Commits on Aug 21, 2023

1. Fix opening new streams over max concurrent …

   There was a bug where opening streams over the max concurrent streams
   was possible if max_concurrent_streams were lowered beyond the current
   number of open streams and there were already new streams adding to the
   pending_send queue.
   
   There was two mechanisms for streams to end up in that queue.
   1. send_headers would push directly onto pending_send when below
      max_concurrent_streams
   2. prioritize would pop from pending_open until max_concurrent_streams
      was reached.
   
   For case 1, a settings frame could be received after pushing many
   streams onto pending_send and before the socket was ready to write
   again. For case 2, the pending_send queue could have Headers frames
   queued going into a Not Ready state with the socket, a settings frame
   could be received, and then the headers would be written anyway after
   the ack.
   
   The fix is therefore also two fold. Fixing case 1 is as simple as
   letting Prioritize decide when to transition streams from `pending_open`
   to `pending_send` since only it knows the readiness of the socket and
   whether the headers can be written immediately. This is slightly
   complicated by the fact that previously SendRequest would block when
   streams would be added as "pending open". That was addressed by
   guessing when to block based on max concurrent streams rather than the
   stream state.
   
   The fix for Prioritize was to conservatively pop streams from
   pending_open when the socket is immediately available for writing a
   headers frame. This required a change to queuing to support pushing on
   the front of pending_send to ensure headers frames don't linger in
   pending_send.
   
   The alternative to this was adding a check to pending_send whether a new
   stream would exceed max concurrent. In that case, headers frames would
   need to carefully be reenqueued. This seemed to impose more complexity
   to ensure ordering of stream IDs would be maintained.

   

   jwilm authored and seanmonstar committed Aug 21, 2023
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