fix(plugin): keep the port when the app holds it but cannot reach herdr

[devswha]

What

plugin start keeps the app on its port when the app's own server holds that port but cannot reach herdr. It reports that state and exits 1 instead of moving the app to another port.

Why

Since #415, settlePort() waits up to 20 s while the recorded pid is alive, then falls through to freePort (#428). When the server that holds the port is ours and only herdr is away, that move starts a second launcher on a fallback port, rewrites plugin-port and the pid file, and blames "another program". The supervisor lock is per port, so nothing stops the second launcher, and a later stop stops only that one.

How

After the wait, and before it picks another port, start asks /api/health?scope=bridge. That endpoint answers without herdr. If the app answers there, the port is ours: when full health has meanwhile come back, start reports "already running" as before. Otherwise it fails with the address, the recorded pid and the error the app's /api/health gives (herdr's connect_failed), and leaves plugin-port and server.pid alone. A port held by anything else is handled as before. The wait stays bounded by the existing deadline.

This is option 2 from the issue, with option 1's outcome: no move, a clear message, and the kept port untouched.

How verified

• New case in scripts/plugin.test.ts (plugin start moves the app to another port when its own server holds the port and herdr is unreachable #428) reproduces the exact setup: the kept port is held by a fake app whose bridge health answers ok and whose full health answers 502 connect_failed, and the recorded pid is alive. On origin/main it fails with Using port 17317 instead. With the fix it passes, and the whole file passes (12/12). Disabling the new check (if (false && await appOnPort())) turns it red again.
• End to end, isolated: an own herdr session under a scratch XDG_CONFIG_HOME, scratch HOME and plugin state/config dirs, and HOST=127.0.0.2. I ran the issue's steps:
  1. start listened at 127.0.0.2:7317, and health answered 200.
  2. I stopped herdr: /api/health answered 502, and ?scope=bridge answered 200.
  3. start exited 1 after 20 s with herdr web ui is running at http://127.0.0.2:7317 (pid …) but cannot reach herdr: cannot reach herdr at …/herdr.sock: Failed to connect. It keeps this port and answers again once herdr is back. No plugin-port file was written, the pid file still named the first launcher, and only 7317 was listening.
  4. I started herdr again, and 7317 answered 200.
  5. stop stopped the first launcher, and no listener was left.
• bun run check fast passes locally.

Not changed: the older case in the issue's "Related" section (server.pid naming a child that died on the supervisor lock) is left as it was. With this fix, a start that finds the app on the port without a live recorded pid also keeps the port at once instead of moving.

Fixes #428

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📝 Summary

Summary by CodeRabbit

• Bug Fixes
  • When the app is running on its expected port but cannot connect to herdr, the plugin now reports the connection issue and exits instead of trying another port or misreporting a port conflict.
  • The error report includes the available health-check failure details.
  • If herdr becomes available during the check, startup succeeds and the app remains on its original port.

Walkthrough

When the app server holds its port but cannot reach herdr, start reports the health error and exits without selecting another port. If herdr becomes reachable during the check, start reports that the server is already running.

Changes

Plugin start health handling

Layer / File(s)	Summary
Detect the app server and handle health failures scripts/plugin.ts, scripts/plugin.test.ts, CHANGELOG.md	Bridge-scoped health identifies the app server. If full health fails, startup checks for recovery and otherwise reports the failure without selecting another port. Tests cover both the failure and recovery cases. The failure test verifies that the saved port and PID remain unchanged. The changelog describes the behavior.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix · Severity of issue fixed: Medium

Suggested reviewers: yoonwoo-ha

Merge Risk: ⚪ Minimal · up to 1b4dc

No actionable issue remains that would prevent merging after normal checks.

Security Architecture Review

Security architecture risk: ⚪ Minimal · up to 1b4dc

The fix preserves the existing server’s address and process record during dependency failure instead of launching another instance. No material new security exposure was identified in the changed startup paths.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• inferred — The changed decision affects the configured plugin server address, its local ownership records and startup diagnostics. The added requests carry no authentication credentials and do not grant filesystem, terminal or cross-machine authority.

Security Findings and Attack Paths

• inferred — A principal controlling the configured port can imitate bridge health and force the new keep-and-fail outcome. However, that same principal could already prevent real startup at the base by returning successful full health. This is a pre-existing ownership-identity limitation, not an identified expansion of attacker authority or independently attackable scope.

Trust Boundaries and Controls

• observed — Bridge detection accepts JSON ok=true without checking HTTP success, bridge_protocol or process identity. It is an availability heuristic, not authorization. Production protected routes still apply the existing authentication, origin and read-only controls.

Resilience and Maintainability Implications

• observed — The supervisor continues to enforce its existing checkout-and-port lock and remove that lock during shutdown. Launcher PID writes are still separate from this lock; the PR does not make concurrent startup globally atomic. The new failed/running branch avoids those writes and child creation.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 42.86% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 7 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: keeping the app port when the app holds it but cannot reach herdr.
Description check	✅ Passed	The description explains the problem, intended behavior, implementation, testing, and regression coverage. It uses headings that differ from the template, but it includes the required change and valid…
Linked Issues check	✅ Passed	Issue #428 requires start to keep the app port when the app answers bridge health but full health cannot reach herdr. The production summary states that appOnPort checks bridge health after the bo…
Out of Scope Changes check	✅ Passed	The changes stay within issue #428. The production changes implement the port decision and error reporting. The test changes cover the reported regression and recovery behavior. The changelog document…

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Inline comments:
Review comments at @scripts/plugin.ts:
- Line 157: Update healthError and its call from start to bound the diagnostic
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- Line 160: Update healthError and its handling in settlePort/start so a
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[devswha]

Independent review (merge-own lane).

Classification: serious bug fix (#428). With herdr away, a second plugin start moved the app to a fallback port beside the running server, rewrote plugin-port and the pid file, and blamed "another program". A later stop then stopped only the second launcher.

What I checked:

• settlePort() still waits for a live recorded start first. Only after that wait, and before freePort, does it ask /api/health?scope=bridge. The server answers that before it pings herdr and without an auth gate (server/index.ts:1315), so an app that cannot reach herdr still answers ok: true. If full health has come back meanwhile, start reports "running" as before. Otherwise it fails with the address, the recorded pid and the app's own health error, and leaves plugin-port and server.pid unchanged. A port held by anything else (no ok: true on the bridge scope) takes the old path, and so does an older app that ignores scope. Both new fetches are bounded at 1.5 s (healthError was at the 20 s start deadline and was cut to 1.5 s in d9bc824, which I reviewed as a delta).
• The test can fail: with the guard disabled (if (false && await appOnPort())), the #428 case fails with Using port 17317 instead.
• The CHANGELOG entry sits under ## [Unreleased] / ### Fixed with its PR link. scripts/ is not typechecked, so I ran the file that changed.

What I ran:

• HERDR_TEST_MODE=unit bun test ./scripts/plugin.test.ts: 12 pass, 0 fail.
• CodeRabbit's one thread was valid: if herdr came back between the last health check and healthError(), start failed with "its health check answered 200". I fixed it in 1b4dc17 (healthError() answers null for an app that answers, and settlePort() reports it running) and added a test that makes the race certain. It fails without the fix and passes with it (13 pass). The thread is answered and resolved.
• Updated with main (e06edfa, f38888f, then d162b9a after #579-#581; CHANGELOG conflicts resolved); plugin.test.ts 12 pass and bun run check fast 1716 pass, 0 fail on e06edfa. Fast checks and Integration and browser are green on 1b4dc17.