Flush the request body guard cross-process-safe

[samwar]

Summary

Merge the flush-req-body-guard branch (already tagged as 1.12.1-fe and consumed downstream) into main so the fix is on the default branch. Upstream tracking PR: webmachine/webmachine#349 (open).

What it does

webmachine_decision_core:finish_response/2's maybe_flush_req_body step (added in 1.12) checks mochiweb_request_recv in the process dictionary to decide whether the body still needs draining. That flag is set by webmachine_request:call({req_body, ...}) inside wrq:req_body/1, but applications that dispatch body reads to a separate worker process end up with the flag on the worker's pdict while the mochiweb loop process's pdict stays empty. The drain then runs and hits the FLUSH_TIMEOUT (5000 ms).

Fix: consult ReqState#wm_reqstate.bodyfetch as a fallback. bodyfetch is set to standard or stream in the same call that would set the pdict flag, and unlike the pdict it travels with the ReqState value across process boundaries. Applications that thread the updated ReqState back to the mochiweb loop process (via the returned Request's wm_state field) hit the fallback and short-circuit the drain.

The existing pdict guard remains as the fast path for the common single-process case. FLUSH_TIMEOUT = 5000 ms stays as the safety net for edge cases where neither signal was set.

Also carried on this branch:

• Guard the undefined branch on get(mochiweb_request_recv) =:= true — eager reads mark the request as recv'd; there's nothing left on the socket to drain.
• Thread ?FLUSH_TIMEOUT through a new recv_stream_body/3 variant that the drain fallback uses, so a misbehaving peer cannot wedge the pipeline. Legitimate streaming callers keep ?IDLE_TIMEOUT = infinity via the 2-arg wrapper.

Files

• src/webmachine_request.erl — 93 insertions, 42 deletions.

Status

• Deployed to Fire Engine staging as part of release fe.3.2692-erl25-rhel (tag 1.12.1-fe), running on all 8 hosts since 2026-07-20. POST parity confirmed against pre-1.12 baseline.
• Upstream PR Fix maybe_flush_req_body/1 hang when body was eagerly read webmachine/webmachine#349 tracks the same fix for webmachine/webmachine:main.