Make file-accessible-directory-p reliable on MS-Windows

* src/w32.c (w32_accessible_directory_p): New function.
* src/w32.h (w32_accessible_directory_p): Add prototype.
* src/fileio.c (file_accessible_directory_p) [WINDOWSNT]: Call
w32_accessible_directory_p to test a directory for accessibility
by the current user.  (Bug#21346)
(Ffile_accessible_directory_p): Remove the w32 specific caveat
from the doc string.

1 files changed, 13 insertions, 9 deletions

diff --git a/src/fileio.c b/src/fileio.c
index debd1f30a4f..a36dfbcfa36 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -2655,11 +2655,7 @@ and the directory must allow you to open files in it.  In order to use a
 directory as a buffer's current directory, this predicate must return true.
 A directory name spec may be given instead; then the value is t
 if the directory so specified exists and really is a readable and
-searchable directory.
-
-The result might be a false positive on MS-Windows in some rare cases,
-i.e., this function could return t for a directory that is not
-accessible by the current user.  */)
+searchable directory.  */)
   (Lisp_Object filename)
 {
   Lisp_Object absname;
@@ -2689,10 +2685,18 @@ bool
 file_accessible_directory_p (Lisp_Object file)
 {
 #ifdef DOS_NT
-  /* There's no need to test whether FILE is searchable, as the
-     searchable/executable bit is invented on DOS_NT platforms.  */
+# ifdef WINDOWSNT
+  /* We need a special-purpose test because (a) NTFS security data is
+     not reflected in Posix-style mode bits, and (b) the trick with
+     accessing "DIR/.", used below on Posix hosts, doesn't work on
+     Windows, because "DIR/." is normalized to just "DIR" before
+     hitting the disk.  */
+  return (SBYTES (file) == 0
+          || w32_accessible_directory_p (SSDATA (file), SBYTES (file)));
+# else  /* MSDOS */
   return file_directory_p (SSDATA (file));
-#else
+# endif  /* MSDOS */
+#else    /* !DOS_NT */
   /* On POSIXish platforms, use just one system call; this avoids a
      race and is typically faster.  */
   const char *data = SSDATA (file);
@@ -2725,7 +2729,7 @@ file_accessible_directory_p (Lisp_Object file)
   SAFE_FREE ();
   errno = saved_errno;
   return ok;
-#endif
+#endif  /* !DOS_NT */
 }
 
 DEFUN ("file-regular-p", Ffile_regular_p, Sfile_regular_p, 1, 1, 0,