Move the resource pools to a different thread

As the server thread can block for a while when writing big responses.

guix-data-service/web/server.scm

@@ -101,9 +101,37 @@
   (%guix-data-service-metrics-registry registry)
 
   (let ((finished?         (make-condition))
+        (priority-scheduler #f)
         (request-scheduler #f))
     (call-with-sigint
      (lambda ()
+       (call-with-new-thread
+        (lambda ()
+          (run-fibers
+           (lambda ()
+             (let* ((current (current-scheduler))
+                    (schedulers
+                     (cons current (scheduler-remote-peers current))))
+
+               (set! priority-scheduler current)
+
+               (for-each
+                (lambda (i sched)
+                  (spawn-fiber
+                   (lambda ()
+                     (catch 'system-error
+                       (lambda ()
+                         (set-thread-name
+                          (string-append "priority " (number->string i))))
+                       (const #t)))
+                   sched))
+                (iota (length schedulers))
+                schedulers))
+
+             (wait finished?))
+           #:hz 0
+           #:parallelism 1)))
+
        (call-with-new-thread
         (lambda ()
           (run-fibers
@@ -151,6 +179,8 @@
 
           (while (not request-scheduler)
             (sleep 0.1))
+          (while (not priority-scheduler)
+            (sleep 0.1))
 
           (let ((requests-metric
                  (make-counter-metric registry "requests_total")))
@@ -179,7 +209,8 @@ port. Also, the port used can be changed by passing the --port option.\n"
                        #:idle-seconds 30
                        #:destructor
                        (lambda (conn)
-                         (close-postgresql-connection conn "web"))))
+                         (close-postgresql-connection conn "web"))
+                       #:scheduler priority-scheduler))
 
                      (reserved-connection-pool
                       (make-resource-pool
@@ -192,7 +223,8 @@ port. Also, the port used can be changed by passing the --port option.\n"
                        #:idle-seconds 600
                        #:destructor
                        (lambda (conn)
-                         (close-postgresql-connection conn "web-reserved"))))
+                         (close-postgresql-connection conn "web-reserved"))
+                       #:scheduler priority-scheduler))
 
                      (resource-pool-default-timeout 5))