services: virtual-build-machine: Add ‘configuration’ action.

* gnu/services/virtualization.scm (build-vm-shepherd-services): Add
‘configuration’ action.
* doc/guix.texi (Virtualization Services): Document it.

Change-Id: I4734e096d744b3cda0d523692498a73c0029e188

doc/guix.texi

@@ -36396,7 +36396,11 @@ set to a date several years in the past, and on a CPU model that
 corresponds to that date---a model possibly older than that of your
 machine.  This lets you rebuild today software from the past that would
 otherwise fail to build due to a time trap or other issues in its build
-process.
+process.  You can view the VM's config like this:
+
+@example
+herd configuration build-vm
+@end example
 
 You can configure the build VM, as in this example:
 

gnu/services/virtualization.scm

@@ -1342,6 +1342,22 @@ authpriv.*;auth.info                    /var/log/secure
                       (kill (- pid) SIGTERM)
                       (apply throw key args)))))))
          (stop #~(make-kill-destructor))
+         (actions
+          (list (shepherd-action
+                 (name 'configuration)
+                 (documentation
+                  "Display the configuration of this virtual build machine.")
+                 (procedure
+                  #~(lambda (_)
+                      (format #t "CPU: ~a~%"
+                              #$(virtual-build-machine-cpu config))
+                      (format #t "number of CPU cores: ~a~%"
+                              #$(virtual-build-machine-cpu-count config))
+                      (format #t "memory size: ~a MiB~%"
+                              #$(virtual-build-machine-memory-size config))
+                      (format #t "initial date: ~a~%"
+                              #$(date->string
+                                 (virtual-build-machine-date config))))))))
          (auto-start? (virtual-build-machine-auto-start? config)))))
 
 (define (authorize-guest-substitutes-on-host)