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| -rw-r--r-- | tvix/docs/differences-drv-paths.md | 46 |
1 files changed, 46 insertions, 0 deletions
diff --git a/tvix/docs/differences-drv-paths.md b/tvix/docs/differences-drv-paths.md new file mode 100644 index 0000000000..7c811d5cbf --- /dev/null +++ b/tvix/docs/differences-drv-paths.md @@ -0,0 +1,46 @@ +--- +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 |