fix(shared): relay client install waits out a brief Windows file lock

[ScottN-PV]

Problem

On Windows, t3 connect can fail to install the relay client with RelayClientInstallError: Could not stage the relay client. and EBUSY on rename (#16945, seen on three Windows 11 machines). The installer runs the new cloudflared.exe to validate it, then renames it once. If another process still holds the file without delete sharing, that rename fails and the install stops.

Change

The two renames that move the binary into place, first to a staging name and then to its final path, now retry on Windows when they fail with EBUSY, EPERM, or EACCES. Each rename retries up to 40 times, 250 ms apart, then returns its existing error: "Could not stage the relay client." or "Could not activate the relay client." Other errors and other platforms fail at once, as before. A genuine EPERM or EACCES on Windows now takes about 10 seconds to report.

Scope and approval

Fixes #16945. Maintainer triage confirmed the failure on main and named this direction: "Retry both renames on Windows when they fail with EBUSY/EPERM/EACCES, using a short bounded backoff." The change is limited to packages/shared/src/relayClient.ts and its tests.

#16607 restructures the same two renames without retrying them. Whichever lands second needs to keep both changes. #16574 adds the same lock check for settings writes in apps/server.

Verification

Agents ran the real installer on Windows 11 into a scratch base directory. It downloaded the pinned 2026.5.2 release, verified its checksum, and ran cloudflared version. A PowerShell script held the new cloudflared.exe open without delete sharing, which produced the same sharing violation the issue reports.

Installer	Lock held	Result
main	none	Installed.
main	15 s	Failed: "Could not stage the relay client.", EBUSY on rename, as reported.
This branch	6 s	Installed after the lock cleared. The installed binary's SHA-256 matched the pin, and cloudflared version printed 2026.5.2.
This branch	30 s	Failed 10.2 s after activation began, with the same message.

• vp test run src/relayClient.test.ts (packages/shared): 11 pass on Linux. On Windows 7 pass and 4 skip, the existing cases that check POSIX exec bits. Six new cases: for each rename, a lock that clears and a lock that does not (41 attempts, then the error, with nothing left in the install directory); and staging failing at once on Linux or for another code. With relayClient.ts from main, the four lock cases fail. With only the staging rename retrying, both activation cases fail.
• vp test run src/cli/connect.test.ts src/cloud/ManagedEndpointRuntime.test.ts (apps/server): 27 pass.
• @t3tools/shared typecheck, vp lint, and vp fmt --check on the two changed files pass.

Not checked:

• A lock held by Defender or another scanner. The scripted lock reproduces the error, not its cause.
• The t3 connect command and the server's relay client install request. Both call this installer.

Known limitation: when a lock outlasts the wait, the failed install leaves its .install-* folder behind while the file is still held. main does the same.

Model: Claude Opus 5.5, Claude Fable 5.1 (review), GPT-6-Astra (review). Harness: Claude Code, Codex.

[macroscopeapp]

Approvability

Verdict: Approved at 65d2ac6

Macroscope's review found this PR approvable — The PR narrowly fixes transient Windows file-lock failures during relay-client installation by retrying the two activation renames, while preserving non-Windows and non-lock behavior. Focused tests cover successful retries, timeout cleanup, and immediate failure outside the intended case.

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[coderabbitai]

📝 Walkthrough

Walkthrough

The relay client now retries staging and activation renames when Windows reports specified transient lock errors. Tests cover successful retries, persistent failures, and errors that do not qualify for retries.

Changes

Windows relay-client rename retries

Layer / File(s)	Summary
Windows rename retry policy packages/shared/src/relayClient.ts	Both installation renames use a helper that retries EBUSY, EPERM, and EACCES on Windows for up to 40 attempts at 250 ms intervals. Existing error wrapping and staged-file cleanup remain.
Retry and failure behavior tests packages/shared/src/relayClient.test.ts	Tests verify that staging and activation succeed after two transient failures, persistent locks fail after 41 attempts with an empty managed directory, and Linux EBUSY and Windows EXDEV staging failures are not retried.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix · Severity of issue fixed: Medium

Suggested reviewers: juliusmarminge

Merge Risk: 🔵 Low · up to 65d2a

A persistent Windows lock takes one extra attempt and about 250 ms longer to fail. This bounded discrepancy can be corrected before merge or explicitly accepted.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: waiting through brief Windows file locks during relay client installation.
Description check	✅ Passed	The description covers the problem, implementation, scope and approval, verification results, limitations, and agent attribution. It includes focused test evidence and explains the linked issue and ma…
Linked Issues check	✅ Passed	The change satisfies the coding requirements for #16945. packages/shared/src/relayClient.ts retries both the staging and activation renames on Windows for EBUSY, EPERM, and EACCES, with 40 sch…
Out of Scope Changes check	✅ Passed	The reviewed changes are limited to packages/shared/src/relayClient.ts and packages/shared/src/relayClient.test.ts. The implementation helper and tests directly support the retry behavior requeste…
Approvability	✅ Passed	The pull request changes only packages/shared/src/relayClient.ts and packages/shared/src/relayClient.test.ts. It adds bounded Windows rename retries to an existing relay-client installation flow a…

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[coderabbitai]

Actionable comments posted: 1

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Inline comments:
Review comments at @packages/shared/src/relayClient.ts:
- Line 277: Update ACTIVATE_RETRY_COUNT used by Effect.retry so the initial
rename plus retries totals 40 attempts, and adjust the related test assertion to
expect 40 total attempts.

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📥 Commits

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📒 Files selected for processing (2)

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

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[juliusmarminge]

Rebased onto main after #17275 (712926a). The conflicts were both sides adding code in the same places, so both were kept; the relay client suite (16 tests) passes.