Add buffering to the log handling for jobs

To increase the log output speed, avoiding inserting one character at a time
in to the database.

guix-data-service/jobs/load-new-guix-revision.scm

@@ -40,6 +40,7 @@
     (current-output-port))
 
   (define id 0)
+  (define buffer "")
 
   (define (insert job_id s)
     (exec-query
@@ -50,9 +51,13 @@
      (list (number->string id) job_id s)))
 
   (define (log-string s)
+    (if (string-contains s "\n")
+        (let ((output (string-append buffer s)))
           (set! id (+ 1 id)) ; increment id
-    (insert job-id s)
+          (set! buffer "") ; clear the buffer
-    (display s output-port))
+          (insert job-id output)
+          (display output output-port))
+        (set! buffer (string-append buffer s))))
 
   ;; TODO, this is useful when re-running jobs, but I'm not sure that should
   ;; be a thing, jobs should probably be only attempted once.
@@ -61,9 +66,10 @@
    "DELETE FROM load_new_guix_revision_job_log_parts WHERE job_id = $1"
    (list job-id))
 
+  (let ((port
          (make-soft-port
           (vector (lambda (c)
-             (log-string (string c)))
+                    (set! buffer (string-append buffer (string c))))
                   log-string
                   (lambda ()
                     (force-output output-port))
@@ -72,7 +78,9 @@
                     ;; close port
                     #f)
                   #f) ; number of characters that can be read
-   "w"))
+          "w")))
+    (setvbuf port 'line)
+    port))
 
 (define* (log-for-job conn job-id
                       #:key