When we do something like 'nix flake update --override-input nixpkgs
...', the override is now kept on subsequent calls. (If you don't want
this behaviour, you can use --no-write-lock-file.)
This fetchers copies a plain directory (i.e. not a Git/Mercurial
repository) to the store (or does nothing if the path is already a
store path).
One use case is to pin the 'nixpkgs' flake used to build the current
NixOS system, and prevent it from being garbage-collected, via a
system registry entry like this:
{
"from": {
"id": "nixpkgs",
"type": "indirect"
},
"to": {
"type": "path",
"path": "/nix/store/rralhl3wj4rdwzjn16g7d93mibvlr521-source",
"lastModified": 1585388205,
"rev": "b0c285807d6a9f1b7562ec417c24fa1a30ecc31a"
},
"exact": true
}
Note the fake "lastModified" and "rev" attributes that ensure that the
flake gives the same evaluation results as the corresponding
Git/GitHub inputs.
This allows querying the location of function arguments. E.g.
builtins.unsafeGetAttrPos "x" (builtins.functionArgs ({ x }: null))
=> { column = 57; file = "/home/infinisil/src/nix/inst/test.nix"; line = 1; }
An example use is for pinning the "nixpkgs" entry the system-wide
registry to a particular store path. Inexact matches
(e.g. "nixpkgs/master") should still use the global registry.
One application for this is pinning the 'nixpkgs' flake to the exact
revision used to build the NixOS system, e.g.
{
"flakes": [
{
"from": {
"id": "nixpkgs",
"type": "indirect"
},
"to": {
"owner": "NixOS",
"repo": "nixpkgs",
"type": "github",
"rev": "b0c285807d6a9f1b7562ec417c24fa1a30ecc31a"
}
}
],
"version": 2
}
Using std::filesystem means also having to link with -lstdc++fs on
some platforms and it's hard to discover for what platforms this is
needed. As all the functionality is already implemented as utilities,
use those instead.
Due to fetchGit not checking if rev is an ancestor of ref (there is even
a FIXME comment about it in the code), the cache repo might not have the
ref even though it has the rev. This doesn't matter when submodule =
false, but the submodule = true code blows up because it tries to fetch
the (missing) ref from the cache repo.
Fix this in the simplest way possible: fetch all refs from the local
cache repo when submodules = true.
TODO: Add tests.
Julian Stecklina