Clean up startup for the pusher (#6558)

* Remove redundant python2 support code

`str.decode()` doesn't exist on python3, so presumably this code was doing
nothing

* Filter out pushers with corrupt data

When we get a row with unparsable json, drop the row, rather than returning a
row with null `data`, which will then cause an explosion later on.

* Improve logging when we can't start a pusher

Log the ID to help us understand the problem

* Make email pusher setup more robust

We know we'll have a `data` member, since that comes from the database. What we
*don't* know is if that is a dict, and if that has a `brand` member, and if
that member is a string.

synapse/storage/data_stores/main/pusher.py

@@ -15,8 +15,7 @@
 # limitations under the License.
 
 import logging
-
-import six
+from typing import Iterable, Iterator
 
 from canonicaljson import encode_canonical_json, json
 
@@ -27,21 +26,16 @@ from synapse.util.caches.descriptors import cachedInlineCallbacks, cachedList
 
 logger = logging.getLogger(__name__)
 
-if six.PY2:
-    db_binary_type = six.moves.builtins.buffer
-else:
-    db_binary_type = memoryview
-
 
 class PusherWorkerStore(SQLBaseStore):
-    def _decode_pushers_rows(self, rows):
+    def _decode_pushers_rows(self, rows: Iterable[dict]) -> Iterator[dict]:
+        """JSON-decode the data in the rows returned from the `pushers` table
+
+        Drops any rows whose data cannot be decoded
+        """
         for r in rows:
             dataJson = r["data"]
-            r["data"] = None
             try:
-                if isinstance(dataJson, db_binary_type):
-                    dataJson = str(dataJson).decode("UTF8")
-
                 r["data"] = json.loads(dataJson)
             except Exception as e:
                 logger.warning(
@@ -50,12 +44,9 @@ class PusherWorkerStore(SQLBaseStore):
                     dataJson,
                     e.args[0],
                 )
-                pass
+                continue
 
-            if isinstance(r["pushkey"], db_binary_type):
-                r["pushkey"] = str(r["pushkey"]).decode("UTF8")
-
-        return rows
+            yield r
 
     @defer.inlineCallbacks
     def user_has_pusher(self, user_id):