Merge branch 'master' into python-tests

Add (guix ssh) module.

* guix/scripts/offload.scm (connect-to-remote-daemon)
(store-import-channel, store-export-channel, send-files)
(retrieve-files): Move to (guix ssh).
(nonce): Add optional 'name' parameter and use it.
(retrieve-files*): New procedure.
(transfer-and-offload): Use it instead of 'retrieve-files', and add
first parameter to 'send-files'.
(assert-node-can-import): Likewise.
(assert-node-can-export): Use 'retrieve-files' instead of
'store-export-channel'.
* guix/ssh.scm: New file.
* configure.ac: Use 'GUIX_CHECK_GUILE_SSH' and define 'HAVE_GUILE_SSH'
Automake conditional.
* Makefile.am (MODULES) [HAVE_GUILE_SSH]: Add guix/ssh.scm.

Merge branch 'master' into staging

offload: Allow testing machines that match a regexp.

* guix/scripts/offload.scm (check-machine-availability): Add 'pred'
parameter and honor it.
(guix-offload): for the "test" sub-command, accept an extra 'regexp'
parameter.  Pass a second argument to 'check-machine-availability'.

offload: Test each machine only once.

* guix/scripts/offload.scm (check-machine-availability)[build-machine=?]:
New procedure.
Add call to 'delete-duplicates'.

offload: Do not read ~/.ssh/known_hosts.

* guix/scripts/offload.scm (open-ssh-session): Pass #:knownhosts to
'make-session'.

Merge remote-tracking branch 'origin/master' into staging

offload: Increase the connection timeout.

* guix/scripts/offload.scm (open-ssh-session): Set #:timeout to 10.

offload: Send the build log to the right file descriptor.

This fixes a regression introduced in
21531add3205e400707c8fbfd841845f9a71863a whereby the build log would no
longer be sent to FD 4, thereby leading the daemon to not see the build
log.

* guix/scripts/offload.scm (transfer-and-offload): Parameterize
CURRENT-BUILD-OUTPUT-PORT.

offload: Fix plural of some messages.

* guix/scripts/offload.scm (send-files): Use 'N_' for possibly plural
message.  Write "store item" instead of "store file".
(retrieve-files): Likewise.

offload: Make the compression methods configurable.

* guix/scripts/offload.scm (<build-machine>)[compression]
[compression-level]: New fields.
(open-ssh-session): Honor them.
* doc/guix.texi (Daemon Offload Setup): Document them.

offload: Add "test" sub-command.

* guix/scripts/offload.scm (assert-node-repl, assert-node-has-guix)
(nonce, assert-node-can-import, assert-node-can-export)
(check-machine-availability): New procedures.
(%random-state): New variable.
(guix-offload): Add case for "test".
* doc/guix.texi (Daemon Offload Setup): Document it.  Remove obsolete
bit about remote invocation of 'guix build'.

Merge branch 'master' into staging

offload: Do not abort when a machine is unreachable.

* guix/scripts/offload.scm (machine-load): Wrap 'open-ssh-session' call
in 'false-if-exception'; return +inf.0 if it returns #f.

offload: Gracefully report connection failures.

* guix/scripts/offload.scm (open-ssh-session): Check the return value of
'connect!'.  Call 'leave' when it's not 'ok.

offload: Warn about the lack of zlib support.

* guix/scripts/offload.scm (guix-offload): Print a warning when
'zlib-support?' returns false.

offload: Remove redundant call to 'topologically-sorted' in 'send-file'.

* guix/scripts/offload.scm (send-files): Remove call to
'topologically-sorted'.

Merge branch 'master' into staging

Merge branch 'master' into python-build-system

offload: Call 'machine-load' only once per machine.

This fixes a longstanding issue where 'choose-build-machine' would make
on average O(N log(N)) calls to 'machine-load', plus an extra call for
the selected machine, instead of N calls.

* guix/scripts/offload.scm (machine-load): Add comment.
(machine-power-factor, machine-less-loaded-or-faster?): Remove.
(choose-build-machine)[machines+slots]: Rename to...
[machines+slots+loads]: ... this.
[undecorate]: Adjust accordingly.
[machine-less-loaded-or-faster?]: New procedure.
Remove extra 'machine-load' call in body.