MDEV-23705 Assertion 'table->data_dir_path || !space'

After DISCARD TABLESPACE, the tablespace of a table will no longer
exist, and dict_get_and_save_data_dir_path() would invoke
dict_get_first_path() to read an entry from SYS_DATAFILES.
For some reason, DISCARD TABLESPACE would not to remove the entry
from there.

dict_get_and_save_data_dir_path(): If the tablespace has been
discarded, do not bother trying to read the name.

Side note: The tables SYS_TABLESPACES and SYS_DATAFILES are
redundant and subject to removal in MDEV-22343.

mysql-test/suite/innodb/r/truncate.result

@@ -39,3 +39,14 @@ TRUNCATE t1;
 SELECT * FROM t1;
 a
 DROP TEMPORARY TABLE t1;
+#
+# MDEV-23705 Assertion 'table->data_dir_path || !space'
+#
+CREATE TABLE t(c INT) ENGINE=InnoDB;
+ALTER TABLE t DISCARD TABLESPACE;
+RENAME TABLE t TO u;
+TRUNCATE u;
+Warnings:
+Warning	1814	Tablespace has been discarded for table `u`
+TRUNCATE u;
+DROP TABLE u;

mysql-test/suite/innodb/t/truncate.test

@@ -50,3 +50,13 @@ INSERT INTO t1 VALUES(1);
 TRUNCATE t1;
 SELECT * FROM t1;
 DROP TEMPORARY TABLE t1;
+
+--echo #
+--echo # MDEV-23705 Assertion 'table->data_dir_path || !space'
+--echo #
+CREATE TABLE t(c INT) ENGINE=InnoDB;
+ALTER TABLE t DISCARD TABLESPACE;
+RENAME TABLE t TO u;
+TRUNCATE u;
+TRUNCATE u;
+DROP TABLE u;

storage/innobase/dict/dict0load.cc

@@ -2739,7 +2739,8 @@ dict_get_and_save_data_dir_path(
 {
 	ut_ad(!dict_table_is_temporary(table));
 
-	if (!table->data_dir_path && table->space) {
+	if (!table->data_dir_path && table->space
+	    && !dict_table_is_discarded(table)) {
 		char*	path = fil_space_get_first_path(table->space);
 
 		if (!dict_mutex_own) {