Fix a race condition in parallel table loading.

ThreadSanitizer reported that a data race occurs when we attach to a
database and load its tables concurrently. Remove the custom code that
waits for all tables to load, and replace it with a local thread pool
and its wait() method.

This changes observable behavior: before, we would load the tables
sequentially when loading the system database, or performing an ATTACH
DATABASE query. Now we always load tables of ordinary databases in
parallel.

dbms/src/Core/Defines.h

@@ -139,3 +139,7 @@
 /// This number is only used for distributed version compatible.
 /// It could be any magic number.
 #define DBMS_DISTRIBUTED_SENDS_MAGIC_NUMBER 0xCAFECABE
+
+/// A macro for suppressing warnings about unused variables. Useful for
+/// structured bindings which have no standard way to declare this.
+#define UNUSED_VARIABLE(X) (void) (X)

dbms/src/Databases/DatabaseDictionary.cpp

@@ -27,7 +27,7 @@ DatabaseDictionary::DatabaseDictionary(const String & name_)
 {
 }
 
-void DatabaseDictionary::loadTables(Context &, ThreadPool *, bool)
+void DatabaseDictionary::loadTables(Context &, bool)
 {
 }
 

dbms/src/Databases/DatabaseDictionary.h

@@ -33,7 +33,6 @@ public:
 
     void loadTables(
         Context & context,
-        ThreadPool * thread_pool,
         bool has_force_restore_data_flag) override;
 
     bool isTableExist(

dbms/src/Databases/DatabaseMemory.cpp

@@ -18,7 +18,6 @@ DatabaseMemory::DatabaseMemory(String name_)
 
 void DatabaseMemory::loadTables(
     Context & /*context*/,
-    ThreadPool * /*thread_pool*/,
     bool /*has_force_restore_data_flag*/)
 {
     /// Nothing to load.

dbms/src/Databases/DatabaseMemory.h

@@ -25,7 +25,6 @@ public:
 
     void loadTables(
         Context & context,
-        ThreadPool * thread_pool,
         bool has_force_restore_data_flag) override;
 
     void createTable(

dbms/src/Databases/DatabaseMySQL.h

@@ -46,7 +46,7 @@ public:
         throw Exception("MySQL database engine does not support detach table.", ErrorCodes::NOT_IMPLEMENTED);
     }
 
-    void loadTables(Context &, ThreadPool *, bool) override
+    void loadTables(Context &, bool) override
     {
         /// do nothing
     }

dbms/src/Databases/DatabaseOrdinary.cpp

@@ -119,7 +119,6 @@ DatabaseOrdinary::DatabaseOrdinary(String name_, const String & metadata_path_,
 
 void DatabaseOrdinary::loadTables(
     Context & context,
-    ThreadPool * thread_pool,
     bool has_force_restore_data_flag)
 {
     using FileNames = std::vector<std::string>;
@@ -161,96 +160,68 @@ void DatabaseOrdinary::loadTables(
       */
     std::sort(file_names.begin(), file_names.end());
 
-    size_t total_tables = file_names.size();
+    const size_t total_tables = file_names.size();
     LOG_INFO(log, "Total " << total_tables << " tables.");
 
     AtomicStopwatch watch;
     std::atomic<size_t> tables_processed {0};
-    Poco::Event all_tables_processed;
-    ExceptionHandler exception_handler;
 
-    auto task_function = [&](const String & table)
+    auto loadOneTable = [&](const String & table)
     {
-        SCOPE_EXIT(
-            if (++tables_processed == total_tables)
-                all_tables_processed.set()
-        );
+        loadTable(context, metadata_path, *this, name, data_path, table, has_force_restore_data_flag);
 
         /// Messages, so that it's not boring to wait for the server to load for a long time.
-        if ((tables_processed + 1) % PRINT_MESSAGE_EACH_N_TABLES == 0
+        if (++tables_processed % PRINT_MESSAGE_EACH_N_TABLES == 0
             || watch.compareAndRestart(PRINT_MESSAGE_EACH_N_SECONDS))
         {
             LOG_INFO(log, std::fixed << std::setprecision(2) << tables_processed * 100.0 / total_tables << "%");
             watch.restart();
         }
-
-        loadTable(context, metadata_path, *this, name, data_path, table, has_force_restore_data_flag);
     };
 
-    for (const auto & filename : file_names)
-    {
-        auto task = createExceptionHandledJob(std::bind(task_function, filename), exception_handler);
+    ThreadPool pool(SettingMaxThreads().getAutoValue());
 
-        if (thread_pool)
-            thread_pool->schedule(task);
-        else
-            task();
+    for (const auto & file_name : file_names)
+    {
+        pool.schedule([&]() { loadOneTable(file_name); });
     }
 
-    if (thread_pool)
-        all_tables_processed.wait();
-
-    exception_handler.throwIfException();
+    pool.wait();
 
     /// After all tables was basically initialized, startup them.
-    startupTables(thread_pool);
+    startupTables(pool);
 }
 
 
-void DatabaseOrdinary::startupTables(ThreadPool * thread_pool)
+void DatabaseOrdinary::startupTables(ThreadPool & thread_pool)
 {
     LOG_INFO(log, "Starting up tables.");
 
-    AtomicStopwatch watch;
-    std::atomic<size_t> tables_processed {0};
-    size_t total_tables = tables.size();
-    Poco::Event all_tables_processed;
-    ExceptionHandler exception_handler;
-
+    const size_t total_tables = tables.size();
     if (!total_tables)
         return;
 
-    auto task_function = [&](const StoragePtr & table)
-    {
-        SCOPE_EXIT(
-            if (++tables_processed == total_tables)
-                all_tables_processed.set()
-        );
+    AtomicStopwatch watch;
+    std::atomic<size_t> tables_processed {0};
 
-        if ((tables_processed + 1) % PRINT_MESSAGE_EACH_N_TABLES == 0
+    auto startupOneTable = [&](const StoragePtr & table)
+    {
+        table->startup();
+
+        if (++tables_processed % PRINT_MESSAGE_EACH_N_TABLES == 0
             || watch.compareAndRestart(PRINT_MESSAGE_EACH_N_SECONDS))
         {
             LOG_INFO(log, std::fixed << std::setprecision(2) << tables_processed * 100.0 / total_tables << "%");
-            watch.restart();
+			watch.restart();
         }
-
-        table->startup();
     };
 
-    for (const auto & name_storage : tables)
+    for (const auto & table : tables)
     {
-        auto task = createExceptionHandledJob(std::bind(task_function, name_storage.second), exception_handler);
-
-        if (thread_pool)
-            thread_pool->schedule(task);
-        else
-            task();
+        thread_pool.schedule([&]() { startupOneTable(table.second); });
     }
 
-    if (thread_pool)
-        all_tables_processed.wait();
-
-    exception_handler.throwIfException();
+    thread_pool.wait();
 }
 
 

dbms/src/Databases/DatabaseOrdinary.h

@@ -19,7 +19,6 @@ public:
 
     void loadTables(
         Context & context,
-        ThreadPool * thread_pool,
         bool has_force_restore_data_flag) override;
 
     void createTable(
@@ -73,7 +72,7 @@ private:
     const String data_path;
     Poco::Logger * log;
 
-    void startupTables(ThreadPool * thread_pool);
+    void startupTables(ThreadPool & thread_pool);
 
     ASTPtr getCreateTableQueryImpl(const Context & context, const String & table_name, bool throw_on_error) const;
 };

dbms/src/Databases/IDatabase.h

@@ -56,11 +56,10 @@ public:
     /// Get name of database engine.
     virtual String getEngineName() const = 0;
 
-    /// Load a set of existing tables. If thread_pool is specified, use it.
+    /// Load a set of existing tables.
     /// You can call only once, right after the object is created.
     virtual void loadTables(
         Context & context,
-        ThreadPool * thread_pool,
         bool has_force_restore_data_flag) = 0;
 
     /// Check the existence of the table.

dbms/src/Interpreters/InterpreterCreateQuery.cpp

@@ -159,7 +159,7 @@ BlockIO InterpreterCreateQuery::createDatabase(ASTCreateQuery & create)
         if (need_write_metadata)
             Poco::File(metadata_file_tmp_path).renameTo(metadata_file_path);
 
-        database->loadTables(context, thread_pool, has_force_restore_data_flag);
+        database->loadTables(context, has_force_restore_data_flag);
     }
     catch (...)
     {

dbms/src/Interpreters/InterpreterCreateQuery.h

@@ -31,11 +31,6 @@ public:
 
     static ASTPtr formatIndices(const IndicesDescription & indices);
 
-    void setDatabaseLoadingThreadpool(ThreadPool & thread_pool_)
-    {
-        thread_pool = &thread_pool_;
-    }
-
     void setForceRestoreData(bool has_force_restore_data_flag_)
     {
         has_force_restore_data_flag = has_force_restore_data_flag_;
@@ -61,9 +56,6 @@ private:
     ASTPtr query_ptr;
     Context & context;
 
-    /// Using while loading database.
-    ThreadPool * thread_pool = nullptr;
-
     /// Skip safety threshold when loading tables.
     bool has_force_restore_data_flag = false;
     /// Is this an internal query - not from the user.

dbms/src/Interpreters/loadMetadata.cpp

@@ -33,7 +33,6 @@ static void executeCreateQuery(
     Context & context,
     const String & database,
     const String & file_name,
-    ThreadPool * pool,
     bool has_force_restore_data_flag)
 {
     ParserCreateQuery parser;
@@ -45,8 +44,6 @@ static void executeCreateQuery(
 
     InterpreterCreateQuery interpreter(ast, context);
     interpreter.setInternal(true);
-    if (pool)
-        interpreter.setDatabaseLoadingThreadpool(*pool);
     interpreter.setForceRestoreData(has_force_restore_data_flag);
     interpreter.execute();
 }
@@ -56,7 +53,6 @@ static void loadDatabase(
     Context & context,
     const String & database,
     const String & database_path,
-    ThreadPool * thread_pool,
     bool force_restore_data)
 {
     /// There may exist .sql file with database creation statement.
@@ -73,7 +69,8 @@ static void loadDatabase(
     else
         database_attach_query = "ATTACH DATABASE " + backQuoteIfNeed(database);
 
-    executeCreateQuery(database_attach_query, context, database, database_metadata_file, thread_pool, force_restore_data);
+    executeCreateQuery(database_attach_query, context, database,
+                       database_metadata_file, force_restore_data);
 }
 
 
@@ -92,9 +89,6 @@ void loadMetadata(Context & context)
     Poco::File force_restore_data_flag_file(context.getFlagsPath() + "force_restore_data");
     bool has_force_restore_data_flag = force_restore_data_flag_file.exists();
 
-    /// For parallel tables loading.
-    ThreadPool thread_pool(SettingMaxThreads().getAutoValue());
-
     /// Loop over databases.
     std::map<String, String> databases;
     Poco::DirectoryIterator dir_end;
@@ -113,10 +107,8 @@ void loadMetadata(Context & context)
         databases.emplace(unescapeForFileName(it.name()), it.path().toString());
     }
 
-    for (const auto & elem : databases)
-        loadDatabase(context, elem.first, elem.second, &thread_pool, has_force_restore_data_flag);
-
-    thread_pool.wait();
+    for (const auto & [name, path] : databases)
+        loadDatabase(context, name, path, has_force_restore_data_flag);
 
     if (has_force_restore_data_flag)
     {
@@ -138,7 +130,7 @@ void loadMetadataSystem(Context & context)
     if (Poco::File(path).exists())
     {
         /// 'has_force_restore_data_flag' is true, to not fail on loading query_log table, if it is corrupted.
-        loadDatabase(context, SYSTEM_DATABASE, path, nullptr, true);
+        loadDatabase(context, SYSTEM_DATABASE, path, true);
     }
     else
     {

dbms/src/Interpreters/tests/create_query.cpp

@@ -84,7 +84,7 @@ try
     context.setPath("./");
     auto database = std::make_shared<DatabaseOrdinary>("test", "./metadata/test/", context);
     context.addDatabase("test", database);
-    database->loadTables(context, nullptr, false);
+    database->loadTables(context, false);
     context.setCurrentDatabase("test");
 
     InterpreterCreateQuery interpreter(ast, context);

dbms/src/Interpreters/tests/select_query.cpp

@@ -39,7 +39,7 @@ try
 
     DatabasePtr system = std::make_shared<DatabaseOrdinary>("system", "./metadata/system/", context);
     context.addDatabase("system", system);
-    system->loadTables(context, nullptr, false);
+    system->loadTables(context, false);
     attachSystemTablesLocal(*context.getDatabase("system"));
     context.setCurrentDatabase("default");