[24.11] release: forbid use of lib.fileset in Nixpkgs
#369977
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I don’t love this, and I’m not convinced it doesn’t have the same pitfalls as filesets, but see the `flattenReferencesGraph` commit for reasoning. It may be better to ban the relevant builtins entirely and just move things into subdirectories when needed, but I didn’t want to do more surgery to this part of the tree than necessary to solve the immediate problem. (cherry picked from commit 2eae7d6)
(cherry picked from commit f6ce575)
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Due to Nix bug <NixOS/nix#11503>, `builtins.filterSource` and chroot stores interact in a confusing and broken way that breaks `lib.fileset`. This means that uses of the API inside Nixpkgs keep breaking the NixOS installer, blocking the channel. The resulting error messages are inscrutable (they look like “the installer test is trying to download `curl`…?” and eventually bottom out in a derivation that has the wrong `outPath` because of the chroot store causing an incorrect `lib.fileset` result). Whenever this happens, someone (well, in practice K900 or I) has to bisect the change that introduced it and remove the use of `lib.fileset`. This has happened at least three times in the past four months (I believe I might actually be missing one here, but these are the ones I remember and could easily dig up): * <NixOS#340046> * <NixOS#352491> * <NixOS#369459> The options I see here are: 1. Forbid use of `lib.fileset` within Nixpkgs until the Nix bug is fixed. This is the approach taken here. External users of Nixpkgs can continue to use the API as normal, but using it from within something that affects any release jobset `outPath`s will cause an evaluation failure with a hopefully‐helpful error message. 2. Forbid `lib.fileset` and also all of the other library APIs that use `builtins.filterSource`. I’m happy to do this, but so far none of those have broken the installer, so I decided to start small and worry about the others if they end up causing a problem in practice. 3. Forbid `builtins.filterSource` directly. This is hard and would require more invasive `builtins.scopedImport` crimes to do at evaluation time. I think this would realistically have to be done in something like nixpkgs-vet instead and I didn’t have much luck shoehorning a check like this into that codebase when I tried. 4. Fix the Nix bug. This would be great! But also it doesn’t seem to be happening any time soon, it seems difficult to fix in a way that doesn’t subtly break compatibility with the previous semantics, and arguably the fix would need backporting all the way back to 2.3 given our minimum version policy. 5. Do nothing; have people continue to innocuously use `lib.fileset` throughout Nixpkgs, breaking the installer whenever one of them sneaks in to that closure, causing the channel to be blocked and requiring expensive bisections to narrow down the inscrutable test failure to the package using `lib.fileset`, which then needs moving back off it. This sucks for the people who keep having to track it down, holds back important channel bumps, and the criteria for when it’s okay to use `lib.fileset` are not realistically possible to teach to all contributors. I'd be happy to work on (2) as an alternative; (3) would be difficult and seems like overkill, (4) is not really something I trust myself to do and wouldn’t address the immediate problem, and (5) isn’t sustainable. I think that the current approach here is the best trade‐off for now, as `lib.fileset` seems to be the only prominent user of the `builtins.filterSource` API that works with full store paths, exposing it to the Nix bug. It’s unfortunate to lose the nice API, but since we can’t rely on it to produce correct results and the channels keep getting blocked as a result, I don’t think we really have an alternative right now. (cherry picked from commit 8725e46)
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Jan 1, 2025
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Manual backport of #369694.
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