Test nixpkgs_package is invalidated on a change in Nix files

This test runs Bazel in the Bazel sandbox to ensure some dependencies
are well tracked at the workspace level. So, we create a workspace,
build it, then change the content of some workspace files and check if
a rebuild of this workspace take into account these changes.

This is a regression test for the issue #113.

nixpkgs/BUILD.bazel

@@ -6,6 +6,8 @@ exports_files([
     "nixpkgs.bzl",
 ])
 
+filegroup(name = "srcs", srcs = glob(["**"]), visibility = ["//visibility:public"])
+
 # @bazel_tools//tools does not define a bzl_library itself, instead we are
 # supposed to define our own using the @bazel_tools//tools:bzl_srcs filegroup.
 # See https://github.com/bazelbuild/skydoc/issues/166

nixpkgs/nixpkgs.bzl

@@ -4,7 +4,9 @@ load("@bazel_tools//tools/cpp:cc_configure.bzl", "cc_autoconf_impl")
 load("@bazel_tools//tools/cpp:lib_cc_configure.bzl", "get_cpu_value")
 
 def _nixpkgs_git_repository_impl(repository_ctx):
-    repository_ctx.file("BUILD")
+    repository_ctx.file(
+        "BUILD",
+        content = 'filegroup(name = "srcs", srcs = glob(["**"]), visibility = ["//visibility:public"])')
 
     # Make "@nixpkgs" (syntactic sugar for "@nixpkgs//:nixpkgs") a valid
     # label for default.nix.

tests/BUILD.bazel

@@ -88,3 +88,15 @@ go_binary(
     name = "go-test",
     srcs = ["go-test.go"]
 )
+
+sh_test(
+    name = "run-test-invalid-nixpkgs-package",
+    timeout = "short",
+    srcs = ["test_invalid_nixpkgs_package.sh"],
+    deps = ["@bazel_tools//tools/bash/runfiles"],
+    data = [
+        "//nixpkgs:srcs",
+        "//tests/invalid_nixpkgs_package:srcs",
+        "@remote_nixpkgs//:srcs"
+    ],
+)

tests/invalid_nixpkgs_package/BUILD.bazel

@@ -0,0 +1 @@
+filegroup(name = "srcs", srcs = glob(["**"]), visibility = ["//visibility:public"])

tests/invalid_nixpkgs_package/default.nix

@@ -0,0 +1,6 @@
+{ ... }:
+let
+  # To not predict from where the nix command is executed, we replace
+  # with sed the import path with an absolute path.
+  pkgs = import REPLACE-WITH-ABSPATH { };
+in import ./hello.nix { inherit pkgs; }

tests/invalid_nixpkgs_package/hello-1.nix

@@ -0,0 +1,5 @@
+{ pkgs }:
+
+{
+  hello = pkgs.writeShellScriptBin "hello" "echo hello-1";
+}

tests/invalid_nixpkgs_package/hello-2.nix

@@ -0,0 +1,5 @@
+{ pkgs }:
+
+{
+  hello = pkgs.writeShellScriptBin "hello" "echo hello-2";
+}

tests/invalid_nixpkgs_package/nested-build.bazel

@@ -0,0 +1,6 @@
+genrule(
+    name = "hello-output",
+    outs = ["hello-output.txt"],
+    cmd = "$(execpath @hello//:bin) > \"$@\"",
+    tools = [ "@hello//:bin" ]
+)

tests/invalid_nixpkgs_package/workspace.bazel

@@ -0,0 +1,19 @@
+workspace(name = "io_tweag_rules_nixpkgs")
+
+load(
+    "//nixpkgs:nixpkgs.bzl",
+    "nixpkgs_local_repository",
+    "nixpkgs_package",
+)
+
+nixpkgs_local_repository(
+    name = "nixpkgs",
+    nix_file = "//:default.nix",
+    nix_file_deps = [ "//:hello.nix" ],
+)
+
+nixpkgs_package(
+    name = "hello",
+    attribute_path = "hello",
+    repository = "@nixpkgs",
+)

tests/test_invalid_nixpkgs_package.sh

@@ -0,0 +1,36 @@
+#!/bin/sh
+set -e
+
+ln -s tests/invalid_nixpkgs_package/workspace.bazel WORKSPACE
+ln -s tests/invalid_nixpkgs_package/nested-build.bazel BUILD
+ln -s tests/invalid_nixpkgs_package/default.nix default.nix
+
+# We need to provide a nixpkgs to create an output store path which is
+# a folder (because nixpkgs_package requires the output store path to
+# be a directory).
+#
+# It would be much more simple to use the mkdir bash builtin because
+# we could create the output store path without having to import
+# nixpkgs. However, the mkdir bash builtin is segfaulting:/
+sed "s;REPLACE-WITH-ABSPATH;$(pwd)/external/remote_nixpkgs;" -i default.nix
+
+# First we build the :hello-output target with the content of
+# hello-1.nix in hello.nix. Building this deriviation creates a file
+# containing "hello-1".
+cp tests/invalid_nixpkgs_package/hello-1.nix hello.nix
+bazel build //:hello-output
+if [[ $(cat bazel-bin/hello-output.txt) != "hello-1" ]]; then
+    exit 1
+fi
+
+# Then, we override the content of the hello.nix file to ensure Bazel
+# rebuilds the :hello-output target when a Nix files is modified. The
+# hello.nix file now builds a derivation creating a file with content
+# "hello-2".
+cp tests/invalid_nixpkgs_package/hello-2.nix hello.nix
+bazel build //:hello-output
+content=$(cat bazel-bin/hello-output.txt)
+if [[ $content != "hello-2" ]]; then
+    echo 'error: the content of bazel-bin/hello-output.txt must be hello-2 instead of' "$content"
+    exit 1
+fi