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self-references may not be discoverable by grep #1

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Apr 18, 2018
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self-references may not be discoverable by grep #1

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4 changes: 2 additions & 2 deletions rfcs/0017-intensional-store.md
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Expand Up @@ -25,7 +25,7 @@ One paragraph explanation of the feature.
Terms used:
* derivation: a `nix-build` output product depending on some inputs and resulting in a file or directory under `/nix/store`
* dependent derivation: a derivation built using the currently considered derivation
* `$out`: name of the location where a derivation is built, e.g., `zyb6qaasr5yhh2r4484x01cy87xzddn7-unit-script-1.12`
* `$out`: name of the location where a derivation is installed first, e.g., `zyb6qaasr5yhh2r4484x01cy87xzddn7-unit-script-1.12`
* calculated based on the hashes of all the inputs, including build tools
* `$cas`: output hash, the total hash of all the files under $out, with the derivation name appended, e.g., `qqzyb6bsr5yhh2r5624x01cy87xzn7aa-unit-script-1.12`

Expand All @@ -37,7 +37,7 @@ After building a derivation, `$cas` is calculated, and `$out` is renamed to `$ca

This means that if 2 different derivations of the same input have a different `$out` but the same `$cas`, any dependent builds will not need to rebuild due to the inputs being different. For example, the 12MB input `poppler-data` is often the same across multiple different input derivations, so many `$out`s for `poppler-data` all result in the same `$cas`. Similarly, a compiler flag change might leave most derivations unchanged.

In order to know which `$out`s refer to a particular `$cas`, symlinks can be used (`$out` pointing to `$cas`), or that data can be stored in the store database. The database can help with doing reverse lookups from `$cas` to all the `$out`s.
In order to know which `$out`s refer to a particular `$cas`, symlinks can be used (`$out` pointing to `$cas`), or that data can be stored in the store database. The database can help with doing reverse lookups from `$cas` to all the `$out`s. Using symlinks will have a benefit of handling the case when self-references that are not discoverable via grep (e.g. filtered by ```xxd(1)```).

## Calculating `$cas`

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