linkat: Make implementation robust against system behaviour variations.

* m4/link-follow.m4 (gl_FUNC_LINK_FOLLOWS_SYMLINK): Define
LINK_FOLLOWS_SYMLINKS to -1 if it needs a runtime test in the Solaris
way, and to -2 if it needs a generic runtime test.
* lib/linkat.c (solaris_optimized_link_immediate,
solaris_optimized_link_follow): New functions.
* tests/test-linkat.c (EXPECT_LINK_HARDLINKS_SYMLINKS): New macro.
(check_same_link): Use it.

1 files changed, 39 insertions, 21 deletions

diff --git a/m4/link-follow.m4 b/m4/link-follow.m4
index 256b2a99d8..95264db261 100644
--- a/m4/link-follow.m4
+++ b/m4/link-follow.m4
@@ -1,4 +1,4 @@
-# serial 15
+# serial 16
 dnl Run a program to determine whether link(2) follows symlinks.
 dnl Set LINK_FOLLOWS_SYMLINKS accordingly.
 
@@ -12,7 +12,8 @@ dnl LINK_FOLLOWS_SYMLINKS is 0, link matches linkat(,0), and
 dnl linkat(,AT_SYMLINK_FOLLOW) requires a readlink. If it is 1,
 dnl link matches linkat(,AT_SYMLINK_FOLLOW), and there is no way
 dnl to do linkat(,0) on symlinks (on all other file types,
-dnl link() is sufficient).  If it is -1, use a runtime test.
+dnl link() is sufficient).  If it is -1, use a Solaris specific
+dnl runtime test.  If it is -2, use a generic runtime test.
 AC_DEFUN([gl_FUNC_LINK_FOLLOWS_SYMLINK],
 [dnl
   AC_CHECK_FUNCS_ONCE([readlink])
@@ -22,12 +23,27 @@ AC_DEFUN([gl_FUNC_LINK_FOLLOWS_SYMLINK],
   dnl linkat variants.  So, we set LINK_FOLLOWS_SYMLINKS to 0.
   gl_link_follows_symlinks=0 # assume GNU behavior
   if test $ac_cv_func_readlink = yes; then
-    AC_CACHE_CHECK([whether link(2) dereferences a symlink],
-                    gl_cv_func_link_follows_symlink,
-    [
-      # Create a regular file.
-      echo > conftest.file
-      AC_RUN_IFELSE([AC_LANG_SOURCE([[
+    dnl Solaris has an __xpg4 variable in libc, and it determines the
+    dnl behaviour of link(): It dereferences a symlink if and only if
+    dnl __xpg4 != 0.
+    AC_CACHE_CHECK([for __xpg4], [gl_cv_have___xpg4],
+      [AC_LINK_IFELSE(
+         [AC_LANG_PROGRAM(
+            [[extern int __xpg4;]],
+            [[return __xpg4;]])],
+         [gl_cv_have___xpg4=yes],
+         [gl_cv_have___xpg4=no])
+      ])
+    if test $gl_cv_have___xpg4 = yes; then
+      gl_link_follows_symlinks=-1
+    else
+      AC_CACHE_CHECK([whether link(2) dereferences a symlink],
+                     [gl_cv_func_link_follows_symlink],
+        [
+         # Create a regular file.
+         echo > conftest.file
+         AC_RUN_IFELSE(
+           [AC_LANG_SOURCE([[
 #       include <sys/types.h>
 #       include <sys/stat.h>
 #       include <unistd.h>
@@ -62,20 +78,22 @@ AC_DEFUN([gl_FUNC_LINK_FOLLOWS_SYMLINK],
              the link call followed the symlink.  */
           return SAME_INODE (sb_hard, sb_file) ? 1 : 0;
         }
-      ]])],
-        [gl_cv_func_link_follows_symlink=no], dnl GNU behavior
-        [gl_cv_func_link_follows_symlink=yes], dnl Followed link/compile failed
-        [gl_cv_func_link_follows_symlink=unknown] dnl We're cross compiling.
-      )
-      rm -f conftest.file conftest.sym conftest.hard
-    ])
-    case $gl_cv_func_link_follows_symlink in
-      yes) gl_link_follows_symlinks=1 ;;
-      no) ;; # already defaulted to 0
-      *) gl_link_follows_symlinks=-1 ;;
-    esac
+           ]])],
+           [gl_cv_func_link_follows_symlink=no], dnl GNU behavior
+           [gl_cv_func_link_follows_symlink=yes], dnl Followed link/compile failed
+           [gl_cv_func_link_follows_symlink=unknown] dnl We're cross compiling.
+         )
+         rm -f conftest.file conftest.sym conftest.hard
+        ])
+      case $gl_cv_func_link_follows_symlink in
+        yes) gl_link_follows_symlinks=1 ;;
+        no) ;; # already defaulted to 0
+        *) gl_link_follows_symlinks=-2 ;;
+      esac
+    fi
   fi
   AC_DEFINE_UNQUOTED([LINK_FOLLOWS_SYMLINKS], [$gl_link_follows_symlinks],
     [Define to 1 if `link(2)' dereferences symbolic links, 0 if it
-     creates hard links to symlinks, and -1 if unknown.])
+     creates hard links to symlinks, -1 if it depends on the variable __xpg4,
+     and -2 if unknown.])
 ])