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'.

store: 'open-connection' no longer raises '&nar-error' for protocol errors.

* guix/store.scm (open-connection): Guard body against 'nar-error?' and
re-raise as '&nix-connection-error'.
* tests/store.scm ("connection handshake error"): New test.

gnu: make-bootstrap: Produce the correct %glibc-bootstrap-tarball for Hurd systems.

* gnu/packages/make-bootstrap.scm (%glibc-bootstrap-tarball): Make it a procedure.
  (%glibc-stripped): Make it a procedure and move the kernel specific part from
  here to ...
* guix/build/make-bootstrap.scm (make-stripped-libc): ... here. New file.
* Makefile.am (MODULES): Add it.

gnu: cross-base: Add i686-w64-mingw32 target.

* guix/utils.scm (mingw-target?): New function.
* gnu/packages/cross-base.scm (cross-gcc-snippet): New procedure
(cross-gcc): Use it.
(cross-gcc-arguments, cross-gcc-patches, cross-gcc): Support MinGW.
(native-libc, cross-newlib?): New functions.
(cross-libc): Use cross-newlib? to support MinGW.
(%gcc-include-paths, %gcc-cross-include-paths): New variables.

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.

guix system: If the new system generation already exists, overwrite it.

Fixes <http://bugs.gnu.org/25100>.

Before that, 'guix system reconfigure' would fail if the next generation
already existed.

* guix/scripts/system.scm (switch-to-system): Use 'switch-symlink'
instead of 'symlink'.
* doc/guix.texi (Using the Configuration System, Invoking guix system):
Document the behavior.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

refresh: Indicate that PACKAGE... is optional.

* guix/scripts/refresh.scm (show-help): Add brackets around PACKAGE.

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'.

publish: Factorize 'content-length' addition.

* guix/scripts/publish.scm (with-content-length): New procedure.
(http-write) <application/octet-stream>: Use it.

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.

store: Increase buffering for the '%stderr-write' upcall.

* guix/store.scm (process-stderr) <%stderr-write>: Pass #:buffer-size to
'dump-port'.

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'.

refresh: '--list-updaters' shows updater coverage.

* guix/scripts/refresh.scm (list-updaters-and-exit): Compute the
coverage ratio of each updater and print it.  Print the coverage ratio
for all the updaters.
* doc/guix.texi (Invoking guix refresh): Document it.