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, 6 insertions, 0 deletions

diff --git a/mysql-test/t/symlink-aria-11902.test b/mysql-test/t/symlink-aria-11902.test
new file mode 100644
index 00000000000..a2a266cbb25
--- /dev/null
+++ b/mysql-test/t/symlink-aria-11902.test
@@ -0,0 +1,6 @@
+#
+# MDEV-11902 mi_open race condition
+#
+source include/have_maria.inc;
+set default_storage_engine=Aria;
+source symlink-myisam-11902.test;