move closure property to discussion references
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@ -35,21 +35,6 @@ A bare file or symlink can be a root file system object.
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Symlinks pointing outside of their own root, or to a store object without a matching reference, are allowed, but might not function as intended.
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## Reference {#reference}
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A store object can reference other store objects.
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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.
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Building, copying and deleting store objects must be done in a way that obeys this property:
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- Build results must only refer to store objects in the closure of the build inputs.
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- Store objects being copied must refer to objects already in the destination store.
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Recursive copying must either proceed in dependency order or be atomic.
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- We can only safely delete unreferenced objects.
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### Reference scanning
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While references could be arbitrary paths, Nix requires them to be store paths to ensure correctness.
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@ -52,12 +52,23 @@ References are [opaque][opaque-data-type], [unique identifiers][unique-identifie
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The only way to obtain references is by adding or building store objects.
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A reference will always point to exactly one store object.
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An added store object cannot have references, unless it is a build task.
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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.
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Adding, building, copying and deleting store objects must be done in a way that obeys this property:
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- A newly added store object cannot have references, unless it is a build task.
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- Build results must only refer to store objects in the closure of the build inputs.
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Building a store object will add appropriate references, according to provided build instructions.
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These references can only come from declared build inputs, and are not known to build instructions a priori.
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These references can only come from declared build inputs.
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- Store objects being copied must refer to objects already in the destination store.
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Recursive copying must either proceed in dependency order or be atomic.
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- We can only safely delete store objects which are not reachable from any reference still in use.
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A store object cannot be deleted as long as it is reachable from a reference still in use.
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Garbage collection will delete all store objects that cannot be reached from any reference in use.
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<!-- more details in section on garbage collection, link to it once it exists -->
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