faccessat: port to macOS (Bug#29231)

macOS faccessat has the same bug that lstat does: if the file
name ends in '/' it ignores the trailing slash.
Problem reported for Emacs by Vincent Zhang.
* doc/posix-functions/faccessat.texi (faccessat): Document this.
* lib/faccessat.c (_GL_INCLUDING_UNISTD_H): Define and undef
around the initial includes.  Include errno.h, string.h, sys/stat.h.
(orig_faccessat) [HAVE_FACCESSAT]: New function.
Include "unistd.h" after defining it.
(rpl_faccessat) [HAVE_FACCESSAT]: New implementation.
* lib/unistd.in.h (faccessat) [REPLACE_FACCESSAT]:
Handle in the usual way.
* m4/faccessat.m4 (gl_FUNC_FACCESSAT): Replace faccessat if
lstat dereferences symlinks, since faccessat is likely to
have the same problem.
* m4/unistd_h.m4 (gl_UNISTD_H_DEFAULTS): Default REPLACE_ACCESSAT.
* modules/faccessat (Depends-on): Add fstatat.
Depend if REPLACE_FACCESSAT is 1, too.
(configure.ac): Link if REPLACE_FACCESSAT is 1.
* modules/faccessat-tests (Depends-on): Add symlink.
* modules/unistd (unistd.h): Substitute REPLACE_FACCESSAT.
* tests/test-faccessat.c (main): Test for the bug.

1 files changed, 53 insertions, 7 deletions

diff --git a/lib/faccessat.c b/lib/faccessat.c
index 6cf9c99df2..fa9235820d 100644
--- a/lib/faccessat.c
+++ b/lib/faccessat.c
@@ -16,10 +16,31 @@
 
 /* written by Eric Blake */
 
+/* If the user's config.h happens to include <unistd.h>, let it include only
+   the system's <unistd.h> here, so that orig_faccessat doesn't recurse to
+   rpl_faccessat.  */
+#define _GL_INCLUDING_UNISTD_H
 #include <config.h>
 
 #include <unistd.h>
+#include <errno.h>
 #include <fcntl.h>
+#include <string.h>
+#include <sys/stat.h>
+#undef _GL_INCLUDING_UNISTD_H
+
+#if HAVE_FACCESSAT
+static int
+orig_faccessat (int fd, char const *name, int mode, int flag)
+{
+  return faccessat (fd, name, mode, flag);
+}
+#endif
+
+/* Write "unistd.h" here, not <unistd.h>, otherwise OSF/1 5.1 DTK cc
+   eliminates this include because of the preliminary #include <unistd.h>
+   above.  */
+#include "unistd.h"
 
 #ifndef HAVE_ACCESS
 /* Mingw lacks access, but it also lacks real vs. effective ids, so
@@ -28,6 +49,29 @@
 # define access euidaccess
 #endif
 
+#if HAVE_FACCESSAT
+
+int
+rpl_faccessat (int fd, char const *file, int mode, int flag)
+{
+  int result = orig_faccessat (fd, file, mode, flag);
+
+  if (result == 0 && file[strlen (file) - 1] == '/')
+    {
+      struct stat st;
+      result = fstatat (fd, file, &st, 0);
+      if (result == 0 && !S_ISDIR (st.st_mode))
+        {
+          errno = ENOTDIR;
+          return -1;
+        }
+    }
+
+  return result;
+}
+
+#else /* !HAVE_FACCESSAT */
+
 /* Invoke access or euidaccess on file, FILE, using mode MODE, in the directory
    open on descriptor FD.  If possible, do it without changing the
    working directory.  Otherwise, resort to using save_cwd/fchdir, then
@@ -36,10 +80,12 @@
    Note that this implementation only supports AT_EACCESS, although some
    native versions also support AT_SYMLINK_NOFOLLOW.  */
 
-#define AT_FUNC_NAME faccessat
-#define AT_FUNC_F1 euidaccess
-#define AT_FUNC_F2 access
-#define AT_FUNC_USE_F1_COND AT_EACCESS
-#define AT_FUNC_POST_FILE_PARAM_DECLS , int mode, int flag
-#define AT_FUNC_POST_FILE_ARGS        , mode
-#include "at-func.c"
+# define AT_FUNC_NAME faccessat
+# define AT_FUNC_F1 euidaccess
+# define AT_FUNC_F2 access
+# define AT_FUNC_USE_F1_COND AT_EACCESS
+# define AT_FUNC_POST_FILE_PARAM_DECLS , int mode, int flag
+# define AT_FUNC_POST_FILE_ARGS        , mode
+# include "at-func.c"
+
+#endif