- spack.util.lock behaves the same as llnl.util.lock, but Lock._lock and
Lock._unlock do nothing.
- can be disabled with a control variable.
- configuration options can enable/disable locking:
- `locks` option in spack configuration controls whether Spack will use filesystem locks or not.
- `-l` and `-L` command-line options can force-disable or force-enable locking.
- Spack will check for group- and world-writability before disabling
locks, and it will not allow a group- or world-writable instance to
have locks disabled.
- update documentation
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