[tiny-ac]

Adds SHA-256 file digests, a cache of them, and the digest of a remote compile's input root, REAPI's name for the directory tree of every file the compile reads. Nothing calls them yet; a later PR sends them to a REAPI server.

DigestCache hashes a file again when its size, modification time, ctime or inode changes. It doesn't cache files changed in the last 2 s, since on a filesystem with coarse timestamps a same-size edit can leave the size, modification time and ctime unchanged. When full (100,000 entries), it drops every entry that isn't pinned. digest_and_pin pins an entry, save writes the pinned entries to a file, and load reads them back as pinned.

InputRoot::build lays out the files as REAPI's canonical Directory tree and returns the root's digest. It fails on conflicting or invalid paths.

Changes

• src/reapi/digest.rs: of_bytes, of_file and DigestCache.
• src/reapi/input_root.rs: InputRoot and InputRootFile.
• Cargo.toml: reapi enables the existing optional sha2 dependency.

Cost

Without the reapi feature, nothing new is compiled. With it, only sha2 is added, which dist-client already uses.

Known limits

On Windows the cache checks a file's creation time instead of its ctime and inode, so an in-place edit that keeps the size and modification time goes unnoticed. sccache's preprocessor cache has the same limit.

Validation

• fmt, and clippy with -D warnings and --features reapi, at each commit.
• cargo test --features reapi --lib reapi::: 27 new tests, 13 for digests (one Unix-only, one Linux-only) and 14 for the input root.
• cargo check --no-default-features --features reapi.