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, 19 insertions, 11 deletions

diff --git a/lib/spack/llnl/util/lock.py b/lib/spack/llnl/util/lock.py
index ed797d2ebf..0d072418cc 100644
--- a/lib/spack/llnl/util/lock.py
+++ b/lib/spack/llnl/util/lock.py
@@ -195,11 +195,13 @@ class Lock(object):
 
         """
         if self._reads == 0 and self._writes == 0:
-            tty.debug('READ LOCK: {0.path}[{0._start}:{0._length}] [Acquiring]'
-                      .format(self))
+            self._debug(
+                'READ LOCK: {0.path}[{0._start}:{0._length}] [Acquiring]'
+                .format(self))
             self._lock(fcntl.LOCK_SH, timeout=timeout)   # can raise LockError.
-            tty.debug('READ LOCK: {0.path}[{0._start}:{0._length}] [Acquired]'
-                      .format(self))
+            self._debug(
+                'READ LOCK: {0.path}[{0._start}:{0._length}] [Acquired]'
+                .format(self))
             self._reads += 1
             return True
         else:
@@ -218,12 +220,13 @@ class Lock(object):
 
         """
         if self._writes == 0:
-            tty.debug(
+            self._debug(
                 'WRITE LOCK: {0.path}[{0._start}:{0._length}] [Acquiring]'
                 .format(self))
             self._lock(fcntl.LOCK_EX, timeout=timeout)   # can raise LockError.
-            tty.debug('WRITE LOCK: {0.path}[{0._start}:{0._length}] [Acquired]'
-                      .format(self))
+            self._debug(
+                'WRITE LOCK: {0.path}[{0._start}:{0._length}] [Acquired]'
+                .format(self))
             self._writes += 1
             return True
         else:
@@ -243,8 +246,9 @@ class Lock(object):
         assert self._reads > 0
 
         if self._reads == 1 and self._writes == 0:
-            tty.debug('READ LOCK: {0.path}[{0._start}:{0._length}] [Released]'
-                      .format(self))
+            self._debug(
+                'READ LOCK: {0.path}[{0._start}:{0._length}] [Released]'
+                .format(self))
             self._unlock()      # can raise LockError.
             self._reads -= 1
             return True
@@ -265,8 +269,9 @@ class Lock(object):
         assert self._writes > 0
 
         if self._writes == 1 and self._reads == 0:
-            tty.debug('WRITE LOCK: {0.path}[{0._start}:{0._length}] [Released]'
-                      .format(self))
+            self._debug(
+                'WRITE LOCK: {0.path}[{0._start}:{0._length}] [Released]'
+                .format(self))
             self._unlock()      # can raise LockError.
             self._writes -= 1
             return True
@@ -274,6 +279,9 @@ class Lock(object):
             self._writes -= 1
             return False
 
+    def _debug(self, *args):
+        tty.debug(*args)
+
 
 class LockTransaction(object):
     """Simple nested transaction context manager that uses a file lock.