locks: fix bug when creating lockfiles in the current directory.

- Fixes a bug in `llnl.util.lock`

- Locks in the current directory would fail because the parent directory
  was the empty string.

- Fix this and return '.' for the parent of locks in the current
  directory.

lib/spack/llnl/util/lock.py

@@ -150,6 +150,11 @@ def _lock(self, op, timeout=_default_timeout):
 
     def _ensure_parent_directory(self):
         parent = os.path.dirname(self.path)
+
+        # relative paths to lockfiles in the current directory have no parent
+        if not parent:
+            return '.'
+
         try:
             os.makedirs(parent)
         except OSError as e:

lib/spack/spack/test/llnl/util/lock.py

@@ -1047,3 +1047,23 @@ def test_lock_with_no_parent_directory(tmpdir):
         lock = lk.Lock('foo/bar/baz/lockfile')
         with lk.WriteTransaction(lock):
             pass
+
+
+def test_lock_in_current_directory(tmpdir):
+    """Make sure locks work even when their parent directory does not exist."""
+    with tmpdir.as_cwd():
+        # test we can create a lock in the current directory
+        lock = lk.Lock('lockfile')
+        for i in range(10):
+            with lk.ReadTransaction(lock):
+                pass
+            with lk.WriteTransaction(lock):
+                pass
+
+        # and that we can do the same thing after it's already there
+        lock = lk.Lock('lockfile')
+        for i in range(10):
+            with lk.ReadTransaction(lock):
+                pass
+            with lk.WriteTransaction(lock):
+                pass