Fix dist overlay: persistent toolchain dirs and sysroot packaging

[justdoGIT]

Summary

Two independent fixes for sccache-dist that affect Void Linux (and potentially other distros with non-standard sysroot layouts).

Fix 1: OverlayBuilder crash on persistent toolchain dirs

When sccache-dist restarts, the in-memory toolchain_dir_map is wiped, but the on-disk toolchain directories survive. On the next build, prepare_overlay_dirs hits the else branch (toolchain not in the map) and calls fs::create_dir on the existing directory, which returns an AlreadyExists error and fails the build.

Guard create_dir with an exists() check. The archive_id is a content hash, so a pre-existing directory for the same id already holds the correct unpacked toolchain and is safe to reuse.

Fix 2: Package rustlib/ instead of whole sysroot lib dir

On distros where rustc --print sysroot reports /usr (e.g. Void Linux), the ToolchainPackager joins LIBS_DIR ("lib") to the sysroot and packages the entire /usr/lib tree. This drags the full system library set (several GiB) into every distributed toolchain archive, blowing past the toolchain cache size and making toolchain packaging prohibitively slow.

Package the rustlib/ subtree instead, which is the actual Rust standard library. Fall back to the old behavior (whole lib dir) if rustlib/ does not exist, preserving compatibility with non-standard sysroot layouts.

Testing

• Built and deployed sccache-dist with both fixes on a 4-node LAN cluster (Void Linux)
• Verified toolchain packaging completes in seconds instead of timing out
• Verified sccache-dist restarts no longer fail with AlreadyExists errors
• All existing unit tests pass

[Copilot]

🟡 Changes recommended

Missing regression tests and unresolved overlay-directory persistence prevent approval.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

This PR reduces Rust toolchain archive sizes and handles pre-existing overlay toolchain directories.

Changes:

• Packages lib/rustlib with a compatibility fallback.
• Avoids errors when toolchain directories already exist.

File summaries

File	Summary
src/compiler/rust.rs	Narrows Rust packaging to rustlib; regression tests for selection and fallback are needed.
src/bin/sccache-dist/build.rs	Guards existing directories; persistence behavior remains unresolved because cleanup removes them, and regression coverage is needed.

Review details

Suppressed comments (2)

src/bin/sccache-dist/build.rs:197

• This branch is not reached for the described server-restart scenario: OverlayBuilder::new calls cleanup() before creating these directories, and cleanup() recursively removes self.dir. A normal restart therefore deletes toolchains/<archive_id> before prepare_overlay_dirs runs, so this change does not provide the claimed restart persistence. Preserve the toolchain directory while cleaning only transient build data, or otherwise add the actual persistence path before relying on this guard.

                if !toolchain_dir.exists() {
                    fs::create_dir(&toolchain_dir)?;

src/compiler/rust.rs:2296

• Because this changes the bytes produced for an unchanged compiler, the toolchain key also needs invalidation. RustHasher builds weak_toolchain_key from CACHE_VERSION and direct sysroot shared-library digests, while ClientToolchains::put_toolchain reuses the persistent weak_map without calling write_pkg; existing clients will therefore keep selecting the old multi-GiB archive after upgrading. Bump the Rust cache/package version or add a packaging-version component to the weak key so this branch runs for existing caches.

            let rustlib_path = libs_path.join("rustlib");
            if rustlib_path.is_dir() {
                package_builder.add_dir_contents(&rustlib_path)?;

• Files reviewed: 2/2 changed files
• Comments generated: 2
• Review effort level: Lite

---

💡 Add a code-review agent skill or configure MCP servers for context-aware, tailored reviews. Learn more in the docs.

[justdoGIT]

Superseded by #2850 which consolidates all fixes onto a single branch.