filesys.c: Fix readlink for ports on Darwin.

When passed a port, `readlink' relies on the Linux specific behavior of
empty c_path meaning "the fd itself".  That does not work on Darwin.
Since there is no branch that would yield both fd and c_path, fallback
to freadlink when __APPLE__ is defined.

* libguile/filesys.c (do_readlink): Call freadlink for !__APPLE__.
* configure.ac (AC_CHECK_FUNCS): Add freadlink.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

configure.ac

@@ -556,7 +556,7 @@ AC_CHECK_FUNCS([DINFINITY DQNAN cexp chsize clog clog10 ctermid         \
   fstatat futimens openat						\
   sched_getaffinity sched_setaffinity sendfile pipe2			\
   posix_spawn_file_actions_addclosefrom_np                              \
-  clearenv])
+  clearenv freadlink])
 
 # The newlib C library uses _NL_ prefixed locale langinfo constants.
 AC_CHECK_DECLS([_NL_NUMERIC_GROUPING], [], [], [[#include <langinfo.h>]])

libguile/filesys.c

@@ -1192,14 +1192,23 @@ SCM_DEFINE (scm_symlinkat, "symlinkat", 3, 0, 0,
 #undef FUNC_NAME
 #endif /* HAVE_SYMLINKAT */
 
-/* Static helper function for choosing between readlink
+/* Static helper function for choosing between readlink, freadlink,
    and readlinkat. */
 static int
 do_readlink (int fd, const char *c_path, char *buf, size_t size)
 {
-#ifdef HAVE_READLINKAT
+/* Darwin does not accept empty c_path. */
+#if HAVE_READLINKAT && !__APPLE__
   if (fd != -1)
     return readlinkat (fd, c_path, buf, size);
+#elif HAVE_FREADLINK
+  /* There is no branch in s_scm_readlink that would lead to having both
+     FD and non-empty C_PATH.  Therefore if FD is set, we (on Darwin
+     only) use freadlink and ignore C_PATH.  On Linux this case is
+     already handled by readlinkat, but Darwin does not understand empty
+     C_PATH to mean "the fd itself" the way Linux does.  */
+  if (fd != -1)
+    return freadlink (fd, buf, size);
 #else
   (void) fd;
 #endif