fs-util: add access_fd() which is like access() but for fds

Linux doesn't have faccess(), hence let's emulate it. Linux has access()
and faccessat() but neither allows checking the access rights of an fd
passed in directly.

src/basic/fs-util.c

@@ -104,7 +104,6 @@ int rmdir_parents(const char *path, const char *stop) {
         return 0;
 }
 
-
 int rename_noreplace(int olddirfd, const char *oldpath, int newdirfd, const char *newpath) {
         struct stat buf;
         int ret;
@@ -809,3 +808,18 @@ int chase_symlinks(const char *path, const char *original_root, unsigned flags,
 
         return exists;
 }
+
+int access_fd(int fd, int mode) {
+        char p[strlen("/proc/self/fd/") + DECIMAL_STR_MAX(fd) + 1];
+        int r;
+
+        /* Like access() but operates on an already open fd */
+
+        xsprintf(p, "/proc/self/fd/%i", fd);
+
+        r = access(p, mode);
+        if (r < 0)
+                r = -errno;
+
+        return r;
+}

src/basic/fs-util.h

@@ -98,3 +98,5 @@ static inline void unlink_and_free(char *p) {
         free(p);
 }
 DEFINE_TRIVIAL_CLEANUP_FUNC(char*, unlink_and_free);
+
+int access_fd(int fd, int mode);