fix: do not assign capacity for pending streams

[benjaminp]

Prioritize::send_data has a check to prevent assigning capacity to streams that are not yet open. Assigning flow control window to pending streams could starve already open streams.

This change moves the stream.is_pending_open check into Prioritize::try_assign_capacity. This prevents capacity from ever being assigned to pending streams. In particular, neither a client's reserve_capacity call nor a remote's initial window size adjustment will assign capacity to pending streams.

Tests capacity_not_assigned_to_unopened_streams and new_initial_window_size_capacity_not_assigned_to_unopened_streams in flow_control.rs demonstrates the fix.

A number of other tests must be changed because they were assuming that pending streams immediately received connection capacity.

This may be related to #853.

[bvinc]

I am hitting this problem in production, and I've spent weeks tracking it down. I am running a tonic GRPC client that is creating thousands of streams to a server that limits streams to 400, and I keep hitting a deadlock. I caught logs where h2 gives capacity to streams that are pending_open, and it triggers a deadlock where no more progress is made. I ended up with a patch to fix it that looks almost exactly like this, which led to me finding this PR. I support this PR.

Here's my question though: Is it always a bug to assign capacity to a stream that is pending_open? If so, do you think there should be a check inside of try_assign_capacity that returns early if the stream is pending_open? If not, maybe there should at lest be a debug_assert?

[seanmonstar]

Just wanted to chime in here, I think this fix sounds good, but part of me worries that it has deeper repercussions. (Like, does stream.requested_send_capacity being changed without the other parts cause problems?) Much of this code was written years ago, and I don't remember all the nuance.

I hope to find time to thoroughly understand how this affects any other internal invariants... But I appreciate any reports that testing this out with live traffic improved things.