MDEV-5260 discovery with sql is too restrictive

allow ENGINE=FOOBAR in the discovering create table statement,
as long as the discovering engine is FOOBAR too

sql/table.cc

@@ -1997,7 +1997,8 @@ int TABLE_SHARE::init_from_binary_frm_image(THD *thd, bool write,
 }
 
 
-static bool sql_unusable_for_discovery(THD *thd, const char *sql)
+static bool sql_unusable_for_discovery(THD *thd, handlerton *engine,
+                                       const char *sql)
 {
   LEX *lex= thd->lex;
   HA_CREATE_INFO *create_info= &lex->create_info;
@@ -2029,7 +2030,7 @@ static bool sql_unusable_for_discovery(THD *thd, const char *sql)
   if (create_info->data_file_name || create_info->index_file_name)
     return 1;
   // ... engine
-  if (create_info->used_fields & HA_CREATE_USED_ENGINE)
+  if (create_info->db_type && create_info->db_type != engine)
     return 1;
 
   return 0;
@@ -2049,6 +2050,7 @@ int TABLE_SHARE::init_from_sql_statement_string(THD *thd, bool write,
   LEX tmp_lex;
   KEY *unused1;
   uint unused2;
+  handlerton *hton= plugin_hton(db_plugin);
   LEX_CUSTRING frm= {0,0};
 
   DBUG_ENTER("TABLE_SHARE::init_from_sql_statement_string");
@@ -2080,10 +2082,10 @@ int TABLE_SHARE::init_from_sql_statement_string(THD *thd, bool write,
   lex_start(thd);
 
   if ((error= parse_sql(thd, & parser_state, NULL) || 
-              sql_unusable_for_discovery(thd, sql_copy)))
+              sql_unusable_for_discovery(thd, hton, sql_copy)))
     goto ret;
 
-  thd->lex->create_info.db_type= plugin_hton(db_plugin);
+  thd->lex->create_info.db_type= hton;
 
   if (tabledef_version.str)
     thd->lex->create_info.tabledef_version= tabledef_version;

storage/test_sql_discovery/mysql-test/sql_discovery/simple.result

@@ -99,6 +99,10 @@ a
 select * from t2;
 ERROR 42S02: Table 'test.t2' doesn't exist
 drop table t1;
+set @@test_sql_discovery_statement='t1:create table t1 (a int) engine=test_sql_discovery';
+select * from t1;
+a
+drop table t1;
 set @@test_sql_discovery_statement='t1:
 create table t1 (
   a int not null default 5 primary key,
@@ -114,7 +118,7 @@ create table t1 (
 ) comment="abc" default character set utf8 max_rows=100 min_rows=10 checksum=1';
 show status like 'handler_discover';
 Variable_name	Value
-Handler_discover	14
+Handler_discover	15
 show create table t1;
 Table	Create Table
 t1	CREATE TABLE t1 (
@@ -132,7 +136,7 @@ t1	CREATE TABLE t1 (
 ) ENGINE=TEST_SQL_DISCOVERY DEFAULT CHARSET=utf8 MIN_ROWS=10 MAX_ROWS=100 CHECKSUM=1 COMMENT='abc'
 show status like 'handler_discover';
 Variable_name	Value
-Handler_discover	15
+Handler_discover	16
 ----
 t1.frm
 ----
@@ -143,19 +147,19 @@ select * from t1;
 a	b	c	d	e	f
 show status like 'handler_discover';
 Variable_name	Value
-Handler_discover	15
+Handler_discover	16
 flush tables;
 select * from t1;
 a	b	c	d	e	f
 show status like 'handler_discover';
 Variable_name	Value
-Handler_discover	15
+Handler_discover	16
 drop table t1;
 set @@test_sql_discovery_write_frm=0;
 set @@test_sql_discovery_statement='t1:create table t1 (a int)';
 show status like 'handler_discover';
 Variable_name	Value
-Handler_discover	15
+Handler_discover	16
 show create table t1;
 Table	Create Table
 t1	CREATE TABLE t1 (
@@ -163,7 +167,7 @@ t1	CREATE TABLE t1 (
 ) ENGINE=TEST_SQL_DISCOVERY DEFAULT CHARSET=latin1
 show status like 'handler_discover';
 Variable_name	Value
-Handler_discover	16
+Handler_discover	17
 ----
 ----
 show open tables from test;
@@ -173,14 +177,14 @@ select * from t1;
 a
 show status like 'handler_discover';
 Variable_name	Value
-Handler_discover	16
+Handler_discover	17
 flush tables;
 select * from t1;
 a
 show status like 'handler_discover';
 Variable_name	Value
-Handler_discover	17
+Handler_discover	18
 drop table t1;
 show status like 'handler_discover';
 Variable_name	Value
-Handler_discover	17
+Handler_discover	18

storage/test_sql_discovery/mysql-test/sql_discovery/simple.test

@@ -91,6 +91,10 @@ select * from t1;
 select * from t2;
 drop table t1;
 
+set @@test_sql_discovery_statement='t1:create table t1 (a int) engine=test_sql_discovery';
+select * from t1;
+drop table t1;
+
 # and something more complex
 set @@test_sql_discovery_statement='t1:
 create table t1 (