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libfetchers: avoid re-copying substituted inputs

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Previously, Nix would not create a cache entry for substituted/cached
inputs

This led to severe slowdowns in some scenarios where a large input (like
Nixpkgs) had already been unpacked to the store but didn't exist in a
users cache, as described in https://github.com/NixOS/nix/issues/11228

Using the same method as https://github.com/NixOS/nix/pull/12911, we can
create a cache entry for the fingerprint of substituted/cached inputs
and avoid this problem entirely
This commit is contained in:
Seth Flynn 2025-09-22 05:59:11 -04:00
parent ab7feb3898
commit 74305d5260
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10
src/libfetchers/fetchers.cc
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@ -5,6 +5,7 @@
#include "nix/util/json-utils.hh"
#include "nix/fetchers/store-path-accessor.hh"
#include "nix/fetchers/fetch-settings.hh"
#include "nix/fetchers/fetch-to-store.hh"
#include <nlohmann/json.hpp>
@ -336,6 +337,15 @@ std::pair<ref<SourceAccessor>, Input> Input::getAccessorUnchecked(ref<Store> sto
accessor->fingerprint = getFingerprint(store);
// Store a cache entry for the substituted tree so later fetches
// can reuse the existing nar instead of copying the unpacked
// input back into the store on every evaluation.
if (accessor->fingerprint) {
ContentAddressMethod method = ContentAddressMethod::Raw::NixArchive;
auto cacheKey = makeFetchToStoreCacheKey(getName(), *accessor->fingerprint, method, "/");
settings->getCache()->upsert(cacheKey, *store, {}, storePath);
}
accessor->setPathDisplay("«" + to_string() + "»");
return {accessor, *this};