raphaelvigee/scratch driver - #403
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…ctory Adds the declaration half of scratch caching: a target kind that names a mutable, non-hermetic directory persisting across runs, which many targets can share. Nothing mounts, locks or stores anything yet — a declaration is inert on its own, and the engine work that gives it meaning lands on top of this. Declaring it as a target rather than inline at each use site is the point. Settings live in exactly one place, so two consumers cannot disagree about a slot's `access` or `version` — that whole class of error becomes inexpressible rather than something to detect and report. The addr is the cache's identity, so packages give namespacing for free (`//go:cache` and `//rust:cache` need no agreed prefix convention) along with visibility and `heph query revdeps`. It is a builtin driver shaped exactly like `plugingroup`, so no new Starlark global is introduced: `target(driver = "scratch", …)` is the surface, and `#[derive(Spec)]` supplies both the parser and the BUILD-file LSP schema. Two settings are author assertions heph cannot check, and both are documented as such with conservative defaults: - `access = "shared"` says the tool is safe under concurrent access. Go's build cache is the motivating case — forcing it exclusive would serialize a whole Go build. Defaults to `exclusive`. - `platform = "any"` says the contents are portable, which lets one slot serve every machine. It asserts two things, not one: no host dependence *and* no embedded absolute paths. Defaults to `os_arch`, because restoring a host-specific cache onto the wrong host is the one mistake here that is not merely slow. Mount-path validation lives on the declaration rather than at each use, so a bad path is reported once at its source. A mount is a symlink out of the sandbox, so absolute and `..`-escaping paths are rejected; so is mounting at the workspace root, which would replace the consumer's whole tree. Design doc: docs/SCRATCH.md. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01FArWjycMDyWeSfHHtpgtoU
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