fix(rust): backdate extracted CLI mtime to stop build.rs self-invalidation

[redsun82]

Problem

When the bundled-cli cargo feature is off, rust/build.rs extracts the Copilot CLI binary into a cache dir and emits cargo:rerun-if-changed=<extracted binary path> on it. That watch is deliberate: it's how a deleted cached binary forces a re-extract, so cargo doesn't replay a stale has_extracted_cli cfg and leave runtime resolution failing with BinaryNotFound (see src/resolve.rs::extracted_cli_path).

The trap: on a cold cache the same build.rs creates that binary mid-build, seconds after a network download + extract. Cargo stamps the build script's output reference file when the script is spawned — before the binary is written. So the freshly-extracted binary's mtime ends up newer than its own rerun-if-changed reference. The next identical cargo invocation sees the watched file as "changed", reruns build.rs, recompiles the SDK crate, and relinks every downstream crate. A warm cache never reproduces (the file already exists, not created that build) — so it's a cold-cache / CI-only redundant rebuild.

Cargo's own fingerprint log proves it:

stale: changed '<cache>/copilot'
  (vs)  '<target>/build/<crate>-<hash>/output'
FileTime{...942} < FileTime{...950}   # output(reference) < binary

In a downstream consumer's CI this wasted ~9 minutes (a second cargo invocation in the same job recompiled the SDK + a Tauri backend with unchanged sources).

Fix

In extract_to_cache, after writing + chmod'ing the staged binary but before the atomic rename into place, backdate the staged file's mtime to the Unix epoch via std::fs::File::set_modified(SystemTime::UNIX_EPOCH). The same-fs sibling rename preserves the mtime, so the file lands already-backdated and unambiguously older than any real build reference — a second identical invocation becomes a true no-op. Best-effort: on error we emit a cargo:warning and continue (worst case = today's redundant-rebuild behaviour, never a broken build).

Why this is correct for all SDK consumers

• Embed mode (bundled-cli on): unaffected — that path emits no rerun-if-changed on any build-created file, so it has no self-invalidation. No change needed there.
• COPILOT_CLI_EXTRACT_DIR set vs default cache dir: both flow through the same extract_to_cache, so one fix covers both.
• sccache / cross-machine target/ reuse: unaffected/helped — the cache binary isn't a compiler input, and an epoch mtime is deterministic across machines.
• Concurrent/parallel builds: safe — each build backdates its own PID+nanos staging file before the atomic rename; last rename wins and the final file is epoch-dated regardless.
• Windows/NTFS (epoch 1601) & FAT (floor 1980): 1970 is fine on NTFS/APFS/ext4; the platform cache dir is never FAT. A FAT COPILOT_CLI_EXTRACT_DIR would clamp to 1980 — still older than any real reference, still benign.
• Deleted-file recovery + version-bump contract: preserved — a missing file still can't be stat'd, so cargo still treats it as stale and reruns; the cli-version.txt watch and version-keyed cache path are untouched.

Verification

Reproduced the consumer scenario (cli-version.txt present, --no-default-features) with a fresh extract dir:

• Build 1 (cold): extracts → binary mtime is 1970-01-01 (epoch 0).
• Build 2 (identical): Finished in 0.05s, no Compiling github-copilot-sdk, no stale/dirty in the fingerprint log — a true no-op.

cargo +nightly fmt --check, cargo clippy -D warnings (the repo's just lint-rust), and all non-e2e test targets (173 tests) pass. The e2e suite is unrunnable in this environment (requires npm install in nodejs/ for the CLI) — orthogonal to this change.

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com

[Copilot]

Pull request overview

This PR addresses a cold-cache CI issue in the Rust SDK where build.rs self-invalidates: the Copilot CLI binary is created during the build, ends up newer than Cargo’s build-script reference timestamp, and forces an unnecessary rerun/recompile on the next identical cargo invocation.

Changes:

• Backdate the freshly extracted staged Copilot CLI binary’s mtime to SystemTime::UNIX_EPOCH (best-effort), so it cannot appear “newer” than Cargo’s build-script reference file on subsequent builds.
• Emit a cargo:warning (and continue) if the filesystem refuses setting the timestamp, preserving the pre-fix behavior rather than breaking builds.

Show a summary per file

File	Description
rust/build.rs	After extracting the CLI to a staging file, sets its modified time to the Unix epoch before the atomic rename, preventing Cargo rerun-if-changed self-invalidation on cold caches.

Copilot's findings

• Files reviewed: 1/1 changed files
• Comments generated: 0

[SteveSandersonMS]

Thanks!

[redsun82]

@SteveSandersonMS I don't have the permission to merge here, would you mind doing that for me?

[redsun82]

(the last push is a force rebase Copilot decided to do 😅)