nixpkgs: add file that fetches pinned revision

tests/default.nix

@@ -49,6 +49,7 @@ let
     no-flake = callTest ./no-flake.nix { };
     lib-tests = callTest ./lib-tests.nix { };
     maintainers = callTest ./maintainers.nix { };
+    nixpkgs-fetch = callTest ./nixpkgs-fetch.nix { };
     nixpkgs-module = callTest ./nixpkgs-module.nix { };
     plugins-by-name = callTest ./plugins-by-name.nix { };
     generated = callTest ./generated.nix { };

tests/nixpkgs-fetch.nix

@@ -0,0 +1,153 @@
+{
+  lib,
+  self,
+  jq,
+  nix,
+  linkFarmFromDrvs,
+  runCommandLocal,
+  writableTmpDirAsHomeHook,
+}:
+
+# nixpkgs.nix fetches our pinned nixpkgs using the `fetchTree` primop.
+#
+# In this file, we test that it is identical to our `inputs.nixpkgs` flake input.
+#
+# The primop is only available with `experimental-features = flakes` or `experimental-features = fetch-tree`,
+# so we have a `fetchTree` polyfill that delegates to `fetchTarball`.
+#
+# Permutations to test:
+# 1. fetchTree primop
+#   a. attrs
+#   b. outPath derivation
+# 2. fetchTree polyfill
+#   a. attrs
+#   b. outPath derivation
+#
+# As we run this test from a flake, we have `experimental-features = flakes` and `fetchTree` primop is available.
+# Therefore, we can test the `fetchTree` primop easily.
+#
+# By running `nix-instantiate` _within_ a build sandbox, we can configure `experimental-features =`
+# to test the polyfill's attrs. However, we cannot do an actual fetch in the sandbox.
+#
+# Since we can't do an actual fetch in the sandbox, the polyfill's outPath can be tested manually.
+# These should both evaluate to the same derivation:
+#     nix-instantiate --eval nixpkgs.nix --attr outPath --option experimental-features flakes
+#     nix-instantiate --eval nixpkgs.nix --attr outPath --option experimental-features ''
+
+let
+  flake-input = self.inputs.nixpkgs.sourceInfo;
+  pinned-input = import ../nixpkgs.nix;
+in
+linkFarmFromDrvs "nixpkgs-fetch-test" [
+
+  (runCommandLocal "nixpkgs-fetch-test-with-primop"
+    {
+      __structuredAttrs = true;
+      unsafeDiscardReferences.out = true;
+      strictDeps = true;
+
+      hasPrimop = builtins ? fetchTree;
+      expected = flake-input;
+      actual = pinned-input;
+
+      expectedAttrs = removeAttrs flake-input [ "outPath" ];
+      actualAttrs = removeAttrs pinned-input [ "outPath" ];
+
+      nativeBuildInputs = [
+        jq
+      ];
+    }
+    ''
+      echo 'Sanity check that `fetchTree` is enabled'
+      if [ -n "$hasPrimop" ]; then
+        echo "fetchTree is available" >&2
+      else
+        echo "fetchTree is not available" >&2
+        exit 1
+      fi
+
+      if [ "$expected" = "$actual" ]; then
+        echo "Expectation met: '$expected'" >&2
+      else
+        echo "Expected '$actual' to be '$expected'" >&2
+        exit 1
+      fi
+
+      mkdir "$out"
+      jq --sort-keys .expectedAttrs "$NIX_ATTRS_JSON_FILE" > "$out/expected.json"
+      jq --sort-keys .actualAttrs "$NIX_ATTRS_JSON_FILE" > "$out/actual.json"
+
+      # Compare actual.json and expected.json
+      if ! diff --unified "$out/expected.json" "$out/actual.json"; then
+        echo 'Unexpected difference between `expected.json` and `actual.json`' >&2
+        exit 1
+      fi
+    ''
+  )
+
+  (runCommandLocal "nixpkgs-fetch-test-with-polyfill"
+    {
+      __structuredAttrs = true;
+      unsafeDiscardReferences.out = true;
+      strictDeps = true;
+
+      src = lib.fileset.toSource {
+        root = ../.;
+        fileset = lib.fileset.unions [
+          ../flake.lock
+          ../nixpkgs.nix
+        ];
+      };
+
+      expectedAttrs = removeAttrs flake-input [ "outPath" ];
+
+      nativeBuildInputs = [
+        jq
+        nix
+        writableTmpDirAsHomeHook
+      ];
+    }
+    ''
+      # Setup nix environment
+      export TEST_ROOT="$PWD/test-tmp"
+      export NIX_CONF_DIR="$TEST_ROOT/etc"
+      export NIX_LOCALSTATE_DIR="$TEST_ROOT/var"
+      export NIX_LOG_DIR="$TEST_ROOT/var/log/nix"
+      export NIX_STATE_DIR="$TEST_ROOT/var/nix"
+      export PAGER=cat
+
+      mkdir -p "$NIX_CONF_DIR"
+      cat > "$NIX_CONF_DIR/nix.conf" <<EOF
+      store = dummy://
+      experimental-features =
+      EOF
+
+      echo 'Sanity check that `fetchTree` is disabled'
+      echo "NOTE: if this assertion fails, then 'fetch-tree' may no longer be an experimental feature"
+      nix-instantiate --eval --raw --expr '
+        if builtins ? fetchTree then
+          throw "fetchTree is available"
+        else
+          "fetchTree is not available\n"
+      '
+
+      mkdir "$out"
+
+      echo "Evaluating expected"
+      jq --sort-keys .expectedAttrs "$NIX_ATTRS_JSON_FILE" > "$out/expected.json"
+
+      echo "Evaluating actual"
+      nix-instantiate --eval --strict --json \
+        --argstr actual "$src/nixpkgs.nix" \
+        --expr '{ actual }: removeAttrs (import actual) [ "outPath" ]' \
+        | jq --sort-keys . > "$out/actual.json"
+
+      # Compare actual.json and expected.json
+      if ! diff --unified "$out/expected.json" "$out/actual.json"; then
+        echo 'Unexpected difference between `expected.json` and `actual.json`' >&2
+        exit 1
+      fi
+    ''
+  )
+
+]