Various small fixes suggested in review.

src/ZODB/Connection.py

@@ -794,7 +794,7 @@ class Connection(ExportImport, object):
             raise
 
         if s:
-            if type(s[0]) is bytes:
+            if type(next(iter(s))) is bytes:
                 for oid in s:
                     self._handle_serial(oid)
                 return

src/ZODB/DB.py

@@ -1012,8 +1012,8 @@ class TransactionalUndo(object):
     def tpc_vote(self, transaction):
         result = self._storage.tpc_vote(transaction)
         if result:
-            if isinstance(result[0], bytes):
-                self._oids.update(set(result))
+            if isinstance(next(iter(result)), bytes):
+                self._oids.update(result)
             else:
                 for oid, _ in result:
                     self._oids.add(oid)

src/ZODB/multicommitadapter.py

@@ -11,7 +11,7 @@ class MultiCommitAdapter:
         self._storage = storage
         ifaces = zope.interface.providedBy(storage)
         zope.interface.alsoProvides(self, ifaces)
-        self._resolved = set()
+        self._resolved = set() # {OID}, here to make linters happy
 
     def __getattr__(self, name):
         v = getattr(self._storage, name)
@@ -39,7 +39,7 @@ class MultiCommitAdapter:
     def undo(self, transaction_id, transaction):
         r = self._storage.undo(transaction_id, transaction)
         if r:
-            self._resolved.update(set(r[1]))
+            self._resolved.update(r[1])
 
     def tpc_vote(self, *args):
         s = self._storage.tpc_vote(*args)
@@ -47,7 +47,7 @@ class MultiCommitAdapter:
             if serial == ResolvedSerial:
                 self._resolved.add(oid)
 
-        return list(self._resolved)
+        return self._resolved
 
     def tpc_finish(self, transaction, f=lambda tid: None):
 

src/ZODB/tests/TransactionalUndoStorage.py

@@ -113,7 +113,7 @@ class TransactionalUndoStorage:
                 oids.extend(undo_result[1])
         v = self._storage.tpc_vote(t)
         if v:
-            if isinstance(v[0], bytes):
+            if isinstance(next(iter(v)), bytes):
                 oids.extend(v)
             else:
                 oids.extend(oid for (oid, _) in v)

src/ZODB/tests/blob_transaction.txt

@@ -411,11 +411,6 @@ Similarly, the new object wasn't added to the storage:
     ...
     POSKeyError: 0x...
 
-    >>> blob_storage.loadBlob(blob._p_oid, s2)
-    Traceback (most recent call last):
-    ...
-    POSKeyError: 'No blob file at <BLOB STORAGE PATH>'
-
 .. clean up
 
     >>> tm1.abort()