Review of 62a50eb: the layer lay in the workspace, which every step binds read-write,
so a change's command could swap a kept directory for a symlink and the move, run
outside the sandbox, would write into the shared cache (ADR 0541 §2). The layer now
lies beside the workspace (Workspaces.CacheLayer, removed by Release and Sweep),
Command refuses a layer within reach of the workspace, a writable directory or a
cache, and every move goes through an os.Root on the layer and keeps no symlink. Only
LayerCaches (the build cache) are layered; a module cache keeps its throwaway layer
(issue 1921).
Every command of a check ran over the shared Go build cache beneath a throwaway
tmpfs layer of its own, so the digests, the vet, the unit tests and the integration
compile of one ask each compiled every package again. With Isolation.CacheLayer a
command writes into a layer on disk of its own and Isolated moves it into the ask's
kept layer when it ends; later commands read the kept layer above the shared cache,
which stays unchanged (ADR 0541 §2, ADR 0448 §3). Empty CacheLayer keeps today's
throwaway layer. Ported from feat/try-speed S2 (issue 1921).
jschoubben