2023-08-14 22:05:43 +02:00
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---
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title: ".drvPath inconsistencies"
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author:
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- tazjin
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- flokli
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email:
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- tazjin@tvl.su
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- flokli@flokli.de
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lang: en-GB
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---
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# Why .drvPath differs between Nix and Tvix
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Nix and Tvix currently use a different approach when it comes to tracking input
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references, in order to build the right dependencies in advance.
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Nix is using string contexts, whereas Tvix is doing reference scanning [^inbox-drvpath].
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There are some real-life cases, for example during nixpkgs bootstrapping, where
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multiple different fixed-output derivations are written to produce the same
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hash.
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For example, bootstrap sources that are downloaded early are fetched using
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a special "builder hack", in which the `builder` field of the derivation is
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2024-02-18 10:57:46 +01:00
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populated with the magic string `builtin:fetchurl` and the builder itself will
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2023-08-14 22:05:43 +02:00
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perform a fetch, with everything looking like a normal derivation to the user.
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These bootstrap sources are later on defined *again*, once `curl`is available,
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to be downloaded using the standard pkgs.fetchtarball mechanism, but yielding
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the *same* outputs (as the same files are being fetched).
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In our reference scanning implementation, this output scanning of FOD will
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cause the path of the *first* derivation producing the given fixed output to be
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stored in the `inputDrvs` field of the derivation, while Nix will point to the
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derivation that was actually used.
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This doesn't cause any differences in the calculated *output paths*, as paths to
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fixed-output derivations are replaced with a special
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`fixed:out:${algo}:${digest}:${fodPath}` string that doesn't contain the "path
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to the wrong derivation" anymore.
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As we haven't fully determined if our reference scanning approach is gonna work,
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and comparing output paths is sufficient to determine equality of the build
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instructions, this is left as a future work item.
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[^inbox-drvpath]: https://inbox.tvl.su/depot/20230316120039.j4fkp3puzrtbjcpi@tp/T/#t
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