From 65a810fc0b12e754af47864ab3b9d4b5756cde15 Mon Sep 17 00:00:00 2001 From: Florian Klink Date: Fri, 1 Mar 2024 10:56:17 +0200 Subject: feat(tvix/docs): switch to mdbook Some of the docs are still outdated (like architecture and drv path inconsistencies). Change-Id: I7a6afceb008ef4cd19a764dd6c637b39fa842a2e Reviewed-on: https://cl.tvl.fyi/c/depot/+/11072 Autosubmit: flokli Tested-by: BuildkiteCI Reviewed-by: edef --- tvix/docs/src/differences-drv-paths.md | 35 ++++++++++++++++++++++++++++++++++ 1 file changed, 35 insertions(+) create mode 100644 tvix/docs/src/differences-drv-paths.md (limited to 'tvix/docs/src/differences-drv-paths.md') diff --git a/tvix/docs/src/differences-drv-paths.md b/tvix/docs/src/differences-drv-paths.md new file mode 100644 index 000000000000..f1c90bd52892 --- /dev/null +++ b/tvix/docs/src/differences-drv-paths.md @@ -0,0 +1,35 @@ +# .drvPath inconsistencies / 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 `builtin: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 -- cgit 1.4.1