Fail if neither 'repository' nor 'path' ar set

This ensures that it will not use whatever nixpkgs channel is available, hence build are trully reproducible.

Author: guibou

nixpkgs/nixpkgs.bzl

@@ -49,16 +49,17 @@ def _nixpkgs_package_impl(ctx):
 
   # If neither repository or path are set, leave empty which will use
   # default value from NIX_PATH.
-  path = []
   if ctx.attr.repository and ctx.attr.path:
     fail("'repository' and 'path' fields are mutually exclusive.")
-  if ctx.attr.repository:
+  elif ctx.attr.repository:
     # XXX Another hack: the repository label typically resolves to
     # some top-level package in the external workspace. So we use
     # dirname to get the actual workspace path.
     path = ["-I", "nixpkgs={0}".format(ctx.path(ctx.attr.repository).dirname)]
-  if ctx.attr.path:
+  elif ctx.attr.path:
     path = ["-I", "nixpkgs={0}".format(ctx.attr.path)]
+  else:
+    fail("""You *must* specify one of 'repository' or 'path'.""")
 
   nix_build_path = ctx.which("nix-build")
   if nix_build_path == None: