reverse-proxy/docs/reviews/010-emfile-accept-loop-flood.md

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status: open last_updated: 2026-09-12 reviewed_code:

• src/server.rs
• src/config/static_config.rs
• /opt/reverse-proxy/docker-compose.yml (dev1 deploy)
• /etc/reverse-proxy/config.toml (dev1 deploy) reviewer: code-reviewer based_on: docs/reviews/009-fix-never-deployed-and-streaming-body-bug.md trigger: Production incident on dev1 (2026-09-11 ~21:00-23:00 UTC) — EMFILE busy-loop wrote ~6M ERROR lines (~1.9 GB log) in 2 hours fixes:
• M1: MITIGATED in production 2026-09-12 — container RLIMIT_NOFILE raised to 8192 (compose ulimits) and max_connections lowered 1024 → 800. Code fixes (C1 backoff, C2 validation) remain open.
• C1: OPEN — accept loop busy-spins on accept errors, no backoff.
• C2: OPEN — no startup cross-check that max_connections fits under RLIMIT_NOFILE with headroom.

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Review #010 — EMFILE Accept-Loop Flood (6M ERROR lines, 1.9 GB log in 2 hours)

Summary

On 2026-09-11, between approximately 21:00 and 23:00 UTC, the production reverse proxy on dev1 hit EMFILE ("No file descriptors available", os error 24) and its accept loop busy-spun, emitting ~6M ERROR log lines (~1.9 GB) in roughly 2 hours — approximately 460K ERROR lines in the 22:00 hour alone, sustained at ~127 lines/second.

The accept loop has no error backoff: tcp_listener.accept() fails instantly and repeatedly when the process is out of file descriptors, and the continue on error turns that into a hot spin. Every iteration writes an error! line both to stdout (Docker json-file, unbounded by rotation) and to the access log file. The access log grew to 1.9 GB in a day (vs. ~60 MB on normal days); 99% of it was the EMFILE flood. This is the same failure shape as a task leak: an unbounded resource (log bytes) with no circuit breaker.

Two contributing design gaps made the EMFILE reachable at all:

1. C1: the accept loop treats accept errors as transient ("log and continue"), which is correct for EINTR/EAGAIN but catastrophic for persistent errors like EMFILE/ENFILE — retry-without-delay on those is a busy loop.
2. C2: max_connections = 1024 (the semaphore cap on concurrent TLS connections) equals the container's Docker-default RLIMIT_NOFILE = 1024 — zero headroom for the listener sockets, health/admin listener, log file FD, ACME renewal sockets, stdin/out/err, and epoll instances. The semaphore therefore admits connections right up to (and past, counting non-connection FDs) the FD ceiling.

Production mitigation applied (2026-09-12)

• /opt/reverse-proxy/docker-compose.yml: added ulimits: nofile: {soft: 8192, hard: 8192}.
• /etc/reverse-proxy/config.toml: max_connections 1024 → 800.
• Container restarted; verified: process limit shows 8192, health green, h2 traffic normal, zero EMFILE lines since.

With nofile = 8192 and max_connections = 800, the semaphore now caps connection FDs at ~10% of the limit, leaving ample headroom. This removes the trigger, not the failure mode — see C1/C2 for the code fixes that remain open.

Evidence

Timeline (from /var/log/reverse-proxy/access.log.1, since rotated)

• 20:00 hour: 54 ERROR lines (baseline noise all day was 8–18/2h block)
• 21:00 hour: 5,558,249 ERROR lines (grep count) — flood begins
• 22:00 hour: 457,419 ERROR lines (flood tail)
• First ERROR overall: 2026-09-11T00:15:37Z; last: 2026-09-12T07:30:27Z (the 07:30 last line is the deploy restart cutting off the tail)

Error line shape:

2026-09-11T22:00:00.000011Z ERROR reverse_proxy::server: failed to accept TCP connection error=No file descriptors available (os error 24)
2026-09-11T22:00:00.000024Z ERROR ... (repeat, ~80 µs apart)

Note the ~13 µs spacing between consecutive EMFILE lines at 22:00:00.000 — a pure busy loop, no backoff, no yield.

Rotated-log size comparison

File	Size	Notes
access.log.2.gz (Sep 11→12)	55 MB gz / 1.9 GB raw	flood day
access.log.3–7.gz (Sep 6→11)	4.9–8.1 MB gz each	normal days

Trigger conditions

The EMFILE state was reachable because concurrent TLS connections (throttled only by the 1024 semaphore) plus the proxy's own FDs exceeded the container's Docker-default RLIMIT_NOFILE = 1024. Verified live on dev1 before mitigation:

Max open files   1024  519288  files      (containerized process)

Traffic context: the flood hour followed sustained crawler load (~77K requests from a 63-IP crawler fleet that day, plus a 30K-request poller from 74.7.242.34). A burst of slow/held TLS connections (e.g. crawlers opening connections and stalling handshakes) suffices to exhaust 1024 FDs; TLS handshakes in progress each hold an FD, and connection_idle_timeout only frees them after 60 s.

After mitigation (nofile 8192, max_connections 800): 0 EMFILE lines, 21 FDs in use at idle, health checks green.

Finding C1: Accept loop busy-spins on persistent accept errors [server]

Severity: High — turns any FD/socket exhaustion into a log flood that burns disk and I/O (1.9 GB in 2 hours observed) and starves the accept loop entirely.

Location: src/server.rs, serve_https_listener() (lines ~255–264):

loop {
    tokio::select! {
        accept_result = tcp_listener.accept() => {
            let (tcp_stream, remote_addr) = match accept_result {
                Ok(conn) => conn,
                Err(e) => {
                    error!(error = %e, "failed to accept TCP connection");
                    continue;   // <- busy spin on EMFILE/ENFILE
                }
            };
            ...

Why it matters: accept() fails immediately with EMFILE — there is no blocking wait between retries, so continue yields a ~13 µs error-log cycle. The log write itself (to file + stdout) makes the spin slower and more destructive.

Recommended fix (matching tokio's standard pattern):

Err(e) => {
    // Transient errors: retry immediately (EINTR, EAGAIN under load).
    // Persistent resource errors: back off so we don't busy-spin.
    let kind = e.kind();
    if matches!(kind, std::io::ErrorKind::WouldBlock | std::io::ErrorKind::Interrupted) {
        continue;
    }
    if matches!(kind, std::io::ErrorKind::ConnectionAborted)
        || e.raw_os_error() == Some(libc::EMFILE)
        || e.raw_os_error() == Some(libc::ENFILE)
    {
        error!(error = %e, "failed to accept TCP connection; backing off 1s");
        tokio::time::sleep(Duration::from_secs(1)).await;
        continue;
    }
    error!(error = %e, "failed to accept TCP connection");
    tokio::time::sleep(Duration::from_millis(100)).await;
    continue;
}

Options worth considering while fixing:

• Log the first N occurrences of a repeated accept error at error!, then demote to a periodic (e.g. once/10s) summary until the error clears — bounds log damage from any repeating accept failure.
• On EMFILE specifically, a small sleep (500 ms–1 s) is both correct and cheap: the process cannot make progress on accepts until FDs are released anyway.

Note: the redirect (HTTP:80) listener and health listener should get the same treatment if they share this loop shape.

Finding C2: max_connections is not cross-checked against RLIMIT_NOFILE [config]

Severity: Medium — the semaphore is the intended protection against FD exhaustion, but it can be configured at/above the process's real ceiling, making it a false safety net.

Location: src/config/static_config.rs (default_max_connections() = 1024) and validation in src/config/validation.rs.

Why it matters: max_connections counts only TLS connections; the process also needs FDs for: HTTPS + HTTP + health-check listener sockets, the log file, stdin/stdout/stderr, epoll/timer FDs, and ACME renewal outbound sockets (rustls-acme opens TCP + TLS per ACME interaction). With max_connections == RLIMIT_NOFILE (the dev1 situation, both 1024), the semaphore guarantees the ceiling can be hit under load.

Recommended fix: at startup (and reload), read the process soft RLIMIT_NOFILE (via rlimit crate or getrlimit) and reject config (or warn + clamp) if:

max_connections + reserved_fds > soft_limit

with reserved_fds ~64 (listeners + log + ACME + epoll headroom). Warn prominently when the soft limit is the Docker default 1024; better, the project's Docker/systemd docs should recommend nofile 8192 alongside max_connections = 1024 (or vice versa: derive a sane default from the observed limit).

Finding M1 (mitigation): raise nofile + lower max_connections [deploy]

Status: Applied 2026-09-12 (see Summary). Backups: docker-compose.yml.bak.20260912-080241, config.toml.bak.20260912-080241.

Residual risk after mitigation: none identified for FD exhaustion at current traffic (peak concurrent connections observed ≪ 800); the code findings C1/C2 remain the durable fix.

Traffic-analysis side note (from the same investigation)

The EMFILE hunt began as a traffic-volume estimate for Gitea, which surfaced the flood because yesterday's rotated log was 1.9 GB vs ~60 MB normal. Traffic profile (2026-09-11, 204K REQUEST lines):

• ~77K requests from a 63-IP crawler fleet (57.141.20.x — GitHub's published crawler range), ~1.3K req/IP, commit/blame/raw pages
• ~30K requests from 74.7.242.34 (Microsoft) polling /pulls + /issues on 3 repos (alkdev/alktunnels, alkdev/alktls, alkdev/alkgen) plus 56 on alkdev/typemap; all GET, no POSTs — dashboard-style monitoring, first seen that day
• ~20 HTTPS git fetches, ~22 API calls, zero HTTPS pushes
• Zero POSTs to /user/login; zero 401/403 that day — no credential attacks over HTTPS

None of this traffic is hostile per se, but it does mean the "1024 concurrent connections" ceiling is reachable from ordinary crawler behavior, which is exactly what made the EMFILE state reachable.

Recommended next steps

1. Land C1 (accept-loop error backoff + log de-duplication) — small, high-value; directly prevents recurrence of the log flood even under unknown future failure modes.
2. Land C2 (RLIMIT cross-check at startup) with a prominent warning or hard validation error.
3. Consider a doc note in docs/architecture/operations.md describing the nofile/max_connections relationship (the mitigation values above are a working baseline: nofile 8192, max_connections 800).
4. Docker image: the deployment Dockerfile on dev1 is a two-line stub (FROM + COPY); if the project's own deploy/Dockerfile is ever used, it should also set ulimits guidance or rely on compose as done here.