorked(st.s.adminCommand({setFeatureCompatibilityVersion: oldVersion}));
 
     checkClusterAfterFCVDowngrade();
diff --git a/src/mongo/db/commands/set_feature_compatibility_version_command.cpp b/src/mongo/db/commands/set_feature_compatibility_version_command.cpp
index b137279d485..d7d6d9f6d8e 100644
--- a/src/mongo/db/commands/set_feature_compatibility_version_command.cpp
+++ b/src/mongo/db/commands/set_feature_compatibility_version_command.cpp
@@ -530,6 +530,26 @@ private:
                 requestedVersion);
 
         if (serverGlobalParams.clusterRole == ClusterRole::ConfigServer) {
+            {
+                // TODO SERVER-xyz review/adapt this scope before v6.0 branches out
+                // Make sure no collection is currently being defragmented
+                DBDirectClient client(opCtx);
+
+                const BSONObj collBeingDefragmentedQuery = BSON(
+                    CollectionType::kBalancerShouldMergeChunksFieldName << BSON("$exists" << true));
+
+                const bool isDefragmenting = client.count(CollectionType::ConfigNS,
+                                                          collBeingDefragmentedQuery,
+                                                          0 /*options*/,
+                                                          1 /* limit */);
+
+                uassert(ErrorCodes::CannotDowngrade,
+                        str::stream() << "Cannot downgrade the cluster when there are collections "
+                                         "being defragmented. Please drain all the defragmentation "
+                                         "processes before downgrading.",
+                        !isDefragmenting);
+            }
+
             // Tell the shards to enter phase-1 of setFCV
             auto requestPhase1 = request;
             requestPhase1.setFromConfigServer(true);
diff --git a/src/mongo/db/s/config/sharding_catalog_manager_collection_operations.cpp b/src/mongo/db/s/config/sharding_catalog_manager_collection_operations.cpp
index 4b0178fb8d6..2851b944304 100644
--- a/src/mongo/db/s/config/sharding_catalog_manager_collection_operations.cpp
+++ b/src/mongo/db/s/config/sharding_catalog_manager_collection_operations.cpp
@@ -48,6 +48,7 @@
 #include "mongo/db/catalog/collection_options.h"
 #include "mongo/db/client.h"
 #include "mongo/db/commands.h"
+#include "mongo/db/commands/feature_compatibility_version.h"
 #include "mongo/db/internal_transactions_feature_flag_gen.h"
 #include "mongo/db/logical_session_cache.h"
 #include "mongo/db/namespace_string.h"
@@ -555,6 +556,14 @@ void ShardingCatalogManager::configureCollectionAutoSplit(
     boost::optional<bool> balancerShouldMergeChunks,
     boost::optional<bool> enableAutoSplitter) {
 
+    // Hold the FCV region to serialize with the setFeatureCompatibilityVersion command
+    FixedFCVRegion fcvRegion(opCtx);
+    uassert(ErrorCodes::IllegalOperation,
+            "_configsvrConfigureAutoSplit can only be run when the cluster is in feature "
+            "compatibility versions greater or equal than 5.3.",
+            serverGlobalParams.featureCompatibility.isGreaterThanOrEqualTo(
+                multiversion::FeatureCompatibilityVersion::kVersion_5_3));
+
     uassert(ErrorCodes::InvalidOptions,
             "invalid collection auto splitter config update",
             maxChunkSizeBytes || balancerShouldMergeChunks || enableAutoSplitter);