9bb3f74062
This question popped up every once in a while. While already explained quite well at https://inbox.tvl.su/depot/20230316120039.j4fkp3puzrtbjcpi@tp/T/#t, it's not easily accessible. Lift it from there into tvix/docs for better visibility. Change-Id: I5f2d4aff31ab4adc421e06a7d36c871f45e09100 Reviewed-on: https://cl.tvl.fyi/c/depot/+/9080 Autosubmit: flokli <flokli@flokli.de> Reviewed-by: raitobezarius <tvl@lahfa.xyz> Tested-by: BuildkiteCI
46 lines
1.9 KiB
Markdown
46 lines
1.9 KiB
Markdown
---
|
|
title: ".drvPath inconsistencies"
|
|
author:
|
|
- tazjin
|
|
- flokli
|
|
email:
|
|
- tazjin@tvl.su
|
|
- flokli@flokli.de
|
|
lang: en-GB
|
|
---
|
|
|
|
# Why .drvPath differs between Nix and Tvix
|
|
|
|
Nix and Tvix currently use a different approach when it comes to tracking input
|
|
references, in order to build the right dependencies in advance.
|
|
Nix is using string contexts, whereas Tvix is doing reference scanning [^inbox-drvpath].
|
|
|
|
There are some real-life cases, for example during nixpkgs bootstrapping, where
|
|
multiple different fixed-output derivations are written to produce the same
|
|
hash.
|
|
|
|
For example, bootstrap sources that are downloaded early are fetched using
|
|
a special "builder hack", in which the `builder` field of the derivation is
|
|
populated with the magic string `builtins:fetchurl` and the builder itself will
|
|
perform a fetch, with everything looking like a normal derivation to the user.
|
|
|
|
These bootstrap sources are later on defined *again*, once `curl`is available,
|
|
to be downloaded using the standard pkgs.fetchtarball mechanism, but yielding
|
|
the *same* outputs (as the same files are being fetched).
|
|
|
|
In our reference scanning implementation, this output scanning of FOD will
|
|
cause the path of the *first* derivation producing the given fixed output to be
|
|
stored in the `inputDrvs` field of the derivation, while Nix will point to the
|
|
derivation that was actually used.
|
|
|
|
This doesn't cause any differences in the calculated *output paths*, as paths to
|
|
fixed-output derivations are replaced with a special
|
|
`fixed:out:${algo}:${digest}:${fodPath}` string that doesn't contain the "path
|
|
to the wrong derivation" anymore.
|
|
|
|
As we haven't fully determined if our reference scanning approach is gonna work,
|
|
and comparing output paths is sufficient to determine equality of the build
|
|
instructions, this is left as a future work item.
|
|
|
|
|
|
[^inbox-drvpath]: https://inbox.tvl.su/depot/20230316120039.j4fkp3puzrtbjcpi@tp/T/#t
|