* guix/scripts/offload.scm (machine-load, machine-less-loaded?,
machine-less-loaded-or-faster?): New procedures.
(choose-build-machine): Use 'machine-less-loaded-or-faster?' when
sorting. Return the head of MACHINES unless it's loaded is >= 2.
* guix/scripts/offload.scm (offload): Add 'log-port' keyword parameter.
Handle log display here. Return the result of (close-pipe pipe).
(process-request): Adjust 'offload' call site accordingly. Call
'retrieve-files' only when 'offload' returns zero; exit when 'offload'
returns non-zero.
* nix/nix-daemon/guix-daemon.cc (GUIX_OPT_NO_BUILD_HOOK): New macro.
(options): Add '--no-build-hook'.
(parse_opt): Handle it.
(main)[HAVE_DAEMON_OFFLOAD_HOOK]: Set 'useBuildHook' by default.
Set $NIX_BUILD_HOOK to our offload hook unless otherwise specified.
[!HAVE_DAEMON_OFFLOAD_HOOK]: Clear 'useBuildHook'.
* pre-inst-env.in: Set and export NIX_BUILD_HOOK.
* nix/scripts/offload.in, guix/scripts/offload.scm: New files.
* guix/ui.scm (show-guix-help)[internal?]: Add "offload".
* config-daemon.ac: Call 'GUIX_CHECK_UNBUFFERED_CBIP'.
Instantiate 'nix/scripts/offload'. Set 'BUILD_DAEMON_OFFLOAD'
conditional, and optionally define 'HAVE_DEAMON_OFFLOAD_HOOK' cpp
macro.
* daemon.am (nodist_pkglibexec_SCRIPTS)[BUILD_DAEMON_OFFLOAD]: Add it.
* Makefile.am (MODULES)[BUILD_DAEMON_OFFLOAD]: Add
'guix/scripts/offload.scm'.
(EXTRA_DIST)[!BUILD_DAEMON_OFFLOAD]: Likewise.
* m4/guix.m4 (GUIX_CHECK_UNBUFFERED_CBIP): New macro.
* doc/guix.texi (Setting Up the Daemon): Move most of the body to...
(Build Environment Setup): ... this. New subsection.
(Daemon Offload Setup): New subsection.
Ludovic Courtès