fd-util: make fd_get_path() support AT_FDCWD

src/basic/fd-util.c

@@ -534,6 +534,11 @@ bool fdname_is_valid(const char *s) {
 int fd_get_path(int fd, char **ret) {
         int r;
 
+        assert(fd >= 0 || fd == AT_FDCWD);
+
+        if (fd == AT_FDCWD)
+                return safe_getcwd(ret);
+
         r = readlink_malloc(FORMAT_PROC_FD_PATH(fd), ret);
         if (r == -ENOENT) {
                 /* ENOENT can mean two things: that the fd does not exist or that /proc is not mounted. Let's make

src/test/test-fd-util.c

@@ -649,6 +649,11 @@ TEST(fd_get_path) {
         assert_se(safe_getcwd(&saved_cwd) >= 0);
         assert_se(chdir(t) >= 0);
 
+        assert_se(fd_get_path(AT_FDCWD, &p) >= 0);
+        assert_se(streq(p, t));
+
+        p = mfree(p);
+
         assert_se(q = path_join(t, "regular"));
         assert_se(touch(q) >= 0);
         assert_se(mkdirat_parents(tfd, "subdir/symlink", 0755) >= 0);