From 2fb0446d5b4f410608adcfe3297338bdeef23abe Mon Sep 17 00:00:00 2001
From: Maxime Devos <maximedevos@telenet.be>
Date: Wed, 10 Mar 2021 12:07:01 +0100
Subject: [PATCH 03/17] =?UTF-8?q?Allow=20file=20ports=20in=20=E2=80=98read?=
 =?UTF-8?q?link=E2=80=99.?=
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* configure.ac: Detect whether ‘readlinkat’ is defined.
* libguile/filesys.c (scm_readlink): Support file ports
  when ‘readlinkat’ exists.
* doc/ref/posix.texi (File System): Document it.
---
 configure.ac       |  2 +-
 doc/ref/posix.texi |  3 ++-
 libguile/filesys.c | 45 ++++++++++++++++++++++++++++++++++++---------
 3 files changed, 39 insertions(+), 11 deletions(-)

diff --git a/configure.ac b/configure.ac
index a23249d95..0f032272a 100644
--- a/configure.ac
+++ b/configure.ac
@@ -481,7 +481,7 @@ AC_CHECK_HEADERS([assert.h crt_externs.h])
 #   sendfile - non-POSIX, found in glibc
 #
 AC_CHECK_FUNCS([DINFINITY DQNAN cexp chsize clog clog10 ctermid         \
-  fesetround ftime ftruncate fchown fchmod fchdir			\
+  fesetround ftime ftruncate fchown fchmod fchdir readlinkat		\
   getcwd geteuid getsid							\
   gettimeofday getuid getgid gmtime_r ioctl lstat mkdir mkdtemp mknod   \
   nice readlink rename rmdir setegid seteuid                            \
diff --git a/doc/ref/posix.texi b/doc/ref/posix.texi
index 54c564cb0..68de5a2ed 100644
--- a/doc/ref/posix.texi
+++ b/doc/ref/posix.texi
@@ -795,7 +795,8 @@ file it points to.  @var{path} must be a string.
 @deffn {Scheme Procedure} readlink path
 @deffnx {C Function} scm_readlink (path)
 Return the value of the symbolic link named by @var{path} (a
-string), i.e., the file that the link points to.
+string, or a port if the @code{readlinkat} system call is supported),
+i.e., the file that the link points to.
 @end deffn
 
 @findex fchown
diff --git a/libguile/filesys.c b/libguile/filesys.c
index a8879f0e1..06a550fc8 100644
--- a/libguile/filesys.c
+++ b/libguile/filesys.c
@@ -1081,10 +1081,25 @@ SCM_DEFINE (scm_symlink, "symlink", 2, 0, 0,
 #undef FUNC_NAME
 #endif /* HAVE_SYMLINK */
 
-SCM_DEFINE (scm_readlink, "readlink", 1, 0, 0, 
+/* Static helper function for choosing between readlink
+   and readlinkat. */
+static int
+do_readlink (int fd, const char *c_path, char *buf, size_t size)
+{
+#ifdef HAVE_READLINKAT
+  if (fd != -1)
+    return readlinkat (fd, c_path, buf, size);
+#else
+  (void) fd;
+#endif
+  return readlink (c_path, buf, size);
+}
+
+SCM_DEFINE (scm_readlink, "readlink", 1, 0, 0,
             (SCM path),
-	    "Return the value of the symbolic link named by @var{path} (a\n"
-	    "string), i.e., the file that the link points to.")
+            "Return the value of the symbolic link named by @var{path} (a\n"
+            "string, or a port if the @code{readlinkat} system call is supported),\n"
+            "i.e., the file that the link points to.")
 #define FUNC_NAME s_scm_readlink
 {
   int rv;
@@ -1092,20 +1107,32 @@ SCM_DEFINE (scm_readlink, "readlink", 1, 0, 0,
   char *buf;
   SCM result;
   char *c_path;
-  
-  scm_dynwind_begin (0);
-
-  c_path = scm_to_locale_string (path);
-  scm_dynwind_free (c_path);
+  int fdes;
 
+  scm_dynwind_begin (0);
+#ifdef HAVE_READLINKAT
+  if (SCM_FPORTP (path))
+    {
+      SCM_VALIDATE_OPINPORT (SCM_ARG1, path);
+      c_path = "";
+      fdes = SCM_FPORT_FDES (path);
+    }
+  else
+#endif
+    {
+      fdes = -1;
+      c_path = scm_to_locale_string (path);
+      scm_dynwind_free (c_path);
+    }
   buf = scm_malloc (size);
 
-  while ((rv = readlink (c_path, buf, size)) == size)
+  while ((rv = do_readlink (fdes, c_path, buf, size)) == size)
     {
       free (buf);
       size *= 2;
       buf = scm_malloc (size);
     }
+  scm_remember_upto_here_1 (path);
   if (rv == -1)
     {
       int save_errno = errno;
-- 
2.30.2