Separate state for tracking downstream end_stream in filter manager and HCM

[yanavlasov]

Commit Message:
Fix two bugs in processing of deferred requests.

1. Filter manager (FM) uses presence of the timestamp of receipt of the downstream end_stream to set the end_stream during filter chain resumption. If a complete request (with end_stream= true) was deferred, its filter chain iteration would start with the downstream end_stream flag set. If there is a decoder filter that pauses and resumes filer chain iteration, all filters after resumption will see end_stream as true.

For example if a filter pauses in decodeHeaders() and then resumes in decodeData() all filters after that will have decodeHeaders() called with end_stream == true. This can cause request to be proxied by the router without the body.

2. The request trailers were also immediately available to filter manager when processing of a deferred request begins. If there is a filter that pauses and resumes iteration it makes filter manager incorrectly process trailers as though they were added by the filter that paused iteration. This causes trailers to be processed by filter chain twice triggering asserts.

The same problem is also present when HCM recreates the stream with body (i.e. for internal redirect).

This change makes filter manager use its own, already existing, flag observed_decode_end_stream_ for tracking state of downstream end_stream flag when resuming the filter chain, instead of the timestamp of the end_stream receipt from downstream client.

HCM continues to use the presence of the end_stream timestamp as the indicator of completed client's stream.

This change can be reverted by setting the envoy.reloadable_features.use_filter_manager_state_for_downstream_end_stream runtime flag to false.

Risk Level: Medium, affects deferred request processing
Testing: Unit tests
Docs Changes: N/A
Release Notes: Yes
Platform Specific Features: N/A
Runtime guard: envoy.reloadable_features.use_filter_manager_state_for_downstream_end_stream
Fixes #35814

[repokitteh-read-only]

CC @envoyproxy/runtime-guard-changes: FYI only for changes made to (source/common/runtime/runtime_features.cc).

🐱

Caused by: #35815 was opened by yanavlasov.

see: more, trace.

[RyanTheOptimist]

Wow, great catch! LGTM

[RyanTheOptimist]

Nice comment! Thanks for adding this.

[KBaichoo]

Nice work!