From e90586c0a40cf59df9d39407dcc49d5944a8b853 Mon Sep 17 00:00:00 2001 From: Valentin Gagarin Date: Fri, 29 Apr 2022 01:15:33 +0200 Subject: [PATCH] add motivation for references --- doc/manual/src/architecture/store/objects.md | 15 +++++++++++++-- 1 file changed, 13 insertions(+), 2 deletions(-) diff --git a/doc/manual/src/architecture/store/objects.md b/doc/manual/src/architecture/store/objects.md index 7f341c87c..30683d22d 100644 --- a/doc/manual/src/architecture/store/objects.md +++ b/doc/manual/src/architecture/store/objects.md @@ -37,9 +37,20 @@ Symlinks pointing outside of their own root, or to a store object without a matc ## Reference {#reference} -A store object can refer to other store objects or itself. +A store object can reference other store objects. -Nix collects these references by scanning file contents for [store paths](./paths.md) when a new store object is created. +Nix stores have the *closure property*: for each store object in the store, all the store objects it references must also be in the store. + +Building, copying and deleting store objects must be done in a way that obeys this property: + +- We can only safely delete unreferenced objects. + +- When copying, to maintain correctness, either the result must be "revealed" atomically to the destination store, or objects must be copied in reference-dependency order. + +- Newly built store objects must only refer to store objects in the closure of the build inputs. + This ensures the purity of the build. + +### Reference scanning While references could be arbitrary paths, Nix requires them to be store paths to ensure correctness. Anything outside a given store is not under control of Nix, and therefore cannot be guaranteed to be present when needed.