Re: unlink test failure on MacOS X

[Bruno Haible]

Jim Meyering wrote:
> > +              (umask 077 && mkdir "$tmp")
> > +            }; then
> > +           mkdir "$tmp/subdir"
> > +           export tmp
> > +           AC_RUN_IFELSE(
> > +             [AC_LANG_SOURCE([[
> > +                #include <stdlib.h>
> > +                #include <unistd.h>
> > +                int main ()
> > +                {
> > +                  if (chdir (getenv ("tmp")) != 0)
> > +                    return 1;
> > +                  return unlink ("..") == 0;
> > +                }
> 
> This all looks fine.
> The only question I had was why bother to create "$tmp/subdir".
> At first I though it was to ensure $tmp is not empty,

Oops, the test program was supposed to chdir into $tmp/subdir, not $tmp.

Then, the directory that is argument of unlink() is non-empty anyway.
The reason for the subdir is:
  1) If the test is being run as root, possibly on a file system where
     unlink() of a directory actually works, I find it preferable to remove
     a directory that contains only an empty subdirectory, rather than
     removing the entire /tmp with all its contents.
  2) The MacOS X kernel code indicates that things work differently when
     the VFS node is sticky. I don't know whether it's the same or a different
     meaning of "sticky". But since /tmp always the sticky bit set, it's better
     to avoid it.
  3) /tmp is not owned by the current user, so unlinking it may fail for another
     reason.
  4) It's also imagineable that /tmp be a mount point (although on the machine
     I use, it's not). Again, unlink() may behave differently on a mount point
     than on a regular directory.

Thanks for the review!

Bruno


2010-03-20  Bruno Haible  <address@hidden>

        * m4/unlink.m4 (gl_FUNC_UNLINK): Fix last commit.
        Reported by Jim Meyering.

--- m4/unlink.m4.orig   Sat Mar 20 17:46:37 2010
+++ m4/unlink.m4        Sat Mar 20 17:45:52 2010
@@ -36,6 +36,11 @@
     [gl_cv_func_unlink_parent_fails],
     [case "$host_os" in
        darwin*)
+         dnl Try to unlink a subdirectory of /tmp, because /tmp is usually on a
+         dnl HFS mount on MacOS X. Use a subdirectory, owned by the current
+         dnl user, because otherwise unlink() may fail due to permissions
+         dnl reasons, and because when running as root we don't want to risk
+         dnl destroying the entire /tmp.
          if {
               # Use the mktemp program if available. If not available, hide 
the error
               # message.
@@ -52,14 +57,15 @@
               (umask 077 && mkdir "$tmp")
             }; then
            mkdir "$tmp/subdir"
-           export tmp
+           GL_SUBDIR_FOR_UNLINK="$tmp/subdir"
+           export GL_SUBDIR_FOR_UNLINK
            AC_RUN_IFELSE(
              [AC_LANG_SOURCE([[
                 #include <stdlib.h>
                 #include <unistd.h>
                 int main ()
                 {
-                  if (chdir (getenv ("tmp")) != 0)
+                  if (chdir (getenv ("GL_SUBDIR_FOR_UNLINK")) != 0)
                     return 1;
                   return unlink ("..") == 0;
                 }
@@ -67,8 +73,8 @@
              [gl_cv_func_unlink_parent_fails=yes],
              [gl_cv_func_unlink_parent_fails=no],
              [gl_cv_func_unlink_parent_fails="guessing no"])
+           unset GL_SUBDIR_FOR_UNLINK
            rm -rf "$tmp"
-           unset tmp
          else
            gl_cv_func_unlink_parent_fails="guessing no"
          fi