bug#72547: [PATCH 4/9] 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.
---
 configure.ac       |  2 +-
 libguile/filesys.c | 13 +++++++++++--
 2 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/configure.ac b/configure.ac
index 262b171f1..e6ed14220 100644
--- a/configure.ac
+++ b/configure.ac
@@ -532,7 +532,7 @@ 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>]])
diff --git a/libguile/filesys.c b/libguile/filesys.c
index 8657fedc3..8585005a9 100644
--- a/libguile/filesys.c
+++ b/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 the 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
-- 
2.45.2