du: handle bind-mounted directory cycles gracefully

du: handle bind-mounted directory cycles gracefully

Before this change, a directory cycle induced by a bind mount
would be treated as a fatal error, i.e., probable disk corruption.
However, such cycles are relatively common, and can be detected
efficiently, so now du emits a descriptive warning and arranges
to exit nonzero.

* NEWS (Bug fixes): Mention it.
* src/du.c: Include "mountlist.h".
(di_mnt): New global set.
(di_files): Rename global from di_set, now that there are two.
(fill_mount_table): New function.
(hash_ins): Add DI_SET parameter.
(process_file): Look up each dir dev/ino pair in the new set.
(main): Allocate, initialize, and free the new set.
* tests/du/bind-mount-dir-cycle: Add a test for the fix.
* tests/Makefile.am (TESTS): Add it.
* THANKS.in: Update.
This implements the proposal in http://bugs.gnu.org/11844.
Originally reported in http://bugs.debian.org/563254 by Alan Jenkins
and more recently as http://bugzilla.redhat.com/836557

Improved by: Jim Meyering