MDEV-8069 DROP or rebuild of a large table may lock up InnoDB

Problematic mutex is dict_sys.mutex.
Idea of the patch: unlink() fd under that mutex while
it's still open. This way unlink() will be fast and
actual file removal will happen on close().
And close() will be called outside of dict_sys.mutex.
This should be safe against crash which may happen between
unlink() and close(): file will be removed by OS anyway.
The same applies to both *nix and Windows.

I created and removed a 4G file on some NVMe SSD on ext4:

write(3, "\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1"..., 1048576) = 1048576 <0.000519>
fdatasync(3)                            = 0 <3.533763>
close(3)                                = 0 <0.000011>
unlink("file")                          = 0 <0.411563>

write(3, "\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1\1"..., 1048576) = 1048576 <0.000520>
fdatasync(3)                            = 0 <3.544938>
unlink("file")                          = 0 <0.000029>
close(3)                                = 0 <0.407057>

Such systems can benefit of this patch.

fil_node_t::detach(): closes fil_node_t but not file handle,
  returns that file handle

fil_node_t::prepare_to_close_or_deatch(): 'closes' fil_node_t

fil_node_t:close_to_free(): new argument detach_handle

fil_system_t::detach(): now can detach file handles

fil_delete_tablespace(): now can detach file handles

row_drop_table_for_mysql(): performs actual file removal

storage/innobase/fil/fil0fil.cc

@@ -511,32 +511,41 @@ static bool fil_node_open_file(fil_node_t* node)
 /** Close the file handle. */
 void fil_node_t::close()
 {
-	bool	ret;
+  prepare_to_close_or_detach();
 
-	ut_ad(mutex_own(&fil_system.mutex));
-	ut_a(is_open());
-	ut_a(n_pending == 0);
-	ut_a(n_pending_flushes == 0);
-	ut_a(!being_extended);
-	ut_a(!needs_flush
-	     || space->purpose == FIL_TYPE_TEMPORARY
-	     || srv_fast_shutdown == 2
-	     || !srv_was_started);
+  /* printf("Closing file %s\n", name); */
+  int ret= os_file_close(handle);
+  ut_a(ret);
+  handle= OS_FILE_CLOSED;
+}
 
-	ret = os_file_close(handle);
-	ut_a(ret);
+pfs_os_file_t fil_node_t::detach()
+{
+  prepare_to_close_or_detach();
 
-	/* printf("Closing file %s\n", name); */
+  pfs_os_file_t result= handle;
+  handle= OS_FILE_CLOSED;
+  return result;
+}
 
-	handle = OS_FILE_CLOSED;
-	ut_ad(!is_open());
-	ut_a(fil_system.n_open > 0);
-	fil_system.n_open--;
+void fil_node_t::prepare_to_close_or_detach()
+{
+  ut_ad(mutex_own(&fil_system.mutex));
+  ut_a(is_open());
+  ut_a(n_pending == 0);
+  ut_a(n_pending_flushes == 0);
+  ut_a(!being_extended);
+  ut_a(!needs_flush || space->purpose == FIL_TYPE_TEMPORARY ||
+       srv_fast_shutdown == 2 || !srv_was_started);
 
-	if (fil_space_belongs_in_lru(space)) {
-		ut_a(UT_LIST_GET_LEN(fil_system.LRU) > 0);
-		UT_LIST_REMOVE(fil_system.LRU, this);
-	}
+  ut_a(fil_system.n_open > 0);
+  fil_system.n_open--;
+
+  if (fil_space_belongs_in_lru(space))
+  {
+    ut_a(UT_LIST_GET_LEN(fil_system.LRU) > 0);
+    UT_LIST_REMOVE(fil_system.LRU, this);
+  }
 }
 
 /** Tries to close a file in the LRU list. The caller must hold the fil_sys
@@ -917,7 +926,7 @@ fil_space_extend(
 }
 
 /** Prepare to free a file from fil_system. */
-inline void fil_node_t::close_to_free()
+pfs_os_file_t fil_node_t::close_to_free(bool detach_handle)
 {
   ut_ad(mutex_own(&fil_system.mutex));
   ut_a(magic_n == FIL_NODE_MAGIC_N);
@@ -958,15 +967,24 @@ inline void fil_node_t::close_to_free()
     }
     ut_a(!n_pending_flushes);
     ut_a(!being_extended);
+    if (detach_handle)
+    {
+      auto result= handle;
+      handle= OS_FILE_CLOSED;
+      return result;
+    }
     bool ret= os_file_close(handle);
     ut_a(ret);
     handle= OS_FILE_CLOSED;
     break;
   }
+
+  return OS_FILE_CLOSED;
 }
 
 /** Detach a tablespace from the cache and close the files. */
-inline void fil_system_t::detach(fil_space_t *space)
+std::vector<pfs_os_file_t> fil_system_t::detach(fil_space_t *space,
+                                                bool detach_handle)
 {
   ut_ad(mutex_own(&fil_system.mutex));
   HASH_DELETE(fil_space_t, hash, spaces, space->id, space);
@@ -1000,9 +1018,18 @@ inline void fil_system_t::detach(fil_space_t *space)
       n_open--;
     }
 
+  std::vector<pfs_os_file_t> handles;
+  handles.reserve(UT_LIST_GET_LEN(space->chain));
+
   for (fil_node_t* node= UT_LIST_GET_FIRST(space->chain); node;
        node= UT_LIST_GET_NEXT(chain, node))
-    node->close_to_free();
+  {
+    auto handle= node->close_to_free(detach_handle);
+    if (handle != OS_FILE_CLOSED)
+      handles.push_back(handle);
+  }
+
+  return handles;
 }
 
 /** Free a tablespace object on which fil_system_t::detach() was invoked.
@@ -2180,11 +2207,14 @@ bool fil_table_accessible(const dict_table_t* table)
 /** Delete a tablespace and associated .ibd file.
 @param[in]	id		tablespace identifier
 @param[in]	if_exists	whether to ignore missing tablespace
+@param[in,out]	detached_handles	return detached handles if not nullptr
 @return	DB_SUCCESS or error */
-dberr_t fil_delete_tablespace(ulint id, bool if_exists)
+dberr_t fil_delete_tablespace(ulint id, bool if_exists,
+			      std::vector<pfs_os_file_t>* detached_handles)
 {
 	char* path = NULL;
 	ut_ad(!is_system_tablespace(id));
+	ut_ad(!detached_handles || detached_handles->empty());
 
 	dberr_t err;
 	fil_space_t *space = fil_check_pending_operations(id, false, &path);
@@ -2260,7 +2290,11 @@ dberr_t fil_delete_tablespace(ulint id, bool if_exists)
 		ut_a(s == space);
 		ut_a(!space->referenced());
 		ut_a(UT_LIST_GET_LEN(space->chain) == 1);
-		fil_system.detach(space);
+		auto handles = fil_system.detach(space,
+						 detached_handles != nullptr);
+		if (detached_handles) {
+			*detached_handles = std::move(handles);
+		}
 		mutex_exit(&fil_system.mutex);
 
 		log_mutex_enter();

storage/innobase/include/fil0fil.h

@@ -653,11 +653,19 @@ struct fil_node_t {
 
   /** Close the file handle. */
   void close();
-  /** Prepare to free a file from fil_system. */
-  inline void close_to_free();
+  /** Same as close() but returns file handle instead of closing it. */
+  pfs_os_file_t detach() MY_ATTRIBUTE((warn_unused_result));
+  /** Prepare to free a file from fil_system.
+  @param detach_handle whether to detach instead of closing a handle
+  @return detached handle or OS_FILE_CLOSED */
+  pfs_os_file_t close_to_free(bool detach_handle= false);
 
   /** Update the data structures on I/O completion */
   inline void complete_io(bool write= false);
+
+private:
+  /** Does stuff common for close() and detach() */
+  void prepare_to_close_or_detach();
 };
 
 /** Value of fil_node_t::magic_n */
@@ -973,8 +981,12 @@ struct fil_system_t {
   }
 #endif
 public:
-  /** Detach a tablespace from the cache and close the files. */
-  inline void detach(fil_space_t *space);
+  /** Detach a tablespace from the cache and close the files.
+  @param space tablespace
+  @param detach_handle whether to detach or close handles
+  @return detached handles or empty vector */
+  std::vector<pfs_os_file_t> detach(fil_space_t *space,
+                                    bool detach_handle= false);
 
 	ib_mutex_t	mutex;		/*!< The mutex protecting the cache */
 	fil_space_t*	sys_space;	/*!< The innodb_system tablespace */
@@ -1285,8 +1297,11 @@ bool fil_table_accessible(const dict_table_t* table)
 /** Delete a tablespace and associated .ibd file.
 @param[in]	id		tablespace identifier
 @param[in]	if_exists	whether to ignore missing tablespace
+@param[out]	leaked_handles	return detached handles here
 @return	DB_SUCCESS or error */
-dberr_t fil_delete_tablespace(ulint id, bool if_exists= false);
+dberr_t
+fil_delete_tablespace(ulint id, bool if_exists= false,
+                      std::vector<pfs_os_file_t> *detached_handles= nullptr);
 
 /** Prepare to truncate an undo tablespace.
 @param[in]	space_id	undo tablespace id

storage/innobase/row/row0mysql.cc

@@ -3374,6 +3374,8 @@ row_drop_table_for_mysql(
 		DBUG_RETURN(DB_TABLE_NOT_FOUND);
 	}
 
+	std::vector<pfs_os_file_t> detached_handles;
+
 	const bool is_temp_name = strstr(table->name.m_name,
 					 "/" TEMP_FILE_PREFIX);
 
@@ -3757,7 +3759,8 @@ row_drop_table_for_mysql(
 		ut_ad(!filepath);
 
 		if (space->id != TRX_SYS_SPACE) {
-			err = fil_delete_tablespace(space->id);
+			err = fil_delete_tablespace(space->id, false,
+						    &detached_handles);
 		}
 		break;
 
@@ -3837,6 +3840,11 @@ row_drop_table_for_mysql(
 		row_mysql_unlock_data_dictionary(trx);
 	}
 
+	for (const auto& handle : detached_handles) {
+		ut_ad(handle != OS_FILE_CLOSED);
+		os_file_close(handle);
+	}
+
 	trx->op_info = "";
 
 	srv_wake_master_thread();