obs: permanent structured emit-decision debug logging#37
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Add gated ngx_log_debug at the output emit-decision point in ngx_http_zstd_filter_compress: zstd emit?: outsz:N rc0:N last:N ctx_last:N ctx_flush:N action:N zstd emit: suppressed empty non-terminal buffer NGX_DEBUG-gated (the ngx_log_debug* macros compile to nothing in release builds), so zero runtime cost when off; visible with `error_log ... debug`. Behaviour is unchanged — this only observes the existing guard, it does not alter it. Rationale: this module has a recurring truncation / zero-size-buffer / terminal-frame bug class (a209f96, 2af5889, PR#23/#49, the 2026-05 bug B). Every prior diagnosis required temporarily patching in debug lines and rebuilding, repeatedly. This makes the single most diagnostic probe permanent: the exact state the emit guard saw. Verified during bug-B work — it immediately exposed the forced-flush mechanism without a patch/rebuild loop. Builds clean under the project -Werror flags (nginx 1.31.0).
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What
Adds two gated
ngx_log_debugcalls at the output emit-decision point inngx_http_zstd_filter_compress:NGX_DEBUG-gated (thengx_log_debug*macros expand to nothing in release builds) → zero runtime cost when off, visible witherror_log ... debug. No behaviour change — it only observes the existing guard.Why
This module has a recurring truncation / zero-size-buffer / terminal-frame bug class (
a209f96,2af5889, PR#23/#49, the in-progress bug B). Every prior diagnosis required temporarily patching debug lines in and rebuilding — repeatedly, this session included. This makes the single most diagnostic probe permanent: exactly what state the emit guard saw and why it suppressed/forwarded a buffer. During bug-B investigation it immediately exposed the forced-flush-under-proxy_buffering offmechanism with no patch/rebuild loop.Scope
Observability only. This is not the bug-B fix (that is unsolved and tracked separately in
BUG-B-PLAN.md/BUG-B-RUNBOOK.md). Shipping this independently so the diagnostic exists in tree for the ongoing investigation and future regressions.Verification
-Werror -Wallflags (nginx 1.31.0).ngx_log_debugblocks).