link: work around IRIX bug

On IRIX 6.5, link(file, "dangling") creates the target of dangling
as a link to file, rather than failing with EEXIST.

* m4/link.m4 (gl_FUNC_LINK): Expose the bug.
* lib/link.c (rpl_link): Work around it.
* tests/test-link.h (test_link): Enhance test.
* doc/posix-functions/link.texi (link): Document the bug.

Signed-off-by: Eric Blake <eblake@redhat.com>

ChangeLog

@@ -1,5 +1,11 @@
 2011-06-22  Eric Blake  <eblake@redhat.com>
 
+	link: work around IRIX bug
+	* m4/link.m4 (gl_FUNC_LINK): Expose the bug.
+	* lib/link.c (rpl_link): Work around it.
+	* tests/test-link.h (test_link): Enhance test.
+	* doc/posix-functions/link.texi (link): Document the bug.
+
 	getopt: silence clang warning
 	* lib/getopt.c (_getopt_internal_r): Avoid unlikely NULL
 	dereference.

doc/posix-functions/link.texi

@@ -15,6 +15,10 @@ mingw.
 This function fails to reject trailing slashes on non-directories on
 some platforms:
 FreeBSD 7.2, Solaris 11 2010-11, Cygwin 1.5.x.
+@item
+When the second argument is a dangling symlink, some platforms follow
+that link and create the destination rather than failing:
+IRIX 6.5.
 @end itemize
 
 Portability problems not fixed by Gnulib:

lib/link.c

@@ -155,9 +155,20 @@ link (const char *file1, const char *file2)
 int
 rpl_link (char const *file1, char const *file2)
 {
+  size_t len1;
+  size_t len2;
+  struct stat st;
+
+  /* Don't allow IRIX to dereference dangling file2 symlink.  */
+  if (!lstat (file2, &st))
+    {
+      errno = EEXIST;
+      return -1;
+    }
+
   /* Reject trailing slashes on non-directories.  */
-  size_t len1 = strlen (file1);
-  size_t len2 = strlen (file2);
+  len1 = strlen (file1);
+  len2 = strlen (file2);
   if ((len1 && file1[len1 - 1] == '/')
       || (len2 && file2[len2 - 1] == '/'))
     {
@@ -165,7 +176,6 @@ rpl_link (char const *file1, char const *file2)
          If stat() fails, then link() should fail for the same reason
          (although on Solaris 9, link("file/","oops") mistakenly
          succeeds); if stat() succeeds, require a directory.  */
-      struct stat st;
       if (stat (file1, &st))
         return -1;
       if (!S_ISDIR (st.st_mode))
@@ -178,7 +188,6 @@ rpl_link (char const *file1, char const *file2)
     {
       /* Fix Cygwin 1.5.x bug where link("a","b/.") creates file "b".  */
       char *dir = strdup (file2);
-      struct stat st;
       char *p;
       if (!dir)
         return -1;

m4/link.m4

@@ -1,4 +1,4 @@
-# link.m4 serial 6
+# link.m4 serial 7
 dnl Copyright (C) 2009-2011 Free Software Foundation, Inc.
 dnl This file is free software; the Free Software Foundation
 dnl gives unlimited permission to copy and/or distribute it,
@@ -11,7 +11,7 @@ AC_DEFUN([gl_FUNC_LINK],
   if test $ac_cv_func_link = no; then
     HAVE_LINK=0
   else
-    AC_CACHE_CHECK([whether link handles trailing slash correctly],
+    AC_CACHE_CHECK([whether link obeys POSIX],
       [gl_cv_func_link_works],
       [touch conftest.a
        # Assume that if we have lstat, we can also check symlinks.
@@ -28,6 +28,10 @@ AC_DEFUN([gl_FUNC_LINK],
 #if HAVE_LSTAT
              if (!link ("conftest.lnk/", "conftest.b"))
                result |= 2;
+             if (rename ("conftest.a", "conftest.b"))
+               result |= 4;
+             if (!link ("conftest.b", "conftest.lnk"))
+               result |= 8;
 #endif
              return result;
            ]])],

tests/test-link.h

@@ -166,6 +166,9 @@ test_link (int (*func) (char const *, char const *), bool print)
   ASSERT (func (BASE "b", BASE "link/") == -1);
   ASSERT (errno == ENOTDIR || errno == ENOENT || errno == EEXIST
           || errno == EINVAL);
+  errno = 0;
+  ASSERT (func (BASE "b", BASE "link") == -1);
+  ASSERT (errno == EEXIST);
   ASSERT (rename (BASE "b", BASE "a") == 0);
   errno = 0;
   ASSERT (func (BASE "link/", BASE "b") == -1);