Not substituting builds with "preferLocalBuild = true" was a bad idea,
because it didn't take the cost of dependencies into account. For
instance, if we can't substitute a fetchgit call, then we have to
download/build git and all its dependencies.

Partially reverts 55586527 and adds a
new derivation attribute "allowSubstitutes" to specify whether a
derivation may be substituted.

Nixpkgs' writeTextAsFile does this:

  mv "$textPath" "$n"

Since $textPath was owned by root, if $textPath is on the same
filesystem as $n, $n will be owned as root. As a result, the build
result was rejected as having suspicious ownership.

http://hydra.nixos.org/build/22836807

The tarball cache is stored in $HOME, so the test should set up its
own $HOME.

Hello!

The patch below adds a ‘verifyStore’ RPC with the same signature as the
current LocalStore::verifyStore method.

Thanks,
Ludo’.

>From aef46c03ca77eb6344f4892672eb6d9d06432041 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ludovic=20Court=C3=A8s?= <ludo@gnu.org>
Date: Mon, 1 Jun 2015 23:17:10 +0200
Subject: [PATCH] Add a 'verifyStore' remote procedure call.

Change-Id: I68a54a0c3f97da2d062f43b638de817fd40f2dcd

This relaxes restricted mode to allow access to anything in the
store. In the future, it would be better to allow access to only paths
that have been constructed in the current evaluation (so a hard-coded
/nix/store/blabla in a Nix expression would still be
rejected). However, note that reading /nix/store itself is still
rejected, so you can't use this so get access to things you don't know
about.

Closes #454, #455.

Also, make sure --delete-older-than doesn't delete the current
generation.

And don't try to delete generations from unwritable directories.

it an option. :)

* only the last generation can be lazy
* depend on the '--lazy-generation' flag to be set

It has been obsolete since Nix 1.2.

Closes #417.

new generations if a generation already exists.

Alternatively or additionally I propose a mode where only the *last* generation will be sparse.