This simplifies OS configuration: users no longer need to be aware of
what a given service depends on.
See the discussion at
<https://lists.gnu.org/archive/html/guix-devel/2018-01/msg00114.html>.
* gnu/services.scm (missing-target-error): New procedure.
(service-back-edges): Use it.
(instantiate-missing-services): New procedure.
* gnu/system.scm (operating-system-services): Call
'instantiate-missing-services'.
* tests/services.scm ("instantiate-missing-services")
("instantiate-missing-services, no default value"): New tests.
* gnu/services/version-control.scm (cgit-service-type)[extensions]: Add
FCGIWRAP-SERVICE-TYPE.
* gnu/tests/version-control.scm (%cgit-os): Remove NGINX-SERVICE-TYPE
and FCGIWRAP-SERVICE-TYPE instances.
* doc/guix.texi (Log Rotation): Remove 'mcron-service-type' in example.
(Miscellaneous Services): Remove 'nginx-service-type' and
'fcgiwrap-service-type' in Cgit example.
Previously tests sometimes start before the shepherd was listening,
leading to test failures.
* gnu/tests/version-control.scm (run-cgit-test)[test]: Add "shepherd
socket ready" test. Rename to two "service running" tests for clarity.
* gnu/tests/version-control.scm (README-contents): New variable.
(%make-git-repository): Add a 'README' file to the repo.
(%test-repository-service): New variable.
(%cgit-os): Use it.
(run-cgit-test): Test /test/tree/README and /test/tree/does-not-exist.
Ludovic Courtès