SERVER-51880: Remove all {dropped:true} entries from config.collection on FCV upgrade

4 files changed, 108 insertions, 1 deletions

diff --git a/jstests/multiVersion/upgrade_downgrade_sharded_cluster.js b/jstests/multiVersion/upgrade_downgrade_sharded_cluster.js
new file mode 100644
index 00000000000..77a7c19e014
--- /dev/null
+++ b/jstests/multiVersion/upgrade_downgrade_sharded_cluster.js
@@ -0,0 +1,75 @@
+/**
+ * This test checks several upgrade/downgrade routines related to sharding, namely:
+ *  - The entries in the config server's config.collections that were marked as 'dropped: true' are
+ * deleted. (SERVER-52630: Remove once 5.0 becomes the LastLTS)
+ */
+(function() {
+"use strict";
+
+load('./jstests/multiVersion/libs/multi_cluster.js');  // for upgradeCluster()
+
+function setupInitialStateOnOldVersion() {
+    let csrs_config_db = st.configRS.getPrimary().getDB('config');
+
+    // Setup sharded collections
+    assert.commandWorked(st.s.adminCommand({enableSharding: 'sharded'}));
+    assert.commandWorked(st.s.adminCommand({shardCollection: 'sharded.foo', key: {x: 1}}));
+    assert.commandWorked(st.s.adminCommand({shardCollection: 'sharded.bar', key: {x: 1}}));
+
+    assert.neq(null, csrs_config_db.collections.findOne({_id: 'sharded.foo'}));
+    assert.neq(null, csrs_config_db.collections.findOne({_id: 'sharded.bar'}));
+
+    // Drop a collection so that it's metadata is left over on the config server's
+    // config.collections as 'dropped: true'
+    st.s.getDB('sharded').foo.drop();
+    assert.eq(true, csrs_config_db.collections.findOne({_id: 'sharded.foo'}).dropped);
+    assert.neq(null, csrs_config_db.collections.findOne({_id: 'sharded.bar'}));
+}
+
+function runChecksAfterUpgrade() {
+    let csrs_config_db = st.configRS.getPrimary().getDB('config');
+    // Check that the left over metadata at csrs config.collections has been cleaned up.
+    assert.eq(null, csrs_config_db.collections.findOne({_id: 'sharded.foo'}));
+    assert.neq(null, csrs_config_db.collections.findOne({_id: 'sharded.bar'}));
+}
+
+function runChecksAfterDowngrade() {
+}
+
+// Start a sharded cluster on version 4.4
+let old_version = '4.4';
+var st = new ShardingTest({
+    shards: 2,
+    mongos: 1,
+    other: {
+        mongosOptions: {binVersion: old_version},
+        configOptions: {binVersion: old_version},
+        shardOptions: {binVersion: old_version},
+
+        rsOptions: {binVersion: old_version},
+        rs: true,
+    }
+});
+st.configRS.awaitReplication();
+
+// Setup initial conditions on 4.4
+setupInitialStateOnOldVersion();
+
+// Upgrade the entire cluster to the latest version.
+jsTest.log('upgrading cluster');
+st.upgradeCluster('latest');
+assert.commandWorked(st.s.adminCommand({setFeatureCompatibilityVersion: latestFCV}));
+
+// Tests after upgrade
+runChecksAfterUpgrade();
+
+// Downgrade back to 4.4
+jsTest.log('downgrading cluster');
+assert.commandWorked(st.s.adminCommand({setFeatureCompatibilityVersion: old_version}));
+st.upgradeCluster(old_version);
+
+// Tests after downgrade to 4.4
+runChecksAfterDowngrade();
+
+st.stop();
+})();
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 c2e9cfe5cec..3a69b2340f3 100644
--- a/src/mongo/db/commands/set_feature_compatibility_version_command.cpp
+++ b/src/mongo/db/commands/set_feature_compatibility_version_command.cpp
@@ -243,8 +243,20 @@ public:
                 _deleteHaystackIndexesOnUpgrade(opCtx);
             }
 
-            // Upgrade shards before config finishes its upgrade.
             if (serverGlobalParams.clusterRole == ClusterRole::ConfigServer) {
+                if (actualVersion ==
+                        ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo44 ||
+                    actualVersion ==
+                        ServerGlobalParams::FeatureCompatibility::Version::kUpgradingFrom44To47 ||
+                    actualVersion ==
+                        ServerGlobalParams::FeatureCompatibility::Version::kUpgradingFrom44To48 ||
+                    actualVersion ==
+                        ServerGlobalParams::FeatureCompatibility::Version::kUpgradingFrom44To49) {
+                    // SERVER-52630: Remove once 5.0 becomes the LastLTS
+                    ShardingCatalogManager::get(opCtx)->removeDroppedCollectionsMetadata(opCtx);
+                }
+
+                // Upgrade shards before config finishes its upgrade.
                 uassertStatusOK(
                     ShardingCatalogManager::get(opCtx)->setFeatureCompatibilityVersionOnShards(
                         opCtx, CommandHelpers::appendMajorityWriteConcern(request.toBSON({}))));
diff --git a/src/mongo/db/s/config/sharding_catalog_manager.cpp b/src/mongo/db/s/config/sharding_catalog_manager.cpp
index ff8e7c04f8d..bb1c707407c 100644
--- a/src/mongo/db/s/config/sharding_catalog_manager.cpp
+++ b/src/mongo/db/s/config/sharding_catalog_manager.cpp
@@ -436,6 +436,15 @@ Status ShardingCatalogManager::setFeatureCompatibilityVersionOnShards(OperationC
     return Status::OK();
 }
 
+void ShardingCatalogManager::removeDroppedCollectionsMetadata(OperationContext* opCtx) {
+    const auto catalogClient = Grid::get(opCtx)->catalogClient();
+    uassertStatusOK(
+        catalogClient->removeConfigDocuments(opCtx,
+                                             CollectionType::ConfigNS,
+                                             BSON("dropped" << true),
+                                             ShardingCatalogClient::kMajorityWriteConcern));
+}
+
 Lock::ExclusiveLock ShardingCatalogManager::lockZoneMutex(OperationContext* opCtx) {
     Lock::ExclusiveLock lk(opCtx->lockState(), _kZoneOpLock);
     return lk;
diff --git a/src/mongo/db/s/config/sharding_catalog_manager.h b/src/mongo/db/s/config/sharding_catalog_manager.h
index 5ae612814fd..06022f7b242 100644
--- a/src/mongo/db/s/config/sharding_catalog_manager.h
+++ b/src/mongo/db/s/config/sharding_catalog_manager.h
@@ -444,6 +444,17 @@ public:
      */
     Status setFeatureCompatibilityVersionOnShards(OperationContext* opCtx, const BSONObj& cmdObj);
 
+    /**
+     * Removes all entries from the config server's config.collections where 'dropped' is true.
+     *
+     * Before v5.0, when a collection was dropped, its entry in config.collections remained, tagged
+     * as 'dropped: true'. As those are no longer needed, this method cleans up the leftover
+     * metadata.
+     *
+     * It shall be called on FCV 4.4 to 5.0 upgrade.
+     */
+    void removeDroppedCollectionsMetadata(OperationContext* opCtx);
+
     //
     // For Diagnostics
     //