Decrease max ACK batch size to 2500

The previous limit of 3000 seems to be too optimistic, and the request
size limit is still hit. Reducing the batch size to 2500 fixes the
problem.

pubsub/google/cloud/pubsub_v1/subscriber/_protocol/dispatcher.py

@@ -37,15 +37,15 @@
 """The maximum amount of time in seconds to wait for additional request items
 before processing the next batch of requests."""
 
-_ACK_IDS_BATCH_SIZE = 3000
+_ACK_IDS_BATCH_SIZE = 2500
 """The maximum number of ACK IDs to send in a single StreamingPullRequest.
 
 The backend imposes a maximum request size limit of 524288 bytes (512 KiB) per
 acknowledge / modifyAckDeadline request. ACK IDs have a maximum size of 164
-bytes, thus we cannot send more than o 524288/164 ~= 3197 ACK IDs in a single
+bytes, thus we cannot send more than o 524288/176 ~= 2979 ACK IDs in a single
 StreamingPullRequest message.
 
-Accounting for some overhead, we should thus only send a maximum of 3000 ACK
+Accounting for some overhead, we should thus only send a maximum of 2500 ACK
 IDs at a time.
 """
 

pubsub/tests/unit/pubsub_v1/subscriber/test_dispatcher.py

@@ -102,9 +102,9 @@ def test_ack_splitting_large_payload():
     dispatcher_ = dispatcher.Dispatcher(manager, mock.sentinel.queue)
 
     items = [
-        # use realistic lengths for ACK IDs (max 164 bytes)
-        requests.AckRequest(ack_id=str(i).zfill(164), byte_size=0, time_to_ack=20)
-        for i in range(6001)
+        # use realistic lengths for ACK IDs (max 176 bytes)
+        requests.AckRequest(ack_id=str(i).zfill(176), byte_size=0, time_to_ack=20)
+        for i in range(5001)
     ]
     dispatcher_.ack(items)
 
@@ -187,9 +187,9 @@ def test_modify_ack_deadline_splitting_large_payload():
     dispatcher_ = dispatcher.Dispatcher(manager, mock.sentinel.queue)
 
     items = [
-        # use realistic lengths for ACK IDs (max 164 bytes)
-        requests.ModAckRequest(ack_id=str(i).zfill(164), seconds=60)
-        for i in range(6001)
+        # use realistic lengths for ACK IDs (max 176 bytes)
+        requests.ModAckRequest(ack_id=str(i).zfill(176), seconds=60)
+        for i in range(5001)
     ]
     dispatcher_.modify_ack_deadline(items)