distro-build: add #:pause-before' and #:pause-after' options

I'm hoping this will help reduce failures in my snapshot builds.

pkgs/distro-build/config.rkt

@@ -145,6 +145,8 @@
     [(#:site-dest) (path-string? val)]
     [(#:pdf-doc?) (boolean? val)]
     [(#:max-snapshots) (real? val)]
+    [(#:pause-before) (and (real? val) (not (negative? val)))]
+    [(#:pause-after) (and (real? val) (not (negative? val)))]
     [(#:readme) (or (string? val)
                     (and (procedure? val)
                          (procedure-arity-includes? val 1)))]

pkgs/distro-build/doc.txt

@@ -239,6 +239,14 @@ Site-configuration keywords (where <string*> means no spaces, etc.):
   #:max-snapshots <number> --- number of snapshots to keep, used by
     the `snapshot-site' makefile target
 
+  #:pause-before <nonnegative-number> --- a pause in seconds to
+    wait before starting a machine, which may help a virtual machine
+    avoid confusion from being stopped and started too quickly; the
+    default is 0
+
+  #:pause-after <nonnegative-number> --- a pause in seconds to
+    wait after stopping a machine;  the default is 0
+
   #:custom <hash-table> --- a hash table mapping arbitrary keywords to
     arbitrary values; when a value for `#:custom' is overriden in a
     nested configuration, the new table is merged with the overriden

pkgs/distro-build/drive-clients.rkt

@@ -458,6 +458,7 @@
            (limit-and-report-failure
             c 2
             (lambda ()
+              (sleep (get-opt c '#:pause-before 0))
               ;; start client, if a VM:
               (start-client c (or (get-opt c '#:max-vm) 1))
               ;; catch failure in build step proper, so we
@@ -467,6 +468,7 @@
                 c 1
                 (lambda () (client-build c)))
                ;; stop client, if a VM:
-               (stop-client c))))))]))))
+               (stop-client c)
+               (sleep (get-opt c '#:pause-after 0)))))))]))))
 
 (display-time)