libfetchers: avoid re-copying substituted inputs

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 NixOS#11228

Using the same method as NixOS#12911, we can
create a cache entry for the fingerprint of substituted/cached inputs
and avoid this problem entirely

Author: getchoo

src/libfetchers/fetchers.cc

@@ -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 +337,14 @@ std::pair<ref<SourceAccessor>, Input> Input::getAccessorUnchecked(ref<Store> sto
 
             accessor->fingerprint = getFingerprint(store);
 
+            // NOTE: This avoids https://github.com/NixOS/nix/issues/11228!
+            // See https://github.com/NixOS/nix/pull/12911 & https://github.com/NixOS/nix/pull/14041
+            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};