add motivation for references

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Valentin Gagarin 2022-04-29 01:15:33 +02:00
parent 07d490fd89
commit e90586c0a4

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@ -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.