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doc: Add 'shepherd-service' example.

* doc/guix.texi (Shepherd Services): Add example.
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Ludovic Courtès 2021-03-19 18:26:59 +01:00
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@ -34060,6 +34060,38 @@ This is the list of modules that must be in scope when @code{start} and
@end table
@end deftp
The example below defines a Shepherd service that spawns
@command{syslogd}, the system logger from the GNU Networking Utilities
(@pxref{syslogd invocation, @command{syslogd},, inetutils, GNU
Inetutils}):
@example
(let ((config (plain-file "syslogd.conf" "@dots{}")))
(shepherd-service
(documentation "Run the syslog daemon (syslogd).")
(provision '(syslogd))
(requirement '(user-processes))
(start #~(make-forkexec-constructor
(list #$(file-append inetutils "/libexec/syslogd")
"--rcfile" #$config)
#:pid-file "/var/run/syslog.pid"))
(stop #~(make-kill-destructor))))
@end example
Key elements in this example are the @code{start} and @code{stop}
fields: they are @dfn{staged} code snippets that use the
@code{make-forkexec-constructor} procedure provided by the Shepherd and
its dual, @code{make-kill-destructor} (@pxref{Service De- and
Constructors,,, shepherd, The GNU Shepherd Manual}). The @code{start}
field will have @command{shepherd} spawn @command{syslogd} with the
given option; note that we pass @code{config} after @option{--rcfile},
which is a configuration file declared above (contents of this file are
omitted). Likewise, the @code{stop} field tells how this service is to
be stopped; in this case, it is stopped by making the @code{kill} system
call on its PID@. Code staging is achieved using G-expressions:
@code{#~} stages code, while @code{#$} ``escapes'' back to host code
(@pxref{G-Expressions}).
@deftp {Data Type} shepherd-action
This is the data type that defines additional actions implemented by a
Shepherd service (see above).