[feature] stream blobs from a CooperativeTask

Adds a CooperativeTask to stream blobs to client as the transport is
able to handle it, pausing and resuming as necessary.

-- Related: #8809

src/leap/soledad/server/_streaming_resource.py

@@ -22,6 +22,9 @@ A twisted resource that serves download as a single stream of multiple blobs.
 import json
 import struct
 
+from zope.interface import implementer
+from twisted.internet.interfaces import IPushProducer
+from twisted.internet import task
 from twisted.web.server import NOT_DONE_YET
 from twisted.web.resource import Resource
 
@@ -59,17 +62,52 @@ class StreamingResource(Resource):
         db = self._handler
         raw_content = request.content.read()
         blob_ids = json.loads(raw_content)
+        paths = []
         for blob_id in blob_ids:
             path = db._get_path(user, blob_id, namespace)
             size = db.get_blob_size(user, blob_id, namespace)
-            request.write(SIZE_PACKER.pack(size))
+            paths.append((blob_id, path, size))
+        DownstreamProducer(request, paths).start()
+        return NOT_DONE_YET
+
+
+@implementer(IPushProducer)
+class DownstreamProducer(object):
+    chunk_size = 2**14
+
+    def __init__(self, request, paths):
+        self.request = request
+        self.paths = paths
+
+    def start(self):
+        iterator = self._gen_data()
+        self.task = task.cooperate(iterator)
+        self.request.registerProducer(self, streaming=True)
+
+    def resumeProducing(self):
+        return self.task.resume()
+
+    def pauseProducing(self):
+        return self.task.pause()
+
+    def _gen_data(self):
+        request, paths = self.request, self.paths
+        while paths:
+            blob_id, path, size = paths.pop(0)
+            request.write(SIZE_PACKER.pack(size))  # sends file size
             with open(path, 'rb') as blob_fd:
-                # TODO: use a producer
                 blob_fd.seek(-16, 2)
-                request.write(blob_fd.read())  # sends tag
+                request.write(blob_fd.read())  # sends AES-GCM tag
                 blob_fd.seek(0)
                 request.write(' ')
-                request.write(blob_fd.read())
-
+                data = blob_fd.read(self.chunk_size)
+                while data:
+                    yield
+                    request.write(data)
+                    data = blob_fd.read(self.chunk_size)
+        request.unregisterProducer()
         request.finish()
-        return NOT_DONE_YET
+
+    def stopProducing(self):
+        self.request = None
+        return self.task.stop()