flake: instantiate the entrypoint once per system #13

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kiara merged 1 commit from hoist-flake-entrypoint into main

2026-08-12 23:47:35 +02:00

kiara commented

2026-08-12 23:38:08 +02:00

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The flake wrapper applied the entrypoint once per (attr, system) pair. import memoizes the file value, not the application, so none of those applications shared any of the entrypoint's internal work -- each re-ran the whole let, including the pkgs ? import sources.nixpkgs { ... } default argument, i.e. a whole fresh nixpkgs.

Binding one instantiation per system in the let and reading every keyed output out of it collapses that.

Measured on fediversity-ci, nix flake show --no-eval-cache, warm store, second of two runs:

	wall	user+sys	maxrss	nrThunks	gc.totalBytes
before	0.73 s	0.53 s	168 MB	351,775	121.0 MB
after	0.61 s	0.41 s	83 MB	110,647	35.3 MB

Output is unchanged: nix flake show --json --all-systems --no-eval-cache | jq -S . is byte-identical before and after, and every drvPath is unmoved bar the treefmt check's, which hashes the project source and so moves with any edit to flake.nix itself.

This is the same defect that cost the fediversity monorepo far more (its entrypoints carry a whole deployment surface, not just a nixpkgs); the wrapper was copied from https://codeberg.org/kiara/poc-override-nix-deps, which is being fixed at the source too.

The flake wrapper applied the entrypoint once per `(attr, system)` pair. `import` memoizes the *file* value, not the *application*, so none of those applications shared any of the entrypoint's internal work -- each re-ran the whole `let`, including the `pkgs ? import sources.nixpkgs { ... }` default argument, i.e. a whole fresh nixpkgs. Binding one instantiation per system in the `let` and reading every keyed output out of it collapses that. Measured on `fediversity-ci`, `nix flake show --no-eval-cache`, warm store, second of two runs: | | wall | user+sys | maxrss | nrThunks | gc.totalBytes | |---|---|---|---|---|---| | before | 0.73 s | 0.53 s | 168 MB | 351,775 | 121.0 MB | | after | 0.61 s | 0.41 s | 83 MB | 110,647 | 35.3 MB | Output is unchanged: `nix flake show --json --all-systems --no-eval-cache | jq -S .` is byte-identical before and after, and every `drvPath` is unmoved bar the `treefmt` check's, which hashes the project source and so moves with any edit to `flake.nix` itself. This is the same defect that cost the fediversity monorepo far more (its entrypoints carry a whole deployment surface, not just a nixpkgs); the wrapper was copied from https://codeberg.org/kiara/poc-override-nix-deps, which is being fixed at the source too.

kiara added 1 commit

2026-08-12 23:38:08 +02:00

flake: instantiate the entrypoint once per system

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c604d57ce4

`import` memoizes the file value, not the application, so applying the
entrypoint per (attr, system) shared none of its internal work: every
`(attr, system)` pair re-ran the whole `let`, `pkgs` default argument --
a whole nixpkgs -- included. Bind one instantiation per system and read
every keyed output out of it.

Output is unchanged: `nix flake show --json --all-systems` is
byte-identical before and after, and every `drvPath` is unmoved bar the
`treefmt` check's, which hashes the project source and so moves with any
edit to `flake.nix` itself.

kiara merged commit 1e00e34061 into main

2026-08-12 23:47:35 +02:00

kiara deleted branch hoist-flake-entrypoint

2026-08-12 23:47:35 +02:00

kiara referenced this pull request from a commit

2026-08-12 23:47:37 +02:00

Merge pull request 'flake: instantiate the entrypoint once per system' (#13) from hoist-flake-entrypoint into main