fix(connect): Restore the pinned relay client before connecting

[mwolson]

Problem

Fixes #16606. Before #9386 the managed relay client ran without --no-autoupdate and could replace itself in place, so the pinned install path can hold a newer cloudflared release than the one T3 Code pins and checksums. T3 Connect only checks that the file is executable and keeps launching it, so a host can run a release the app never tested, and hosts on the same app version can run different connectors.

Change

Before starting the connector, T3 Connect now checks that the managed relay client is the release it pins. It runs cloudflared version (the pinned Windows release rejects --version) and requires an exact match. On a mismatch it reinstalls the pinned release through the existing checksum-verified, locked installer, so a host ends up with the same binary a fresh install would download. Relay client status checks stay read-only, so they never download or replace a binary, including one a running connector is using.

The check is cached until the binary file changes, so a healthy install is probed once. An automatic repair gets 30 seconds, including any wait behind a manual install that is already running. If it fails or times out, connector startup goes ahead without waiting further (a replacement already in progress still finishes in the background, so the binary is never left half-swapped), the existing binary stays in place, a warning is logged, and further automatic attempts wait five minutes; a manual install still retries immediately. A relay client set through the override or found on PATH is never replaced, and the connector still runs with --no-autoupdate. This stays separate from #13968, which checks the version of a PATH or override binary.

Scope and approval

This fixes #16606, which maintainer triage confirmed on current main and labeled bug and via-triage, pointing to this PR as the fix: #16606 (comment). The change is limited to the relay client's managed-install resolution in packages/shared and the connector startup call that now uses it, plus their tests.

Verification

Focused relay client tests cover a stale managed binary being replaced with the pinned release, a matching binary used without a download, a failed or mismatched download keeping the existing binary, a stalled download, body, validation or activation falling back after 30 seconds, a stalled manual install not blocking connection startup, status checks leaving a stale binary alone, interruption during activation leaving only the managed binary, the cache and five-minute cooldown, and override and PATH binaries left untouched. The new tests fail without the change. The relay client and server cloud tests, lint, typecheck and knip pass.

Three hosts had self-updated binaries at the pinned path (2026.9.3 on a Mac and a Linux host, 2026.8.2 on another Linux host). Putting the pinned 2026.5.2 back at the same path, which is what this change does automatically, kept each one connected through a relaunch. This change has no UI.

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[macroscopeapp]

Approvability

Verdict: Not approved

Macroscope's review found this PR not approvable — The PR adds automatic managed-binary validation and replacement directly to the production connector startup path, including network downloads, executable activation, caching, and concurrency handling. An unresolved high-severity availability concern remains around timeout behavior during stalled activation.

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📥 Commits

Reviewing files that changed from the base of the PR and between 4ce938d and d1f7adb.

📒 Files selected for processing (5)

• apps/server/src/cli/connect.test.ts
• apps/server/src/cloud/ManagedEndpointRuntime.test.ts
• apps/server/src/cloud/ManagedEndpointRuntime.ts
• packages/shared/src/relayClient.test.ts
• packages/shared/src/relayClient.ts

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📝 Walkthrough

Walkthrough

RelayClient adds prepare to validate and repair managed cloudflared binaries. The managed endpoint runtime calls prepare before using the executable. Failed or timed-out repair returns the existing executable status.

Changes

Managed cloudflared validation and repair

Layer / File(s)	Summary
Managed binary identity and validation packages/shared/src/relayClient.ts, packages/shared/src/relayClient.test.ts	RelayClient identifies managed binaries by path and filesystem metadata. It checks the version command and requires the pinned version. Installation only reuses an available managed executable when that check passes. Tests cover identity changes, cached checks, and executable selection.
Serialized repair and install lifecycle packages/shared/src/relayClient.ts, packages/shared/src/relayClient.test.ts	prepare serializes repair, applies a five-minute retry interval, and waits up to 30 seconds. Failed or timed-out repair returns the existing executable status. Activation renames complete without interruption, followed by staged-file cleanup. Tests cover repair failures, concurrency, timeouts, and cleanup.
Managed runtime preparation apps/server/src/cloud/ManagedEndpointRuntime.ts, apps/server/src/cloud/ManagedEndpointRuntime.test.ts, apps/server/src/cli/connect.test.ts	The managed endpoint runtime calls prepare instead of resolve. Runtime tests check preparation timing and returned statuses. CLI tests assert that the download flow does not call prepare.

Priority: ➖ Normal

Estimated code review effort: 4 (Complex) | ~45 minutes

Change: Bug fix · Severity of issue fixed: Medium

Sequence Diagram(s)

sequenceDiagram
  participant ManagedEndpointRuntime
  participant RelayClient
  participant cloudflared
  participant InstallFlow
  ManagedEndpointRuntime->>RelayClient: Call prepare
  RelayClient->>cloudflared: Run version check for managed executable
  cloudflared-->>RelayClient: Return version and exit status
  RelayClient->>InstallFlow: Repair invalid managed executable
  InstallFlow-->>RelayClient: Return repair result
  RelayClient-->>ManagedEndpointRuntime: Return executable status

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Suggested reviewers: juliusmarminge

Merge Risk: ⚪ Minimal · up to d1f7a

Managed relay clients are now checked against the pinned cloudflared release and restored automatically before connecting. If repair fails or times out, the existing binary is kept and retries are throttled. Override and PATH binaries are left untouched. No blocking issues remain.

Security Architecture Review

Security architecture risk: 🔵 Low · up to d1f7a

Automatic repair retains checksum verification, serialized installation, and atomic replacement. Repair failure can still launch the existing connector, so startup does not strictly guarantee the pinned version. This behavior preserves the previous execution exposure rather than establishing a new security finding.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• inferred — The relevant exposure is the host’s managed executable installation and the connector process’s inherited environment and configured tunnel token. Executable compromise could therefore affect host-process permissions and that tunnel. The inspected change does not establish fleet-wide propagation or additional tenant authority.

Security Findings and Attack Paths

• observed — Repair failure deliberately permits startup with a binary that did not pass the pinned-version check. The base already launched the same available managed executable without that check and supplied the same connector credentials. This residual execution policy predates the PR; it is not retained as an introduced or worsened security concern.

Trust Boundaries and Controls

• observed — Network-supplied replacement bytes cross into executable authority only after checksum verification. User-selected override and PATH binaries remain outside automatic replacement. The version probe executes the existing managed file without a shell; it does not authenticate that file against a malicious local writer.

Resilience and Maintainability Implications

• observed — The timeout bounds preparation waiting, not completion of an already-started activation. Activation retains scope ownership and cleanup until rename callbacks finish, avoiding publication cleanup racing an outstanding filesystem operation.

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Caution

Pre-merge checks failed

Please resolve all errors before merging. Addressing warnings is optional.

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❌ Failed checks (1 error)

Check name	Status	Explanation	Resolution
Approvability	❌ Error	The pull request changes an external side effect. ManagedEndpointRuntime.ts now calls relayClient.prepare before connecting. In relayClient.ts, prepare calls installUnlocked when the managed…	This pull request needs a maintainer's review. Review and approve the automatic pre-connect download from GitHub and replacement of the managed relay binary in packages/shared/src/relayClient.ts, triggered by `apps/server/src/cloud/Manage…

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Issue [#16606] requires the managed client to run pinned cloudflared 2026.5.2. prepare checks the managed binary with cloudflared version and repairs a mismatch through the existing checksum-verif…
Out of Scope Changes check	✅ Passed	The changes in packages/shared implement managed-binary validation and repair. The ManagedEndpointRuntime call and its tests connect that preparation step to connector startup. The remaining test …
Title check	✅ Passed	The title clearly and concisely describes restoring the pinned relay client before connecting.
Description check	✅ Passed	The description covers the problem, change, scope and approval, and focused verification. It includes the linked issue and specific test results, and it states that the change has no UI.

Full details: Approvability

Explanation

The pull request changes an external side effect. ManagedEndpointRuntime.ts now calls relayClient.prepare before connecting. In relayClient.ts, prepare calls installUnlocked when the managed binary needs repair; the installer sends an HTTP GET for the pinned release and replaces the managed binary. This matches the rule “Adds or changes an external side effect.”

Resolution

This pull request needs a maintainer's review. Review and approve the automatic pre-connect download from GitHub and replacement of the managed relay binary in packages/shared/src/relayClient.ts, triggered by apps/server/src/cloud/ManagedEndpointRuntime.ts.

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[coderabbitai]

Actionable comments posted: 1

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🤖 Prompt to fix review comments

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Inline comments:
Review comments at @packages/shared/src/relayClient.ts:
- Around line 543-572: Coordinate automatic repair around the managed connector
lifecycle: defer the installUnlocked repair guarded by installSemaphore while
the connector is running, or stop it before replacing the managed binary. Resume
repair only when the executable is no longer in use, avoiding repeated failed
activation attempts and held permits.

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📒 Files selected for processing (2)

• packages/shared/src/relayClient.test.ts
• packages/shared/src/relayClient.ts

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