locks: add configuration and command-line options to enable/disable locks (#7692)

- 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

1 files changed, 11 insertions, 0 deletions

diff --git a/lib/spack/docs/config_yaml.rst b/lib/spack/docs/config_yaml.rst
index da760f05e3..35bc22d72e 100644
--- a/lib/spack/docs/config_yaml.rst
+++ b/lib/spack/docs/config_yaml.rst
@@ -151,6 +151,17 @@ to ``false`` to disable these checks.  Disabling this can expose you to
 attacks.  Use at your own risk.
 
 --------------------
+``locks``
+--------------------
+
+When set to ``true``, concurrent instances of Spack will use locks to
+avoid modifying the install tree, database file, etc. If false, Spack
+will disable all locking, but you must **not** run concurrent instances
+of Spack.  For filesystems that don't support locking, you should set
+this to ``false`` and run one Spack at a time, but otherwise we recommend
+enabling locks.
+
+--------------------
 ``dirty``
 --------------------