mangaBookmark/docs/adr/0005-on-demand-browser.md

ADR-0005: On-demand browser sidecar

Date: 2026-08-09 Status: accepted

Decision

Keep the headless-shell service and its CDP port alive, but start Google Chrome only when the first CDP connection arrives. The entrypoint supervises a socat front-end, serializes browser start/reap state with flock, and tracks each connection with a marker named for its helper PID. A reaper stops Chrome after 300 seconds with no live markers. Marker reconciliation covers a helper killed before its cleanup trap runs.

Chrome runs in its own process group so reap sends the termination signal to Chrome and its renderer children. The explicit /home/chrome/profile user-data directory remains: Chrome remaps remote debugging to loopback on modern builds, and Chrome ignores remote-debugging flags on a default profile. socat therefore continues to front Chrome's loopback CDP port.

The profile is a named Compose volume. Clearance cookies survive both a reap and docker compose up --build; the browser still starts with a fresh debugger UUID, so chromedp must keep endpoint discovery enabled and must not use chromedp.NoModifyURL.

The socat front-end and explicit profile are retained because Chromium remaps a non-loopback debugging address to loopback since M113, while Chrome ignores the remote-debugging flags on a default profile since Chrome 136. Flag tuning is deliberately not adopted: its roughly 30% idle-footprint saving is irrelevant to a browser that exists for seconds per wake and risks an untested fingerprint.

Constraints

The 300-second floor is deliberate. Chromium batches cookie persistence on a roughly 31-second timer, and Go's default HTTP transport can keep the discovery connection parked for about 90 seconds after use. Reaping only with zero live connections holds Chrome through both windows and through the poller's staggered batch plus cover prefetch.

The anti-bot properties remain unchanged: a plausible non-UTC timezone, a Chrome-version-derived User-Agent without HeadlessChrome, and no automation flag. A remote browser restart can surface as context.Canceled, the same error as a caller deadline, so the backend wraps cancellation observed with a closed CDP connection as browser interrupted; the focused test asserts that classification without killing a real browser. The same process-group stop runs during supervisor shutdown, not only during idle reap, so Chrome can flush its cookie batch before a container rebuild or graceful stop.