cleanup: change dd_frm_type() to return the engine name, not legacy_db_type

this simplifies the code and avoids unnecessary conversions back and forth.
and it works even if the engine is not installed.

sql/datadict.cc

@@ -41,23 +41,25 @@ static int read_string(File file, uchar**to, size_t length)
 
   @param[in]  thd               The current session.
   @param[in]  path              path to FRM file.
-  @param[out] dbt   db_type of the table if FRMTYPE_TABLE, otherwise undefined.
+  @param[in/out] engine_name    table engine name (length < NAME_CHAR_LEN)
+
+  engine_name is a LEX_STRING, where engine_name->str must point to
+  a buffer of at least NAME_CHAR_LEN+1 bytes.
 
   @retval  FRMTYPE_ERROR        error
   @retval  FRMTYPE_TABLE        table
   @retval  FRMTYPE_VIEW         view
 */
 
-frm_type_enum dd_frm_type(THD *thd, char *path, enum legacy_db_type *dbt)
+frm_type_enum dd_frm_type(THD *thd, char *path, LEX_STRING *engine_name)
 {
   File file;
   uchar header[10];     //"TYPE=VIEW\n" it is 10 characters
   size_t error;
   frm_type_enum type= FRMTYPE_ERROR;
+  uchar dbt;
   DBUG_ENTER("dd_frm_type");
 
-  *dbt= DB_TYPE_UNKNOWN;
-
   if ((file= mysql_file_open(key_file_frm, path, O_RDONLY | O_SHARE, MYF(0))) < 0)
     DBUG_RETURN(FRMTYPE_ERROR);
   error= mysql_file_read(file, (uchar*) header, sizeof(header), MYF(MY_NABP));
@@ -72,17 +74,24 @@ frm_type_enum dd_frm_type(THD *thd, char *path, enum legacy_db_type *dbt)
 
   type= FRMTYPE_TABLE;
 
-  /*
-    This is just a check for DB_TYPE. We'll return default unknown type
-    if the following test is true (arg #3). This should not have effect
-    on return value from this function (default FRMTYPE_TABLE)
-  */
-  if (!is_binary_frm_header(header))
+  if (!is_binary_frm_header(header) || !engine_name)
     goto err;
 
-  *dbt= (enum legacy_db_type) (uint) *(header + 3);
+  engine_name->length= 0;
+  dbt= header[3];
 
-  if (*dbt >= DB_TYPE_FIRST_DYNAMIC) /* read the true engine name */
+  /* cannot use ha_resolve_by_legacy_type without a THD */
+  if (thd && dbt < DB_TYPE_FIRST_DYNAMIC)
+  {
+    handlerton *ht= ha_resolve_by_legacy_type(thd, (enum legacy_db_type)dbt);
+    if (ht)
+    {
+      *engine_name= hton2plugin[ht->slot]->name;
+      goto err;
+    }
+  }
+
+  /* read the true engine name */
   {
     MY_STAT state;  
     uchar *frm_image= 0;
@@ -110,15 +119,10 @@ frm_type_enum dd_frm_type(THD *thd, char *path, enum legacy_db_type *dbt)
       next_chunk+= connect_string_length + 2;
       if (next_chunk + 2 < buff_end)
       {
-        uint str_db_type_length= uint2korr(next_chunk);
-        LEX_STRING name;
-        name.str= (char*) next_chunk + 2;
-        name.length= str_db_type_length;
-        plugin_ref tmp_plugin= ha_resolve_by_name(thd, &name, false);
-        if (tmp_plugin)
-          *dbt= plugin_data(tmp_plugin, handlerton *)->db_type;
-        else
-          *dbt= DB_TYPE_UNKNOWN;
+        uint len= uint2korr(next_chunk);
+        if (len <= NAME_CHAR_LEN)
+          strmake(engine_name->str, (char*)next_chunk + 2,
+                  engine_name->length= len);
       }
     }
 

sql/datadict.h

@@ -35,12 +35,11 @@ enum frm_type_enum
   Prefer to use ha_table_exists() instead.
   To check whether it's an frm of a view, use dd_frm_is_view().
 */
-frm_type_enum dd_frm_type(THD *thd, char *path, enum legacy_db_type *dbt);
+frm_type_enum dd_frm_type(THD *thd, char *path, LEX_STRING *engine_name);
 
 static inline bool dd_frm_is_view(THD *thd, char *path)
 {
-  enum legacy_db_type not_used;
-  return dd_frm_type(thd, path, &not_used) == FRMTYPE_VIEW;
+  return dd_frm_type(thd, path, NULL) == FRMTYPE_VIEW;
 }
 
 bool dd_recreate_table(THD *thd, const char *db, const char *table_name,

sql/handler.cc

@@ -5025,14 +5025,16 @@ bool ha_table_exists(THD *thd, const char *db, const char *table_name,
     bool exists= true;
     if (hton)
     {
-      enum legacy_db_type db_type;
-      if (dd_frm_type(thd, path, &db_type) != FRMTYPE_VIEW)
+      char engine_buf[NAME_CHAR_LEN + 1];
+      LEX_STRING engine= { engine_buf, 0 };
+
+      if (dd_frm_type(thd, path, &engine) != FRMTYPE_VIEW)
       {
-        handlerton *ht= ha_resolve_by_legacy_type(thd, db_type);
-        if ((*hton= ht))
+        plugin_ref p=  plugin_lock_by_name(thd, &engine,  MYSQL_STORAGE_ENGINE_PLUGIN);
+        *hton= p ? plugin_hton(p) : NULL;
+        if (*hton)
           // verify that the table really exists
-          exists= discover_existence(thd,
-                             plugin_int_to_ref(hton2plugin[ht->slot]), &args);
+          exists= discover_existence(thd, p, &args);
       }
       else
         *hton= view_pseudo_hton;

sql/sql_plugin.cc

@@ -1646,10 +1646,11 @@ int plugin_init(int *argc, char **argv, int flags)
     {
       char path[FN_REFLEN + 1];
       build_table_filename(path, sizeof(path) - 1, "mysql", "plugin", reg_ext, 0);
-      enum legacy_db_type db_type;
-      frm_type_enum frm_type= dd_frm_type(NULL, path, &db_type);
+      char engine_name_buf[NAME_CHAR_LEN + 1];
+      LEX_STRING maybe_myisam= { engine_name_buf, 0 };
+      frm_type_enum frm_type= dd_frm_type(NULL, path, &maybe_myisam);
       /* if mysql.plugin table is MyISAM - load it right away */
-      if (frm_type == FRMTYPE_TABLE && db_type == DB_TYPE_MYISAM)
+      if (frm_type == FRMTYPE_TABLE && !strcasecmp(maybe_myisam.str, "MyISAM"))
       {
         plugin_load(&tmp_root);
         flags|= PLUGIN_INIT_SKIP_PLUGIN_TABLE;