Revision 1578176

Model the FSFS pack lock similarly to our other file based locks in FSFS.
This gives us proper mutex functionality should multiple threads try to
pack the same repo at the same time.

* subversion/libsvn_fs_fs/fs.h
  (fs_fs_shared_data_t): Add a thread mutex alongside the file lock.

* subversion/libsvn_fs_fs/fs.c
  (fs_serialized_init): Initialize the new mutex.

* subversion/libsvn_fs_fs/fs_fs.h
  (svn_fs_fs__get_lock_on_filesystem): Privatize again.
  (svn_fs_fs__with_pack_lock): New lock handling function similar
                               to what we do for the other locks.

* subversion/libsvn_fs_fs/fs_fs.c
  (path_pack_lock): New utility function.
  (svn_fs_fs__get_lock_on_filesystem): Rename back to ...
  (get_lock_on_filesystem): ... this again.
  (with_some_lock_file): Update caller.
  (svn_fs_fs__with_pack_lock): Implement the new lock function.

* subversion/libsvn_fs_fs/pack.c
  (svn_fs_fs__pack): Use the new pack lock handling function.

Suggested by: ivan