WIP: Allow for null packages and formatters #13291
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This is useful for frameworks (such as flake-parts) to avoid unnecessary evaluation. See discussion here: hercules-ci/flake-parts#288 (comment) While digging into this, I discovered that `nix fmt` already handles `null` formatters identically to undefined formatters. I added a couple of tests to demonstrate this behavior. `nix flake show` needs some reworking to avoid crashing with a "error: expected a derivation" when it encounters a `null` formatter or package. My changes here avoid the crash, but has some cosmetic issues, which is why I've labeled this PR as a draft. Cosmetic issues =============== With the following `flake.nix`: ```nix { outputs = _: { packages.x86_64-linux.default = null; }; } ``` `nix flake show` shows a weird empty system: ``` $ nix flake show path:/tmp/tmp.uL0iGuNwXB?lastModified=1748472558&narHash=sha256-tC%2BhdXAyoeFvWHNllJbois8X%2B7wpZ6CJzEzbcaGQxtM%3D └───packages └───x86_64-linux ``` Similarly, `nix flake show --json` includes an empty object: ``` $ nix flake show --json { "packages": { "x86_64-linux": { "default": null } } } ``` `nix build` crashes: ``` $ nix build .#default error: expected flake output attribute 'packages.x86_64-linux.default' to be a derivation or path but found null: null ```
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| @@ -762,6 +763,14 @@ std::vector<Symbol> AttrCursor::getAttrs() | |||
| return attrs; | |||
| } | |||
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| bool AttrCursor::isNull() | |||
| { | |||
| // <<< TODO: caching? >>> | |||
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That's surprising, but yeah, null isn't supported yet.
enum AttrType could be extended.
Not sure if that should be considered a breaking schema change, but it'd be a safe choice to then also update the version in the file name "/eval-cache-v5" to v6.
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I'll give that a shot! I see some type related checks in the other caching codepaths. Do I need to worry about the fact that the attribute I'm caching could change from a derivation to null and back?
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This is useful for frameworks (such as flake-parts) to avoid unnecessary evaluation. See discussion here:
hercules-ci/flake-parts#288 (comment)
While digging into this, I discovered that
nix fmtalready handlesnullformatters identically to undefined formatters. I added a couple of tests to demonstrate this behavior.nix flake showneeds some reworking to avoid crashing with a "error: expected a derivation" when it encounters anullformatter or package. My changes here avoid the crash, but has some cosmetic issues, which is why I've labeled this PR as a draft.Cosmetic issues
With the following
flake.nix:nix flake showshows a weird empty system:Similarly,
nix flake show --jsonincludes an empty object:nix buildcrashes:Motivation
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