* Allow derivations to hint that they should not be built remotely

  using the build hook mechanism, by setting the derivation attribute
  "preferLocalBuild" to true.  This has a few use cases:

  - The user environment builder.  Since it just creates a bunch of
    symlinks without much computation, there is no reason to do it
    remotely.  In fact, doing it remotely requires the entire closure
    of the user environment to be copied to the remote machine, which
    is extremely wasteful.

  - `fetchurl'.  Performing the download on a remote machine and then
    copying it to the local machine involves twice as much network
    traffic as performing the download locally, and doesn't save any
    CPU cycles on the local machine.

1 files changed, 4 insertions, 0 deletions

diff --git a/corepkgs/buildenv/default.nix b/corepkgs/buildenv/default.nix
index 36dd9d0c6..d76f52740 100644
--- a/corepkgs/buildenv/default.nix
+++ b/corepkgs/buildenv/default.nix
@@ -11,4 +11,8 @@ derivation {
   paths = derivations;
   active = map (x: if x ? meta && x.meta ? active then x.meta.active else "true") derivations;
   priority = map (x: if x ? meta && x.meta ? priority then x.meta.priority else "5") derivations;
+
+  # Building user environments remotely just causes huge amounts of
+  # network traffic, so don't do that.
+  preferLocalBuild = true;
 }