MDEV-23569 temporary tables can overwrite existing files

for internal temporary tables: don't use realpath(),
and let them overwrite whatever orphan temp files might've
left in the tmpdir (see main.error_simulation test).

for user created temporary tables: we have to use realpath(),
(see 3a726ab, remember DATA/INDEX DIRECTORY). don't allow
them to overwrite existing files.

This bug was reported by RACK911 LABS

mysql-test/r/temp_table_symlink.result

@@ -0,0 +1,13 @@
+create table d1 (a int);
+create temporary table t1 (a int);
+create temporary table t2 (a int);
+Got one of the listed errors
+create temporary table t3 (a int) engine=Aria;
+Got one of the listed errors
+select * from information_schema.columns where table_schema='test';
+Got one of the listed errors
+flush tables;
+select * from d1;
+a
+drop temporary table t1;
+drop table d1;

mysql-test/t/temp_table_symlink.test

@@ -0,0 +1,32 @@
+source include/not_windows.inc;
+
+#
+# MDEV-23569 temporary tables can overwrite existing files
+#
+
+let datadir=`select @@datadir`;
+create table d1 (a int);
+create temporary table t1 (a int);
+perl;
+chdir "$ENV{MYSQL_TMP_DIR}/mysqld.1/";
+for (<#sql*.MYI>) {
+  /^#sql(.*)_([0-9a-f]+_)([0-9a-f]+)\.MYI$/ or die;
+  symlink "$ENV{datadir}/test/d1.MYI", sprintf "#sql$1_$2%x.MYI", hex($3)+1;
+  symlink "$ENV{datadir}/test/d1.MYI", sprintf "#sql$1_$2%x.MAI", hex($3)+1;
+  symlink "$ENV{datadir}/test/d1.MYI", sprintf "#sql$1_$2%x.MAI", hex($3)+2;
+  symlink "$ENV{datadir}/test/d1.MYI", "#sql_$1_0.MAI";
+}
+EOF
+
+error 1,1030;
+create temporary table t2 (a int);
+error 1,1030;
+create temporary table t3 (a int) engine=Aria;
+error 1,1030;
+select * from information_schema.columns where table_schema='test';
+
+flush tables;
+select * from d1;
+drop temporary table t1;
+remove_files_wildcard $MYSQL_TMP_DIR/mysqld.1 *sql*;
+drop table d1;

storage/maria/ma_create.c

@@ -322,7 +322,7 @@ int maria_create(const char *name, enum data_file_type datafile_type,
   {
     options|= HA_OPTION_TMP_TABLE;
     tmp_table= TRUE;
-    create_mode|= O_NOFOLLOW;
+    create_mode|= O_NOFOLLOW | (internal_table ? 0 : O_EXCL);
     /* "CREATE TEMPORARY" tables are not crash-safe (dropped at restart) */
     ci->transactional= FALSE;
     flags&= ~HA_CREATE_PAGE_CHECKSUM;
@@ -887,8 +887,8 @@ int maria_create(const char *name, enum data_file_type datafile_type,
   {
     char *iext= strrchr(name, '.');
     int have_iext= iext && !strcmp(iext, MARIA_NAME_IEXT);
-    fn_format(kfilename, name, "", MARIA_NAME_IEXT,
-              MY_UNPACK_FILENAME | MY_RETURN_REAL_PATH |
+    fn_format(kfilename, name, "", MARIA_NAME_IEXT, MY_UNPACK_FILENAME |
+              (internal_table ? 0 : MY_RETURN_REAL_PATH) |
               (have_iext ? MY_REPLACE_EXT : MY_APPEND_EXT));
     klinkname_ptr= NullS;
     /*

storage/myisam/mi_create.c

@@ -184,7 +184,7 @@ int mi_create(const char *name,uint keys,MI_KEYDEF *keydefs,
   if (flags & HA_CREATE_TMP_TABLE)
   {
     options|= HA_OPTION_TMP_TABLE;
-    create_mode|= O_NOFOLLOW;
+    create_mode|= O_NOFOLLOW | (internal_table ? 0 : O_EXCL);
   }
   if (flags & HA_CREATE_CHECKSUM || (options & HA_OPTION_CHECKSUM))
   {
@@ -618,8 +618,8 @@ int mi_create(const char *name,uint keys,MI_KEYDEF *keydefs,
   {
     char *iext= strrchr(name, '.');
     int have_iext= iext && !strcmp(iext, MI_NAME_IEXT);
-    fn_format(kfilename, name, "", MI_NAME_IEXT,
-              MY_UNPACK_FILENAME | MY_RETURN_REAL_PATH |
+    fn_format(kfilename, name, "", MI_NAME_IEXT, MY_UNPACK_FILENAME |
+              (internal_table ? 0 : MY_RETURN_REAL_PATH) |
               (have_iext ? MY_REPLACE_EXT : MY_APPEND_EXT));
     klinkname_ptr= 0;
     /* Replace the current file */