ln: replace destination links more atomically

If the file B already exists, commands like 'ln -f A B' and
'cp -fl A B' no longer remove B before creating the new link.
Instead, they arrange for the new link to replace B atomically.
This should fix a race condition reported by Mike Crowe (Bug#25680).
* NEWS, doc/coreutils.texi (cp invocation, ln invocation):
Document this.
* bootstrap.conf (gnulib_modules): Add symlinkat.
* src/copy.c, src/ln.c: Include force-link.h.
* src/copy.c (same_file_ok): It's also OK to remove a destination
symlink when creating symbolic links, or when the source and
destination are on the same file system and when creating hard links.
* src/copy.c (create_hard_link, copy_internal):
* src/ln.c (do_link):
Rewrite using force_linkat and force_symlinkat, to close a window
where the destination temporarily does not exist.
* src/cp.c (main): Do not set x.unlink_dest_before_opening
merely because we are in link-creation mode.
* src/force-link.c, src/force-link.h: New files.
* src/local.mk (copy_sources, src_ln_SOURCES): Add them.
* tests/cp/same-file.sh: Adjust test case to match fixed behavior.

1 files changed, 0 insertions, 5 deletions

diff --git a/src/cp.c b/src/cp.c
index 08055581a..88db3a3e9 100644
--- a/src/cp.c
+++ b/src/cp.c
@@ -1155,11 +1155,6 @@ main (int argc, char **argv)
   if (x.recursive)
     x.copy_as_regular = copy_contents;
 
-  /* If --force (-f) was specified and we're in link-creation mode,
-     first remove any existing destination file.  */
-  if (x.unlink_dest_after_failed_open && (x.hard_link || x.symbolic_link))
-    x.unlink_dest_before_opening = true;
-
   /* Ensure -Z overrides -a.  */
   if ((x.set_security_context || scontext)
       && ! x.require_preserve_context)