load_db_opt was always doing a file access if db.opt file did not exist

Added caching of database directories that did not have a db.opt file.
This was common for older MariaDB installaiton or if a user created
a database with 'mkdir'.

Other things:
- Give a note "no db.opt file" if one uses SHOW CREATE DATABASE one
  a database without a db.opt file.

sql/sql_db.cc

@@ -536,36 +536,53 @@ static bool write_db_opt(THD *thd, const char *path,
 
   DESCRIPTION
 
+  create->default_table_charset is guaranteed to be alway set
+  Required by some callers
+
   RETURN VALUES
   0	File found
-  1	No database file or could not open it
-
+  -1	No database file (file was not found or 'empty' file was cached)
+  1     Could not open it
 */
 
-bool load_db_opt(THD *thd, const char *path, Schema_specification_st *create)
+int load_db_opt(THD *thd, const char *path, Schema_specification_st *create)
 {
   File file;
   char buf[256+DATABASE_COMMENT_MAXLEN];
   DBUG_ENTER("load_db_opt");
-  bool error=1;
+  int error= 0;
   size_t nbytes;
   myf utf8_flag= thd->get_utf8_flag();
 
   bzero((char*) create,sizeof(*create));
-  create->default_table_charset= thd->variables.collation_server;
 
   /* Check if options for this database are already in the hash */
   if (!get_dbopt(thd, path, create))
-    DBUG_RETURN(0);
+  {
+    if (!create->default_table_charset)
+      error= -1;                                // db.opt did not exists
+    goto err1;
+  }
 
   /* Otherwise, load options from the .opt file */
   if ((file= mysql_file_open(key_file_dbopt,
                              path, O_RDONLY | O_SHARE, MYF(0))) < 0)
+  {
+    /*
+      Create an empty entry, to avoid doing an extra file open for every create
+      table.
+    */
+    put_dbopt(path, create);
+    error= -1;
     goto err1;
+  }
 
   IO_CACHE cache;
   if (init_io_cache(&cache, file, IO_SIZE, READ_CACHE, 0, 0, MYF(0)))
-    goto err2;
+  {
+    error= 1;
+    goto err2;                                  // Not cached
+  }
 
   while ((int) (nbytes= my_b_gets(&cache, (char*) buf, sizeof(buf))) > 0)
   {
@@ -586,7 +603,7 @@ bool load_db_opt(THD *thd, const char *path, Schema_specification_st *create)
            default-collation commands.
         */
         if (!(create->default_table_charset=
-        get_charset_by_csname(pos+1, MY_CS_PRIMARY, MYF(utf8_flag))) &&
+              get_charset_by_csname(pos+1, MY_CS_PRIMARY, MYF(utf8_flag))) &&
             !(create->default_table_charset=
               get_charset_by_name(pos+1, MYF(utf8_flag))))
         {
@@ -621,10 +638,11 @@ bool load_db_opt(THD *thd, const char *path, Schema_specification_st *create)
 err2:
   mysql_file_close(file, MYF(0));
 err1:
+  if (!create->default_table_charset)           // In case of error
+    create->default_table_charset= thd->variables.collation_server;
   DBUG_RETURN(error);
 }
 
-
 /*
   Retrieve database options by name. Load database options file or fetch from
   cache.
@@ -651,11 +669,12 @@ err1:
     db_create_info right after that.
 
   RETURN VALUES (read NOTE!)
-    FALSE   Success
-    TRUE    Failed to retrieve options
+  0	File found
+  -1	No database file (file was not found or 'empty' file was cached)
+  1     Could not open it
 */
 
-bool load_db_opt_by_name(THD *thd, const char *db_name,
+int load_db_opt_by_name(THD *thd, const char *db_name,
                          Schema_specification_st *db_create_info)
 {
   char db_opt_path[FN_REFLEN + 1];
@@ -1951,8 +1970,7 @@ bool mysql_upgrade_db(THD *thd, const LEX_CSTRING *old_db)
 
   build_table_filename(path, sizeof(path)-1,
                        old_db->str, "", MY_DB_OPT_FILE, 0);
-  if ((load_db_opt(thd, path, &create_info)))
-    create_info.default_table_charset= thd->variables.collation_server;
+  load_db_opt(thd, path, &create_info);
 
   length= build_table_filename(path, sizeof(path)-1, old_db->str, "", "", 0);
   if (length && path[length-1] == FN_LIBCHAR)

sql/sql_db.h

@@ -37,8 +37,8 @@ bool mysql_opt_change_db(THD *thd,
 bool my_dboptions_cache_init(void);
 void my_dboptions_cache_free(void);
 bool check_db_dir_existence(const char *db_name);
-bool load_db_opt(THD *thd, const char *path, Schema_specification_st *create);
-bool load_db_opt_by_name(THD *thd, const char *db_name,
+int load_db_opt(THD *thd, const char *path, Schema_specification_st *create);
+int load_db_opt_by_name(THD *thd, const char *db_name,
                          Schema_specification_st *db_create_info);
 CHARSET_INFO *get_default_db_collation(THD *thd, const char *db_name);
 bool my_dbopt_init(void);

sql/sql_show.cc

@@ -1435,7 +1435,14 @@ bool mysqld_show_create_db(THD *thd, LEX_CSTRING *dbname,
       DBUG_RETURN(TRUE);
     }
 
-    load_db_opt_by_name(thd, dbname->str, &create);
+    if (load_db_opt_by_name(thd, dbname->str, &create) < 0)
+    {
+      push_warning_printf(thd, Sql_condition::WARN_LEVEL_NOTE,
+                          ER_UNKNOWN_ERROR,
+                          "Database '%.192s' does not have a db.opt file. "
+                          "You can create one with ALTER DATABASE if needed",
+                          dbname->str);
+    }
   }
 
   mysqld_show_create_db_get_fields(thd, &field_list);