gexp: 'gexp-modules' accepts plain Scheme objects.

* guix/gexp.scm (gexp-modules): Return '() when not (gexp? GEXP).
* tests/gexp.scm ("gexp-modules and literal Scheme object"): New test.

Add (guix cache) and use it in (guix scripts substitute).

* guix/cache.scm, tests/cache.scm: New files.
* Makefile.am (MODULES, SCM_TESTS): Add them.
* guix/scripts/substitute.scm (obsolete?): Remove.
(remove-expired-cached-narinfos): Rename to...
(cached-narinfo-expiration-time): ... this.  Remove the removal part and
only keep the expiration time part.
(narinfo-cache-directories): Add optional 'directory' parameter and
honor it.
(maybe-remove-expired-cached-narinfo): Remove.
(cached-narinfo-files): New procedure.
(guix-substitute): Use 'maybe-remove-expired-cache-entries' instead of
'maybe-remove-expired-cached-narinfo'.

publish: Add '--cache' and '--workers'.

Fixes <http://bugs.gnu.org/26201>.
Reported by <dian_cecht@zoho.com>.

These options allow nars to be "baked" off-line and cached instead of
being compressed on the fly.  As a side-effect, this allows us to
provide a 'Content-Length' header for nars.

* guix/scripts/publish.scm (show-help, %options): Add '--cache' and
'--workers'.
(%default-options): Add 'workers'.
(nar-cache-file, narinfo-cache-file, run-single-baker): New procedures.
(single-baker): New macro.
(render-narinfo/cached, bake-narinfo+nar)
(render-nar/cached): New procedures.
(make-request-handler): Add #:cache and #:pool parameters and honor
them.
(run-publish-server): Likewise.
(guix-publish): Honor '--cache' and '--workers'.
* tests/publish.scm ("with cache"): New test.
* doc/guix.texi (Invoking guix publish): Document it.

Add (guix workers).

* guix/workers.scm, tests/workers.scm: New files.
* Makefile.am (MODULES, SCM_TESTS): Add them.
* .dir-locals.el: Add rule for 'eventually'.

services: Service types can now specify a default value for instances.

* gnu/services.scm (&no-default-value): New variable.
(<service-type>)[default-value]: New field.
(<service>): Rename constructor from 'service' to 'make-service'.
(service): New macro.
(%service-with-default-value): New procedure.
(&missing-value-service-error): New error condition.
* tests/services.scm ("services, default value"): New test.
* doc/guix.texi (Service Types and Services): Document 'default-value'.
(Service Reference): Explain default values.

services: 'service-parameters' becomes 'service-value'.

* gnu/services.scm (<service>)[parameters]: Rename to...
[value]: ... this.
Change calls to 'service-parameters' to 'service-value'.
* gnu/system.scm, gnu/tests/base.scm,
guix/scripts/system.scm, tests/services.scm: Likewise.
* doc/guix.texi (Service Reference): Adjust accordingly.

size: Add test to multiple 'store-profile' arguments.

* tests/size.scm ("store-profile with multiple items"): New test.

packages: Add 'package-mapping' and base 'package-input-rewriting' on it.

* guix/packages.scm (package-mapping): New procedure.
(package-input-rewriting): Rewrite in terms of 'package-mapping'.
* tests/packages.scm ("package-mapping"): New test.
* doc/guix.texi (Defining Packages): Document it.

tests: Adjust search-paths test following the introduction of aarch64.

Fixes a regression introduced in
3b88f3767d9f3ad2cc64173525cd53d429bfe7e7.

* tests/search-paths.scm ("evaluate-search-paths, separator is #f"):
Change the expected result to "aarch64-linux".

packages: Catch invalid input errors for structs.

Reported by Thomas Sigurdsen <thomas.sigurdsen@gmail.com>
at <https://lists.gnu.org/archive/html/help-guix/2017-04/msg00007.html>.

* guix/packages.scm (expand-input): Add 'guard' form around call to
'package-source-derivation'.
* tests/packages.scm (dummy): New test.

gexp: 'lower-object' raises an exception when passed an invalid object.

* guix/gexp.scm (&gexp-error, &gexp-input-error): New error conditions.
(lower-object): Raise &gexp-input-error when 'lookup-compiler' returns #f.
* tests/gexp.scm ("lower-object & gexp-input-error?"): New test.
* guix/ui.scm (call-with-error-handling): Add case for 'gexp-input-error?'.

Merge branch 'core-updates'

Merge branch 'master' into core-updates

Most conflicts are from 6fd52309b8f52c9bb59fccffac53e029ce94b698.

derivations: Do not fetch narinfos for non-substitutable items.

This avoids connections to substitute servers for derivations that are
not substitutable anyway, such as profiles.

Reported by Andy Wingo.

* guix/derivations.scm (substitution-oracle): Skip derivations that do
not pass 'substitutable-derivation?'.
* tests/derivations.scm ("substitution-oracle and #:substitute? #f"):
New test.

Merge branch 'master' into core-updates

ui: Support Texinfo markup in package synopses.

* guix/ui.scm (package-field-string): New procedure.
(package-description-string): Use it.
(package-synopsis-string): New procedure.
(package->recutils): Use it.
* guix/scripts/lint.scm (check-synopsis-style)[check-texinfo-markup]:
New procedure.  Use it in checks.
* tests/lint.scm: Test it.
* gnu/packages/perl.scm (perl-try-tiny)[synopsis]: Adjust for the
Texinfo markup.

union: Add create-all-directories? parameter to 'union-build'.

* guix/build/union.scm (union-build): Add create-all-directories? keyword
parameter.
* tests/union.scm ("union-build #:create-all-directories? #t"): New test.

Co-authored-by: Ludovic Courtès <ludo@gnu.org>

Merge remote-tracking branch 'origin/master' into core-updates

tests: Skip 'guix pack' test when networking is unavailable.

* tests/pack.scm ("self-contained-tarball"): Skip unless (network-reachable?).

tests: Fix 'cond-expand' clause for 2.2/2.0.

Fixes a mistake in a9a0227c0127e1813190a7664f7ad858e791480d.

* tests/store.scm ("current-build-output-port, UTF-8 + garbage"):
Inverse the 'cond-expand' clauses since the 'guile-2.0' feature doesn't
exist.