feat: allow disabling response stream pre-fetch

[plamut]

Closes #25.

This PR adds the ability to disable automatically pre-fetching the first item of a stream returned by *-Stream gRPC callables. This hook will be used in PubSub to fix the stalled stream issue, while also not affecting Firestore, since the default behavior is preserved.

I realize the fix is far from ideal, but it's the least ugly among the approaches I tried, e.g. somehow passing the flag through ResumableBidiRpc (it's a messy rabbit hole). On the PubSub side monkeypatching the generated SubscriberClient will be needed, but it's a (relatively) clean one-liner:

diff --git google/cloud/pubsub_v1/gapic/subscriber_client.py google/cloud/pubsub_v1/gapic/subscriber_client.py
index e98a686..1d6c058 100644
--- google/cloud/pubsub_v1/gapic/subscriber_client.py
+++ google/cloud/pubsub_v1/gapic/subscriber_client.py
@@ -1169,6 +1169,8 @@ class SubscriberClient(object):
                 default_timeout=self._method_configs["StreamingPull"].timeout,
                 client_info=self._client_info,
             )
+            # TODO: explain this monkeypatch!
+            self.transport.streaming_pull._prefetch_first_result_ = False
 
         return self._inner_api_calls["streaming_pull"](
             requests, retry=retry, timeout=timeout, metadata=metadata

If/when we merge this, we should also release it, and then we can add != 1.17.0 to the google-api-core version pin in PubSub.

PR checklist

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

[crwilcox]

This seems like a reasonable approach. Before merging I would like to take a moment and make sure firestore is 'fixed' with the original change as well, just to make sure this isn't fixing the wrong thing :)

[plamut]

Sounds good, let me know when you have info on that. :)

[busunkim96]

@crwilcox Anything I can do to help this move forward? I accidentally introduced a dependency conflict with a new release of google-api-python-client

[plamut]

+1, quite a few PubSub users are looking forward to the fix, too.

[space55]

Hi!

Is there any ETA for having this merged? We're currently blocked on a bugfix for this in our servers.

Thanks!

[arithmetic1728]

+1 any updates on this bug fix? @plamut @crwilcox

[plamut]

@space55 @arithmetic1728 The new PubSub release that includes its part of this fix is on the way.

[space55]

Thanks! I'm excited!