libfetchers: avoid re-copying substituted inputs

Previously when creating the accessor for substituted/cached inputs, Nix
would neither:

- Ensure a cache entry exists for the substituted inputs (which are already in the store)
- Use the cache for substituted inputs. Instead, it attempted to find
  the always missing fingerprint of the original, unsubstituted input

Both of these 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 #11228

The work in #12911 already fixed this
in most cases, but here we can ensure it applies to the substituted
store paths used for inputs

Author: getchoo

src/libfetchers/fetchers.cc

@@ -332,9 +332,19 @@ std::pair<ref<SourceAccessor>, Input> Input::getAccessorUnchecked(ref<Store> sto
 
             debug("using substituted/cached input '%s' in '%s'", to_string(), store->printStorePath(storePath));
 
-            auto accessor = makeStorePathAccessor(store, storePath);
-
-            accessor->fingerprint = getFingerprint(store);
+            /*
+             * NOTE: Make sure to use this code path for creating the accessor!! It contains special casing for
+             * correctly caching paths already in the store, like we're dealing with here
+             *
+             * This avoids https://github.com/NixOS/nix/issues/11228, and ensures the workaround introduced in
+             * https://github.com/NixOS/nix/pull/12911 applies to the substituted input
+             */
+            auto input = fromAttrs(*settings, {{"type", "path"}, {"path", store->printStorePath(storePath)}});
+            auto [accessor, result] = input.scheme->getAccessor(store, input);
+
+            // We can then re-use that nicely cached fingerprint in the new accessor, while continuing to return our
+            // original Input and its attributes
+            accessor->fingerprint = result.getFingerprint(store);
 
             accessor->setPathDisplay("«" + to_string() + "»");