Correct whitespaces and add context manager for file locking

tfw/internals/ref_counter/ref_counter.py

@@ -1,4 +1,5 @@
 from os import remove
+from contextlib import contextmanager
 from fcntl import flock, LOCK_EX, LOCK_UN
 
 
@@ -6,9 +7,14 @@ class RefCounter:
     def __init__(self, lockpath):
         self.lockpath = lockpath
         self._lockfile = open(self.lockpath, 'a+')
+        with self.locked():
+            counter = self._read_counter()
+            self._write_counter(counter+1)
+
+    @contextmanager
+    def locked(self):
         flock(self._lockfile, LOCK_EX)
-        counter = self._read_counter()
-        self._write_counter(counter+1)
+        yield
         flock(self._lockfile, LOCK_UN)
 
     def _read_counter(self):
@@ -26,14 +32,13 @@ class RefCounter:
         self._lockfile.flush()
 
     def teardown_instance(self):
-        flock(self._lockfile, LOCK_EX)
-        counter = self._read_counter()
-        if counter <= 1:
-            remove(self.lockpath)
-            self.deallocate()
-        else:
-            self._write_counter(counter-1)
-            flock(self._lockfile, LOCK_UN)
+        with self.locked():
+            counter = self._read_counter()
+            if counter <= 1:
+                remove(self.lockpath)
+                self.deallocate()
+            else:
+                self._write_counter(counter-1)
         self._lockfile.close()
 
     def deallocate(self):

tfw/internals/ref_counter/test_ref_counter.py

@@ -39,6 +39,7 @@ def context():
         mkfifo(pipepath)
         yield CounterContext(join(workdir, 'test.lock'), pipepath)
 
+
 def test_increment_decrement(context):
     counter, processes = 0, []
     for _ in range(5):
@@ -53,6 +54,7 @@ def test_increment_decrement(context):
         proc.send_signal(SIGINT)
         proc.wait()
 
+
 def test_deallocate(context):
     state = False
     def trigger():