Support polling git repositories for new branches/revisions

This is mostly a workaround for the occasional problems with the guix-commits
mailing list, as it can break and then the data service doesn't learn about
new revisions until the problem is fixed.

I think it's still a generally good feature though, and allows deploying the
data service without it consuming emails to learn about new revisions, and is
a step towards integrating some kind of way of notifying the data service to
poll.

guix-data-service/database.scm

@@ -39,6 +39,7 @@
             check-test-database!
 
             with-advisory-session-lock
+            with-advisory-session-lock/log-time
             obtain-advisory-transaction-lock
 
             exec-query-with-null-handling))
@@ -298,6 +299,22 @@
                     "SELECT pg_advisory_unlock($1)"
                     (list lock-number))))))
 
+(define (with-advisory-session-lock/log-time conn lock f)
+  (simple-format #t "debug: Acquiring advisory session lock: ~A\n" lock)
+  (let ((start-time (current-time)))
+    (with-advisory-session-lock
+     conn
+     lock
+     (lambda ()
+       (let ((time-taken (- (current-time) start-time)))
+         (simple-format #t "debug: Finished aquiring lock ~A, took ~A seconds\n"
+                        lock time-taken))
+       (let ((result (f)))
+         (let ((time-spent (- (current-time) start-time)))
+           (simple-format #t "debug: Releasing lock ~A, spent ~A seconds\n"
+                          lock time-spent))
+         result)))))
+
 (define (obtain-advisory-transaction-lock conn lock)
   (let ((lock-number (number->string (symbol-hash lock))))
     (exec-query conn