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Document fetchClosure

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Eelco Dolstra 2022-03-22 23:31:48 +01:00
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7
doc/manual/src/release-notes/rl-next.md
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@ -4,3 +4,10 @@
* `nix store make-content-addressable` has been renamed to `nix store
make-content-addressed`.
* New builtin function `builtins.fetchClosure` that copies a closure
from a binary cache at evaluation time and rewrites it to
content-addressed form (if it isn't already). Like
`builtins.storePath`, this allows importing pre-built store paths;
the difference is that it doesn't require the user to configure
binary caches and trusted public keys.

33
src/libexpr/primops/fetchClosure.cc
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@ -110,6 +110,39 @@ static RegisterPrimOp primop_fetchClosure({
.name = "__fetchClosure",
.args = {"args"},
.doc = R"(
Fetch a Nix store closure from a binary cache, rewriting it into
content-addressed form. For example,
```nix
builtins.fetchClosure {
fromStore = "https://cache.nixos.org";
fromPath = /nix/store/r2jd6ygnmirm2g803mksqqjm4y39yi6i-git-2.33.1;
toPath = /nix/store/ldbhlwhh39wha58rm61bkiiwm6j7211j-git-2.33.1;
}
```
fetches `/nix/store/r2jd...` from the specified binary cache,
and rewrites it into the content-addressed store path
`/nix/store/ldbh...`.
If `fromPath` is already content-addressed, or if you are
allowing impure evaluation (`--impure`), then `toPath` may be
omitted.
To find out the correct value for `toPath` given a `fromPath`,
you can use `nix store make-content-addressed`:
```console
# nix store make-content-addressed /nix/store/r2jd6ygnmirm2g803mksqqjm4y39yi6i-git-2.33.1
rewrote '/nix/store/r2jd6ygnmirm2g803mksqqjm4y39yi6i-git-2.33.1' to '/nix/store/ldbhlwhh39wha58rm61bkiiwm6j7211j-git-2.33.1'
```
This function is similar to `builtins.storePath` in that it
allows you to use a previously built store path in a Nix
expression. However, it is more reproducible because it requires
specifying a binary cache from which the path can be fetched.
Also, requiring a content-addressed final store path avoids the
need for users to configure binary cache public keys.
)",
.fun = prim_fetchClosure,
});