Changed load_before and preftech to use generators rather than callbacks

src/ZEO/asyncio/client.py

@@ -8,6 +8,7 @@ else:
 from ZEO.Exceptions import ClientDisconnected
 from ZODB.ConflictResolution import ResolvedSerial
 import concurrent.futures
+import functools
 import logging
 import random
 import threading
@@ -27,6 +28,37 @@ Fallback = object()
 
 local_random = random.Random() # use separate generator to facilitate tests
 
+def future_generator(func):
+    """Decorates a generator that generates futures
+    """
+
+    @functools.wraps(func)
+    def call_generator(*args, **kw):
+        gen = func(*args, **kw)
+        try:
+            f = next(gen)
+        except StopIteration:
+            gen.close()
+        else:
+            def store(gen, future):
+                @future.add_done_callback
+                def _(future):
+                    try:
+                        try:
+                            result = future.result()
+                        except Exception as exc:
+                            f = gen.throw(exc)
+                        else:
+                            f = gen.send(result)
+                    except StopIteration:
+                        gen.close()
+                    else:
+                        store(gen, f)
+
+            store(gen, f)
+
+    return call_generator
+
 class Protocol(base.Protocol):
     """asyncio low-level ZEO client interface
     """
@@ -249,7 +281,7 @@ class Protocol(base.Protocol):
             self.futures[message_id] = future
             self._write(
                 self.encode(message_id, False, 'loadBefore', (oid, tid)))
-        return future.add_done_callback
+        return future
 
     # Methods called by the server.
     # WARNING WARNING we can't call methods that call back to us
@@ -525,30 +557,28 @@ class Client(object):
 
     # Special methods because they update the cache.
 
+    @future_generator
     def load_before_threadsafe(self, future, oid, tid):
         data = self.cache.loadBefore(oid, tid)
         if data is not None:
             future.set_result(data)
         elif self.ready:
-
-            @self.protocol.load_before(oid, tid)
-            def load_before(load_future):
             try:
-                    data = load_future.result()
+                data = yield self.protocol.load_before(oid, tid)
+            except Exception as exc:
+                future.set_exception(exc)
+            else:
                 future.set_result(data)
                 if data:
                     data, start, end = data
                     self.cache.store(oid, start, end, data)
-                except Exception as exc:
-                    future.set_exception(exc)
         else:
             self._when_ready(self.load_before_threadsafe, future, oid, tid)
 
+    @future_generator
     def _prefetch(self, oid, tid):
-        @self.protocol.load_before(oid, tid)
-        def load_before(load_future):
         try:
-                data = load_future.result()
+            data = yield self.protocol.load_before(oid, tid)
             if data:
                 data, start, end = data
                 self.cache.store(oid, start, end, data)