Allow repository rule to check for sha

Author: leostera

WORKSPACE

@@ -7,6 +7,7 @@ load("//nixpkgs:nixpkgs.bzl", "nixpkgs_git_repository", "nixpkgs_package")
 nixpkgs_git_repository(
   name = "nixpkgs",
   revision = "17.09",
+  sha256 = "405f1d6ba523630c83fbabef93f0da11ea388510a576adf2ded26a744fbf793e",
 )
 
 nixpkgs_package(name = "hello", repository = "@nixpkgs")

nixpkgs/nixpkgs.bzl

@@ -7,12 +7,14 @@ def _nixpkgs_git_repository_impl(ctx):
   ctx.download_and_extract(
     url = "https://github.com/NixOS/nixpkgs/archive/%s.tar.gz" % ctx.attr.revision,
     stripPrefix = "nixpkgs-" + ctx.attr.revision,
+    sha256 = ctx.attr.sha256,
   )
 
 nixpkgs_git_repository = repository_rule(
   implementation = _nixpkgs_git_repository_impl,
   attrs = {
     "revision": attr.string(),
+    "sha256": attr.string(),
   },
   local = False,
 )