sort: avoid undefined operation with destroying locked mutex

This didn't seem to cause any invalid operation on GNU/Linux at least,
but depending on the implementation, mutex deadlocks could occur.
For example this might be the cause of lockups seen on Solaris:
http://lists.gnu.org/archive/html/coreutils/2013-03/msg00048.html

This was identified with valgrind 3.9.0 with this setup:

  seq 200000 > file.sort
  valgrind --tool=drd src/sort file.sort -o file.sort

With that, valgrind would _intermittently_ report the following:

 Destroying locked mutex: mutex 0x5419548, recursion count 1, owner 2.
    at 0x4C2E3F0: pthread_mutex_destroy(in vgpreload_drd-amd64-linux.so)
    by 0x409FA2: sortlines (sort.c:3649)
    by 0x409E26: sortlines (sort.c:3621)
    by 0x40AA9E: sort (sort.c:3955)
    by 0x40C5D9: main (sort.c:4739)
 mutex 0x5419548 was first observed at:
    at 0x4C2DE82: pthread_mutex_init(in vgpreload_drd-amd64-linux.so)
    by 0x409266: init_node (sort.c:3276)
    by 0x4092F4: init_node (sort.c:3286)
    by 0x4090DD: merge_tree_init (sort.c:3234)
    by 0x40AA5A: sort (sort.c:3951)
    by 0x40C5D9: main (sort.c:4739)

 Thread 2:
 The object at address 0x5419548 is not a mutex.
    at 0x4C2F4A4: pthread_mutex_unlock(in vgpreload_drd-amd64-linux.so)
    by 0x4093CA: unlock_node (sort.c:3323)
    by 0x409C85: merge_loop (sort.c:3531)
    by 0x409F8F: sortlines (sort.c:3644)
    by 0x409CE3: sortlines_thread (sort.c:3574)
    by 0x4E44F32: start_thread (in /usr/lib64/libpthread-2.18.so)
    by 0x514EEAC: clone (in /usr/lib64/libc-2.18.so)

* src/sort.c (sortlines): Move pthread_mutex_destroy() out to
merge_tree_destroy(), so that we don't overlap mutex destruction
with threads still operating on the nodes.
(sort): Call the destructors only with "lint" defined, as the
memory used will be deallocated implicitly at process end.
* NEWS: Mention the bug fix.

1 files changed, 4 insertions, 0 deletions

diff --git a/NEWS b/NEWS
index 2bd739683..c7f74210c 100644
--- a/NEWS
+++ b/NEWS
@@ -93,6 +93,10 @@ GNU coreutils NEWS                                    -*- outline -*-
   shuf --repeat no longer dumps core if the input is empty.
   [bug introduced with the --repeat feature in coreutils-8.22]
 
+  sort when using multiple threads now avoids undefined behavior with mutex
+  destruction, which could cause deadlocks on some implementations.
+  [bug introduced in coreutils-8.6]
+
   tail -f now uses polling mode for VXFS to cater for its clustered mode.
   [bug introduced with inotify support added in coreutils-7.5]