Hard-enforce max_active_streams_in at HTTP/2 stream creation

[maskit]

proxy.config.http2.max_active_streams_in has only adjusted the advertised SETTINGS_MAX_CONCURRENT_STREAMS, leaving the proxy unable to bound buffered response memory against clients that open streams faster than the advisory throttle reacts. It now also carries a finite default so the cap is no longer effectively unlimited.

The new knob proxy.config.http2.max_active_streams_policy_in selects enforcement. Value 0 keeps the advisory behavior that lowers advertised concurrency to proxy.config.http2.min_concurrent_streams_in. Value 1 refuses new inbound streams with REFUSED_STREAM once the process-wide active-stream count reaches the limit and leaves the advertised value untouched, so the min_concurrent_streams_in reduction that disrupts some clients no longer applies.

max_active_streams_in now defaults to 200000 rather than 0. Operators should size it to their available memory budget; set it to 0 to disable the cap.

Refusal happens after HPACK decoding so the dynamic table stays in sync with the client and the connection survives. proxy.process.http2.max_active_streams_exceeded_in counts each refusal.

[Copilot]

Pull request overview

This pull request hard-enforces the inbound HTTP/2 proxy.config.http2.max_active_streams_in limit at stream creation time (after HPACK decode), adding a new policy knob to choose between the existing advisory throttling behavior and hard refusal with REFUSED_STREAM. It also updates defaults, documentation, and adds gold tests to validate enforcement and HPACK dynamic-table synchronization across refused streams.

Changes:

• Add proxy.config.http2.max_active_streams_policy_in to select advisory throttling (0) vs hard refusal (1) when the process-wide inbound active-stream cap is reached.
• Change the default proxy.config.http2.max_active_streams_in to 200000 and update HTTP/2 code paths to refuse new inbound streams under the enforce policy while leaving advertised concurrency unchanged.
• Add gold tests (replays + custom HTTP/2 client) to validate enforcement behavior and ensure HPACK stays in sync across refused streams.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
tests/gold_tests/h2/replay/http2_max_active_streams_enforce.replay.yaml	New origin replay for enforce-mode scenario (streams refused at proxy).
tests/gold_tests/h2/replay/http2_max_active_streams_advisory.replay.yaml	New origin replay for advisory-mode scenario (all streams reach origin).
tests/gold_tests/h2/http2_max_active_streams.test.py	New gold test covering policy behavior and HPACK sync expectations.
tests/gold_tests/h2/clients/h2_max_active_streams.py	New custom HTTP/2 client used by the gold test to exercise stream refusal and HPACK dynamic table reuse.
src/records/RecordsConfig.cc	Update defaults and add the new max_active_streams_policy_in records.yaml knob definition.
src/proxy/http2/Http2ConnectionState.cc	Enforce the cap on inbound streams after header decode and bypass advisory throttling of advertised settings under enforce policy.
src/proxy/http2/HTTP2.cc	Initialize/read the new config, update default for max_active_streams_in, and register a new metric counter.
include/proxy/http2/HTTP2.h	Add the new config field and new stats counter to the HTTP/2 stats block.
doc/admin-guide/files/records.yaml.en.rst	Document default change and add docs for the new enforcement policy knob.