store: Change 'export-paths' to always export in topological order.

* guix/store.scm (export-paths): Pass PATHS through
  'topologically-sorted' before iterating.
* tests/store.scm ("export/import paths, ensure topological order"): New
  test.

union: Do not compare directories upon collision.

* guix/build/union.scm (file=?): Return #f if FILE1 and FILE2 are not
  regular files.  Fixes a bug whereby collisions among directories would
  lead to the invocation of 'file=?' and thus 'call-with-input-file' on
  directories.  Reported by Mark H. Weaver <mhw@netris.org>.

download: Provide a 'User-Agent' field in HTTP requests.

Fixes <http://bugs.gnu.org/16703>.
Reported by Raimon Grau <raimonster@gmail.com>.

* guix/build/download.scm (http-fetch)[headers]: New variable.
  Pass it as #:headers or #:extra-headers to 'http-get' and
  'http-get*'.

gnu: linux-initrd: When booting, chdir to the new root before calling 'chroot'.

* guix/build/linux-initrd.scm (boot-system): Add 'chdir' call right
  before 'chroot'.

monads: Add 'text-file*'.

* guix/monads.scm (text-file*): New procedure.
* tests/monads.scm ("text-file*"): New test.
* doc/guix.texi (The Store Monad): Change example since the previous one
  would erroneously fail to retain a reference to Coreutils.  Document
  'text-file*'.

offload: Have 'build-machines' honor its argument.

* guix/scripts/offload.scm (build-machines): Honor FILE.

gnu: linux-initrd: Build /dev/input devices.

* guix/build/linux-initrd.scm (make-essential-device-nodes): Make
  dev/input devices.

gnu: linux-initrd: Make /dev/{mem,kmem}.

* guix/build/linux-initrd.scm (make-essential-device-nodes): Make
  dev/{mem,kmem}.

gnu: linux-initrd: Allow the root file system to be volatile.

* gnu/system/linux-initrd.scm (qemu-initrd): Add 'volatile-root?'
  parameter.
* guix/build/linux-initrd.scm (boot-system): Likewise.  Honor it.

gnu: linux-initrd: Recognize 9p file systems.

* gnu/system/linux-initrd.scm (qemu-initrd)[virtio-9p-modules]: New
  variable.
  [linux-modules]: Append VIRTIO-9P-MODULES when a 9p file system is in
  MOUNTS.
* guix/build/linux-initrd.scm (mount-qemu-9p): New procedure.
  (boot-system): Recognize '9p' in MOUNTS, and use 'mount-qemu-9p'.

gnu: linux-initrd: Properly distinguish between /dev/sda* and /dev/vda*.

* guix/build/linux-initrd.scm (make-essential-device-nodes): Rename
  devices with major = 8 to /dev/sda*.  Make /dev/vda* devices.
* gnu/system/vm.scm (qemu-image): Change '/dev/vda' to '/dev/sda'.
* gnu/system.scm (operating-system-derivation): Likewise.

gnu: linux-initrd: Start a REPL when the root could not be mounted.

* guix/build/linux-initrd.scm (boot-system): Catch errors when mounting
  ROOT and call 'start-repl' upon error.

gnu: linux-initrd: Factorize boot code.

* guix/build/linux-initrd.scm (boot-system): New procedure.
* gnu/system/linux-initrd.scm (qemu-initrd): Add keyword parameters
  'guile-modules-in-chroot?' and 'mounts'.  Change builder to simply
  call 'boot-system'.
  (gnu-system-initrd): Change to a simple call to 'qemu-initrd'.
* gnu/system/vm.scm (expression->derivation-in-linux-vm): Call
  'qemu-initrd' with #:guile-modules-in-chroot?.

monads: Add 'imported-modules' and 'compiled-modules'.

* guix/monads.scm (package-file): Fix typo.
  (imported-modules, compiled-modules): New procedures.

download: Add archive.apache.org to the Apache mirrors.

* guix/download.scm (%mirrors)[apache]: Add archive.apache.org as a last
  resort.

offload: Do not try to retrieve anything upon build failure.

* 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.

derivations: Add #:local-build? parameter for derivations.

* guix/derivations.scm (derivation): Add #:local-build? parameter and
  honor it.
  (build-expression->derivation): Likewise.
* doc/guix.texi (Derivations): Update documentation of these
  procedures.

profiles: Remove misleading message.

Fixes <http://bugs.gnu.org/16484>.
Reported by Andreas Enge <andreas@enge.fr>.

* guix/profiles.scm (profile-derivation)[builder]: Remove "building
  profile '~a' with ~a packages" message.

Add 'guix offload' as a daemon build hook.

* 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.

store: Add 'topologically-sorted'.

* guix/store.scm (topologically-sorted): New procedure.
* tests/store.scm ("topologically-sorted, one item",
  "topologically-sorted, several items", "topologically-sorted, more
  difficult"): New tests.