storage/reflink: split @_unblock into @_coroutinized @_locked

And change the volume lock from an asyncio.Lock to a threading.Lock -
locking is now handled before coroutinization.

This will allow the coroutinized resize() and a new *not* coroutinized
size() getter from one of the next commits ("storage/reflink: preferably
get volume size from image size") to both run under the volume lock.

qubes/storage/reflink.py

@@ -32,6 +32,7 @@ import logging
 import os
 import subprocess
 import tempfile
+import threading
 from contextlib import contextmanager, suppress
 
 import qubes.storage
@@ -117,36 +118,47 @@ class ReflinkPool(qubes.storage.Pool):
             self.dir_path)
 
 
-def _unblock(method):
-    ''' Decorator transforming a synchronous volume method into a
-        coroutine that runs the original method in the event loop's
-        thread-based default executor, under a per-volume lock.
+def _coroutinized(function):
+    ''' Decorator transforming a synchronous function into a coroutine
+        that runs the function in the event loop's thread-based
+        default executor.
     '''
     @asyncio.coroutine
+    @functools.wraps(function)
+    def wrapper(*args, **kwargs):
+        return (yield from asyncio.get_event_loop().run_in_executor(
+            None, functools.partial(function, *args, **kwargs)))
+    return wrapper
+
+def _locked(method):
+    ''' Decorator transforming a synchronous volume method to run
+        under the volume lock.
+    '''
     @functools.wraps(method)
     def wrapper(self, *args, **kwargs):
-        with (yield from self._lock):  # pylint: disable=protected-access
-            return (yield from asyncio.get_event_loop().run_in_executor(
-                None, functools.partial(method, self, *args, **kwargs)))
+        with self._lock:  # pylint: disable=protected-access
+            return method(self, *args, **kwargs)
     return wrapper
 
 class ReflinkVolume(qubes.storage.Volume):
     def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)
-        self._lock = asyncio.Lock()
+        self._lock = threading.Lock()
         self._path_vid = os.path.join(self.pool.dir_path, self.vid)
         self._path_clean = self._path_vid + '.img'
         self._path_dirty = self._path_vid + '-dirty.img'
         self._path_import = self._path_vid + '-import.img'
         self.path = self._path_dirty
 
-    @_unblock
+    @_coroutinized
+    @_locked
     def create(self):
         if self.save_on_stop and not self.snap_on_start:
             _create_sparse_file(self._path_clean, self.size)
         return self
 
-    @_unblock
+    @_coroutinized
+    @_locked
     def verify(self):
         if self.snap_on_start:
             img = self.source._path_clean  # pylint: disable=protected-access
@@ -160,7 +172,8 @@ class ReflinkVolume(qubes.storage.Volume):
         raise qubes.storage.StoragePoolException(
             'Missing image file {!r} for volume {}'.format(img, self.vid))
 
-    @_unblock
+    @_coroutinized
+    @_locked
     def remove(self):
         ''' Drop volume object from pool; remove volume images from
             oldest to newest; remove empty VM directory.
@@ -189,7 +202,8 @@ class ReflinkVolume(qubes.storage.Volume):
     def is_dirty(self):
         return self.save_on_stop and os.path.exists(self._path_dirty)
 
-    @_unblock
+    @_coroutinized
+    @_locked
     def start(self):
         self._cleanup()
         if self.is_dirty():  # implies self.save_on_stop
@@ -203,7 +217,8 @@ class ReflinkVolume(qubes.storage.Volume):
             _create_sparse_file(self._path_dirty, self.size)
         return self
 
-    @_unblock
+    @_coroutinized
+    @_locked
     def stop(self):
         if self.save_on_stop:
             self._commit(self._path_dirty)
@@ -232,7 +247,8 @@ class ReflinkVolume(qubes.storage.Volume):
         for number, timestamp in list(self.revisions.items())[:-keep or None]:
             _remove_file(self._path_revision(number, timestamp))
 
-    @_unblock
+    @_coroutinized
+    @_locked
     def revert(self, revision=None):
         if self.is_dirty():
             raise qubes.storage.StoragePoolException(
@@ -246,7 +262,8 @@ class ReflinkVolume(qubes.storage.Volume):
         _rename_file(path_revision, self._path_clean)
         return self
 
-    @_unblock
+    @_coroutinized
+    @_locked
     def resize(self, size):
         ''' Resize a read-write volume image; notify any corresponding
             loop devices of the size change.
@@ -271,7 +288,8 @@ class ReflinkVolume(qubes.storage.Volume):
                 'Cannot export: {} is not save_on_stop'.format(self.vid))
         return self._path_clean
 
-    @_unblock
+    @_coroutinized
+    @_locked
     def import_data(self):
         if not self.save_on_stop:
             raise NotImplementedError(
@@ -286,9 +304,10 @@ class ReflinkVolume(qubes.storage.Volume):
             _remove_file(self._path_import)
         return self
 
-    import_data_end = _unblock(_import_data_end)
+    import_data_end = _coroutinized(_locked(_import_data_end))
 
-    @_unblock
+    @_coroutinized
+    @_locked
     def import_volume(self, src_volume):
         if not self.save_on_stop:
             return self