Input::fetchToStore(): Don't try to substitute

Having a narHash doesn't mean that we have the other attributes
returned by the fetcher (such as lastModified or rev). For instance,

   $ nix flake metadata github:NixOS/patchelf/7c2f768bf9601268a4e71c2ebe91e2011918a70f
   Last modified: 2024-01-15 10:51:22

but

   $ nix flake metadata github:NixOS/patchelf/7c2f768bf9601268a4e71c2ebe91e2011918a70f?narHash=sha256-PPXqKY2hJng4DBVE0I4xshv/vGLUskL7jl53roB8UdU%3D
   (does not print a "Last modified")

The latter only happens if the store path already exists or is
substitutable, which made this impure behaviour unpredictable.

Fixes NixOS#10601.

src/libfetchers/fetchers.cc

@@ -167,24 +167,6 @@ std::pair<StorePath, Input> Input::fetchToStore(ref<Store> store) const
     if (!scheme)
         throw Error("cannot fetch unsupported input '%s'", attrsToJSON(toAttrs()));
 
-    /* The tree may already be in the Nix store, or it could be
-       substituted (which is often faster than fetching from the
-       original source). So check that. */
-    if (getNarHash()) {
-        try {
-            auto storePath = computeStorePath(*store);
-
-            store->ensurePath(storePath);
-
-            debug("using substituted/cached input '%s' in '%s'",
-                to_string(), store->printStorePath(storePath));
-
-            return {std::move(storePath), *this};
-        } catch (Error & e) {
-            debug("substitution of input '%s' failed: %s", to_string(), e.what());
-        }
-    }
-
     auto [storePath, input] = [&]() -> std::pair<StorePath, Input> {
         try {
             auto [accessor, final] = getAccessorUnchecked(store);

tests/functional/tarball.sh

@@ -33,9 +33,6 @@ test_tarball() {
     nix-build  -o $TEST_ROOT/result -E "import (fetchTree file://$tarball)"
     nix-build  -o $TEST_ROOT/result -E "import (fetchTree { type = \"tarball\"; url = file://$tarball; })"
     nix-build  -o $TEST_ROOT/result -E "import (fetchTree { type = \"tarball\"; url = file://$tarball; narHash = \"$hash\"; })"
-    # Do not re-fetch paths already present
-    nix-build  -o $TEST_ROOT/result -E "import (fetchTree { type = \"tarball\"; url = file:///does-not-exist/must-remain-unused/$tarball; narHash = \"$hash\"; })"
-    expectStderr 102 nix-build  -o $TEST_ROOT/result -E "import (fetchTree { type = \"tarball\"; url = file://$tarball; narHash = \"sha256-xdKv2pq/IiwLSnBBJXW8hNowI4MrdZfW+SYqDQs7Tzc=\"; })" | grep 'NAR hash mismatch in input'
 
     [[ $(nix eval --impure --expr "(fetchTree file://$tarball).lastModified") = 1000000000 ]]