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.

mysys/my_open.c

@@ -15,9 +15,14 @@
 
 #include "mysys_priv.h"
 #include "mysys_err.h"
-#include <my_dir.h>
+#include <m_string.h>
 #include <errno.h>
 
+#if !defined(O_PATH) && defined(O_EXEC) /* FreeBSD */
+#define O_PATH O_EXEC
+#endif
+
+static int open_nosymlinks(const char *pathname, int flags, int mode);
 
 /*
   Open a file
@@ -46,7 +51,10 @@ File my_open(const char *FileName, int Flags, myf MyFlags)
 #if defined(_WIN32)
   fd= my_win_open(FileName, Flags);
 #else
-  fd = open(FileName, Flags, my_umask);
+  if (MyFlags & MY_NOSYMLINKS)
+    fd = open_nosymlinks(FileName, Flags, my_umask);
+  else
+    fd = open(FileName, Flags, my_umask);
 #endif
 
   fd= my_register_filename(fd, FileName, FILE_BY_OPEN,
@@ -174,3 +182,81 @@ void my_print_open_files(void)
 }
 
 #endif
+
+/**
+  like open(), but with symlinks are not accepted anywhere in the path
+
+  This is used for opening symlinked tables for DATA/INDEX DIRECTORY.
+  The paths there have been realpath()-ed. So, we can assume here that
+
+  * `pathname` is an absolute path
+  * no '.', '..', and '//' in the path
+  * file exists
+*/
+static int open_nosymlinks(const char *pathname, int flags, int mode)
+{
+#ifndef O_PATH
+#ifdef HAVE_REALPATH
+  char buf[PATH_MAX+1];
+  if (realpath(pathname, buf) == NULL)
+    return -1;
+  if (strcmp(pathname, buf))
+  {
+    errno= ENOTDIR;
+    return -1;
+  }
+#endif
+  return open(pathname, flags, mode | O_NOFOLLOW);
+#else
+
+  char buf[PATH_MAX+1];
+  char *s= buf, *e= buf+1, *end= strnmov(buf, pathname, sizeof(buf));
+  int fd, dfd= -1;
+
+  if (*end)
+  {
+    errno= ENAMETOOLONG;
+    return -1;
+  }
+
+  if (*s != '/') /* not an absolute path */
+  {
+    errno= ENOENT;
+    return -1;
+  }
+
+  for (;;)
+  {
+    if (*e == '/') /* '//' in the path */
+    {
+      errno= ENOENT;
+      goto err;
+    }
+    while (*e && *e != '/')
+      e++;
+    *e= 0;
+    if (!memcmp(s, ".", 2) || !memcmp(s, "..", 3))
+    {
+      errno= ENOENT;
+      goto err;
+    }
+
+    fd = openat(dfd, s, O_NOFOLLOW | (e < end ? O_PATH : flags), mode);
+    if (fd < 0)
+      goto err;
+
+    if (dfd >= 0)
+      close(dfd);
+
+    dfd= fd;
+    s= ++e;
+
+    if (e >= end)
+      return fd;
+  }
+err:
+  if (dfd >= 0)
+    close(dfd);
+  return -1;
+#endif
+}