fix(server): read desktop telemetry without blocking process exit

[saphid]

Problem

If the desktop app's backend crashes, it can fail to exit. The process stays alive with nothing running, so the desktop supervisor never sees an exit and never restarts it. The window is stuck on "Connecting…" until the app is quit.

Why this qualifies

This is a small, obvious bug fix: a crashed backend must be able to exit so the supervisor can restart it. No issue or prior maintainer discussion exists for it. The change only affects how the backend opens its inherited telemetry descriptor. The telemetry protocol, decoder, control channel, and scoped cleanup are unchanged.

Fix

The desktop app passes host telemetry to the backend on fd 4 and keeps the writing end open. The receiver read fd 4 with fs.createReadStream, and a pending read on a pipe or socket holds a libuv threadpool worker. Node cannot finish exiting while that read is outstanding, even after destroy(), so an uncaught exception or a normal scope shutdown hangs.

The receiver now adopts pipes and sockets as a read-only net.Socket, which uses the event loop instead of a worker. Descriptors that net.Socket rejects with ERR_INVALID_FD_TYPE, such as regular files, still use fs.createReadStream. The change stays inside the receiver's existing Node boundary and adds no diagnostic suppressions.

Evidence

Environment: macOS 26.5.2 (arm64), Node 24.12.0, desktop dev build (vp run dev:desktop, Electron 44.4.2) on fresh local state. Upstream main c5a0c78b7c. The desktop run used the earlier main 845ddd9354. The receiver file is identical on both. Of the 4 commits between them, three only change server test fixtures. The fourth, c5a0c78b7c, adds keyboard machine stepping for new threads. It changes the web and mobile clients (including iOS and Android native keyboard modules), the shared keybinding contract, and the keybindings user guide. None of the four touches the telemetry receiver, the desktop app, or the backend exit path.

Real desktop reproduction. Start the desktop app and wait for "Connected". To simulate a crash, send SIGUSR1 to the backend process to open its Node inspector, then evaluate setTimeout(() => { throw new Error("injected backend crash") }) in it. Watch the backend process and the window.

• Before (main): the backend logs Error: injected backend crash but stays alive. 20 s of sampling saw the same PID in state Ss, no exit, and no restart scheduled log. The window shows a stale "Connected", then "Connecting…" with Continue disabled, and never recovers.
• After (this PR): the backend exits right after the throw. The supervisor logs backend exited unexpectedly; restart scheduled (reason: code=1, delayMs: 500), starts a new backend, and logs backend ready about 3.4 s after the injection was scheduled. The window shows "Connecting…" briefly, reconnects, and Continue works.

	Before (main)	After (this PR)
Connected
After crash
Recording

Full recordings: before (mp4), after (mp4). The machine name is blurred.

Backend-only reproduction. Run the real DesktopTelemetryReceiver in a child process with fd 4 as a pipe. Send a valid desktopTelemetryHello, keep the writer open, then raise an uncaught exception. A 5 s watchdog bounds a hung child.

Before: receivedTelemetry=true uncaughtCrash=true code=null signal=SIGKILL watchdogKilled=true
After:  receivedTelemetry=true uncaughtCrash=true code=1    signal=null    watchdogKilled=false

Tests (from apps/server): vp test run src/resourceTelemetry/DesktopTelemetryReceiver.test.ts

• With main's receiver and this PR's tests: 2 failed, 8 passed. "exits after a crash with the desktop telemetry writer still open" and "closes its receiver scope with the desktop telemetry writer still open" both fail with expected 'SIGKILL' to be null.
• At this head: 10 passed. The new tests also cover fragmented input with EOF and the regular-file fallback. Passing cases resolve on the child's real close event. A generous 60 s bound on the child only fails a hang and never paces a passing run. Five concurrent runs on a loaded machine all passed. With an extra 8 s injected into child startup, all 10 still pass.

Also run at this head on c5a0c78: vp run --filter t3 typecheck, vp lint --report-unused-disable-directives and vp fmt --check on the two changed files, vp run build:desktop, and node scripts/release-smoke.ts. All pass.

Surfaces

• Web: not affected. The descriptor only comes from the desktop bootstrap envelope, so servers started by npx t3 or for app.t3.codes never open one.
• Desktop: affected and fixed. This is the desktop-supervised backend, verified above on macOS.
• Mobile: not affected in code. Mobile clients connected to a desktop-hosted server get the same benefit: the backend restarts instead of hanging.
• Local / remote-relay / tunnel: the fix is in the backend process lifecycle, below the transport layer. Every connection mode recovers when the backend restarts. Only local was exercised.
• Providers (Codex, Claude, Cursor, Grok, OpenCode, Antigravity): not affected. No adapter code changes, and this behavior does not depend on the provider.
• WSL backends: not affected. They receive the bootstrap on stdin and get no telemetry descriptor.

Not checked

• Linux and Windows were not run. Whether net.Socket adopts the inherited descriptor there is untested. Anything it rejects with ERR_INVALID_FD_TYPE falls back to the previous fs.createReadStream path.
• Remote/relay/tunnel reconnection after the restart was not exercised. Only the local desktop window was.
• The packaged (signed) desktop build was not run; the reproduction used the dev desktop build.
• The crash was injected through the Node inspector, not a real backend bug. The exit path is the same uncaught-exception path.

GPT-6.1 Sol, Claude Opus 5.5 and GPT-6 Astra via T3 Code
🤖 Generated with Claude Code

Part of #16932 (with #16626, which frees the browser-channel worker).

[macroscopeapp]

Approvability

Verdict: Approved at c398a69

Macroscope's review found this PR approvable — The PR narrowly fixes desktop backend shutdown by replacing the blocking inherited-pipe read path while preserving the existing fallback and telemetry processing. Regression tests cover crash exit, cleanup, EOF, fragmented input, and regular descriptors, with no product-default or static-analysis override changes.

Notes:

• All code in this push has already been reviewed. Approvability was decided on eligibility alone.

^{You can add or adjust custom eligibility rules. Learn more.}

[coderabbitai]

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• apps/server/src/resourceTelemetry/DesktopTelemetryReceiver.test.ts

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📝 Walkthrough

Walkthrough

The desktop telemetry receiver now opens its inherited descriptor as a socket, with a file-stream fallback for ERR_INVALID_FD_TYPE. Subprocess tests cover crash exit, scope closure, EOF, and telemetry input from a regular file.

Changes

Desktop telemetry descriptor handling

Layer / File(s)	Summary
Inherited descriptor reading and lifecycle tests apps/server/src/resourceTelemetry/DesktopTelemetryReceiver.ts, apps/server/src/resourceTelemetry/DesktopTelemetryReceiver.test.ts	The receiver tries to create a socket for the inherited descriptor and falls back to createReadStream when socket creation fails with ERR_INVALID_FD_TYPE. Subprocess tests check crash exit, scope closure with the writer open, EOF after writer closure, and receipt from a regular file descriptor.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~12 minutes

Change: Bug fix

Merge Risk: 🔵 Low · up to c398a

The desktop telemetry fix is small and tested. The only open concern is whether the new subprocess tests run on older supported Node versions. This is a low merge risk, but confirm the supported Node range before merging.

Security Architecture Review

Security architecture risk: 🔵 Low · up to c398a

The change preserves the existing desktop telemetry interface and control permissions while addressing a backend exit hang. Remaining uncertainty concerns descriptor compatibility and shutdown behavior across desktop platforms, rather than expanded access or authority.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• inferred — The changed exposure is bounded to reading and releasing the existing backend telemetry descriptor and the availability effects inherited by existing consumers. The source comparison does not show a new network entrypoint, descriptor source, or control-channel privilege.

Trust Boundaries and Controls

• observed — Telemetry supplied through the inherited descriptor still passes protocol-version and message decoding checks before affecting receiver state. Those checks and the separate serialized control-write path predate this PR and remain in place; socket adoption does not itself grant write authority to the telemetry reader.

Resilience and Maintainability Implications

• observed — The receiver retains scoped consumption and cleanup, reports stopped on EOF and degraded on processing failure, and preserves stopped health during stale checks. The supplied lifecycle tests do not establish platform-specific ownership after construction failure, repeated or concurrent cleanup behavior, or successful recovery after a supervisor restart.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 3 functions across 4 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly and concisely describes the primary fix: preventing desktop telemetry reads from blocking backend process exit.
Description check	✅ Passed	The description explains the problem, fix, scope, verification results, environment, evidence, and untested platforms. It uses alternative headings such as “Fix,” “Evidence,” and “Not checked,” but in…

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[coderabbitai]

Actionable comments posted: 1

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• 🪄 Fix CodeRabbit comments on this PR

🤖 Prompt to fix review comments

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only still-valid issues, skip the rest with a brief reason, keep changes
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Inline comments:
Review comments at
@apps/server/src/resourceTelemetry/DesktopTelemetryReceiver.test.ts:
- Around line 45-46: Update the Node arguments in runReceiverChild to enable
TypeScript type stripping before the child imports DesktopTelemetryReceiver.ts,
so it reaches ready under Node 22.16.

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[saphid]

Rebased onto current main. Notes on the CodeRabbit summary: (1) the Node 22.16 merge-risk note refers to the withdrawn strip-types finding; CI and development run Node ^24.13.1. (2) Docstring coverage: the touched functions are the existing make (already documented), the test helper runReceiverChild, and the inline descriptor adoption, which has a comment explaining the exit constraint. We are not adding boilerplate docstrings to reach a percentage. (3) Platform descriptor teardown: verified on macOS (unit tests, a backend reproduction, and a real desktop crash-and-restart run); Linux and Windows are listed under Not checked.

Dismissing prior approval to re-evaluate c398a69

[saphid]

Review requested

Date (UTC)	Reviewer	Where
2026-10-03	Julius	Discord DM

_{Logged so this PR shows when a maintainer was asked to review it.}

[saphid]

Review requested

Date (UTC)	Reviewer	Where
2026-10-06	Julius	Discord DM

_{Logged so this PR shows when a maintainer was asked to review it.}

[juliusmarminge]

Note

Grok responding on behalf of Julius.

Closing as superseded by #17386, which already landed the same net.Socket fix for DesktopTelemetryReceiver (and the browser-channel pipe reader) on main. Thanks for the solid repro and evidence — the merged PR covers this path.

[juliusmarminge]

Note

Grok responding on behalf of Julius.

Closing as superseded by #17386, which already landed the same net.Socket fix for DesktopTelemetryReceiver (and the browser channel) on main. Thanks for the work — the root cause is covered.