SERVER-34483 Avoid taking DBLocks when clearing in-memory database cache in FCV op observer

Author: EshaMaharishi

jstests/sharding/database_versioning_upgrade_downgrade.js

@@ -32,7 +32,7 @@
     assert.eq(null, db1EntryOriginal.version);
     assert.eq(null, db2EntryOriginal.version);
 
-    // Ensure the shard does not have a cached entries in-memory or on-disk.
+    // Ensure the shard does not have cached entries in-memory or on-disk.
     checkInMemoryDatabaseVersion(st.rs0.getPrimary(), db1Name, {});
     checkInMemoryDatabaseVersion(st.rs0.getSecondary(), db1Name, {});
     checkInMemoryDatabaseVersion(st.rs0.getPrimary(), db2Name, {});
@@ -104,10 +104,9 @@
     assert.docEq(db1EntryOriginal, db1EntryFCV36);
     assert.docEq(db2EntryOriginal, db2EntryFCV36);
 
-    // The shard's in-memory database cache should have been cleared, but its on-disk cache left
-    // untouched.
-    checkInMemoryDatabaseVersion(st.rs0.getPrimary(), db1Name, {});
-    checkInMemoryDatabaseVersion(st.rs0.getSecondary(), db1Name, {});
+    // The shard's in-memory and on-disk database caches should have been left untouched.
+    checkInMemoryDatabaseVersion(st.rs0.getPrimary(), db1Name, db1EntryFCV40.version);
+    checkInMemoryDatabaseVersion(st.rs0.getSecondary(), db1Name, db1EntryFCV40.version);
     checkInMemoryDatabaseVersion(st.rs0.getPrimary(), db2Name, {});
     checkInMemoryDatabaseVersion(st.rs0.getSecondary(), db2Name, {});
     checkOnDiskDatabaseVersion(st.shard0, db1Name, db1EntryFCV40);

src/mongo/db/commands/feature_compatibility_version.cpp

@@ -42,7 +42,7 @@
 #include "mongo/db/operation_context.h"
 #include "mongo/db/repl/optime.h"
 #include "mongo/db/repl/storage_interface.h"
-#include "mongo/db/s/database_sharding_state.h"
+#include "mongo/db/s/collection_sharding_state.h"
 #include "mongo/db/s/sharding_state.h"
 #include "mongo/db/server_parameters.h"
 #include "mongo/db/service_context.h"
@@ -167,26 +167,7 @@ void FeatureCompatibilityVersion::onInsertOrUpdate(OperationContext* opCtx, cons
             (newVersion ==
                  ServerGlobalParams::FeatureCompatibility::Version::kFullyDowngradedTo36 ||
              newVersion == ServerGlobalParams::FeatureCompatibility::Version::kFullyUpgradedTo40)) {
-            // Clear the in-memory cached database versions and collections metadata.
-            // TODO: Once SERVER-34431 goes in, just clear the DatabaseShardingStateMap.
-            std::vector<std::string> dbNames;
-            getGlobalServiceContext()->getGlobalStorageEngine()->listDatabases(&dbNames);
-            for (const auto& dbName : dbNames) {
-                if (dbName == "admin") {
-                    // The 'admin' database is already locked, since the FCV document is in
-                    // admin.system.version. Just skip 'admin', since it is not versioned.
-                    continue;
-                }
-                AutoGetDb autoDb(opCtx, dbName, MODE_X);
-                if (autoDb.getDb()) {
-                    DatabaseShardingState::get(autoDb.getDb()).setDbVersion(opCtx, boost::none);
-                    for (const auto& collection : *autoDb.getDb()) {
-                        CollectionShardingState::get(opCtx, collection->ns())
-                            ->refreshMetadata(opCtx, nullptr);
-                    }
-                }
-            }
-
+            CollectionShardingState::resetAll(opCtx);
             Grid::get(opCtx)->catalogCache()->purgeAllDatabases();
         }
 

src/mongo/db/s/collection_sharding_state.cpp

@@ -124,6 +124,16 @@ class CollectionShardingStateMap {
         return *it->second;
     }
 
+    void resetAll() {
+        stdx::lock_guard<stdx::mutex> lg(_mutex);
+        for (auto it = _collections.begin(); it != _collections.end(); ++it) {
+            // This is a hack to get around CollectionShardingState::refreshMetadata() requiring
+            // the X lock: markNotShardedAtStepdown() doesn't have a lock check. Temporary
+            // measure until SERVER-31595 removes the X lock requirement.
+            it->second->markNotShardedAtStepdown();
+        }
+    }
+
     void report(OperationContext* opCtx, BSONObjBuilder* builder) {
         BSONObjBuilder versionB(builder->subobjStart("versions"));
 
@@ -175,6 +185,11 @@ CollectionShardingState* CollectionShardingState::get(OperationContext* opCtx,
     return &collectionsMap.getOrCreate(ns);
 }
 
+void CollectionShardingState::resetAll(OperationContext* opCtx) {
+    auto& collectionsMap = CollectionShardingStateMap::get(opCtx->getServiceContext());
+    collectionsMap.resetAll();
+}
+
 void CollectionShardingState::report(OperationContext* opCtx, BSONObjBuilder* builder) {
     auto& collectionsMap = CollectionShardingStateMap::get(opCtx->getServiceContext());
     collectionsMap.report(opCtx, builder);

src/mongo/db/s/collection_sharding_state.h

@@ -71,6 +71,7 @@ class CollectionShardingState : public Decorable<CollectionShardingState> {
     static CollectionShardingState* get(OperationContext* opCtx, const NamespaceString& nss);
     static CollectionShardingState* get(OperationContext* opCtx, const std::string& ns);
 
+    static void resetAll(OperationContext* opCtx);
     static void report(OperationContext* opCtx, BSONObjBuilder* builder);
 
     /**