MDEV-11902 mi_open race condition

TOCTOU bug. The path is checked to be valid, symlinks are resolved.
Then the resolved path is opened. Between the check and the open,
there's a window when one can replace some path component with a
symlink, bypassing validity checks.

Fix: after we resolved all symlinks in the path, don't allow open()
to resolve symlinks, there should be none.

Compared to the old MyISAM/Aria code:
* fastpath. Opening of not-symlinked files is just one open(),
  no fn_format() and lstat() anymore.
* opening of symlinked tables doesn't do fn_format() and lstat() either.
  it also doesn't to realpath() (which was lstat-ing every path
  component), instead if opens every path component with O_PATH.
* share->data_file_name stores realpath(path) not readlink(path). So,
  SHOW CREATE TABLE needs to do lstat/readlink() now (see ::info()),
  and certain error messages (cannot open file "XXX") show the real
  file path with all symlinks resolved.

1 files changed, 39 insertions, 0 deletions

diff --git a/mysql-test/r/symlink-aria-11902.result b/mysql-test/r/symlink-aria-11902.result
new file mode 100644
index 00000000000..f704bb86ea7
--- /dev/null
+++ b/mysql-test/r/symlink-aria-11902.result
@@ -0,0 +1,39 @@
+set default_storage_engine=Aria;
+call mtr.add_suppression("File.*t1.* not found");
+create table mysql.t1 (a int, b char(16), index(a));
+insert mysql.t1 values (100, 'test'),(101,'test');
+create table t1 (a int, b char(16), index(a))
+data directory="MYSQLTEST_VARDIR/tmp/foo";
+insert t1 values (200, 'some'),(201,'some');
+select * from t1;
+a	b
+200	some
+201	some
+flush tables;
+set debug_sync='mi_open_datafile SIGNAL ok WAIT_FOR go';
+select * from t1;
+set debug_sync='now WAIT_FOR ok';
+set debug_sync='now SIGNAL go';
+ERROR HY000: File 'MYSQLTEST_VARDIR/tmp/foo/t1.MAD' not found (Errcode: 20)
+flush tables;
+drop table if exists t1;
+create table t1 (a int, b char(16), index (a))
+index directory="MYSQLTEST_VARDIR/tmp/foo";
+insert t1 values (200, 'some'),(201,'some');
+explain select a from t1;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	index	NULL	a	5	NULL	2	Using index
+select a from t1;
+a
+200
+201
+flush tables;
+set debug_sync='mi_open_kfile SIGNAL waiting WAIT_FOR run';
+select a from t1;
+set debug_sync='now WAIT_FOR waiting';
+set debug_sync='now SIGNAL run';
+ERROR HY000: Can't find file: 't1' (errno: 20)
+flush tables;
+drop table if exists t1;
+drop table mysql.t1;
+set debug_sync='RESET';