From 4210699949668655ee3142cd2a9a8997dfa83faa Mon Sep 17 00:00:00 2001
From: Christopher Baines <mail@cbaines.net>
Date: Sun, 29 Jun 2025 23:22:31 +0200
Subject: [PATCH] Don't use the after-gc-hook for monitoring garbage collection

This seems to be happening not in the thread I expect, so avoid using the
hook.
---
 scripts/guix-data-service.in | 45 +++++++++++++++++++-----------------
 1 file changed, 24 insertions(+), 21 deletions(-)

diff --git a/scripts/guix-data-service.in b/scripts/guix-data-service.in
index 0f97fae..44248cb 100644
--- a/scripts/guix-data-service.in
+++ b/scripts/guix-data-service.in
@@ -231,31 +231,34 @@
 
          (set-thread-name "gc watcher")
 
-         (add-hook!
-          after-gc-hook
-          (let ((last-gc-time-taken
-                 (assq-ref (gc-stats) 'gc-time-taken)))
-            (lambda ()
-              (let* ((stats
-                      (gc-stats))
-                     (gc-time-taken
+         (let loop ((last-gc-times
+                     (or (assq-ref (gc-stats) 'times) 0))
+                    (last-gc-time-taken
+                     (assq-ref (gc-stats) 'gc-time-taken)))
+           (let* ((stats (gc-stats))
+                  (gc-times (or (assq-ref stats 'gc-times) 0))
+                  (gc-time-taken (assq-ref stats 'gc-time-taken)))
+             (when (> gc-times last-gc-times)
+               (let ((gc-time-taken
                       (assq-ref stats 'gc-time-taken))
                      (time-since-last
                       (/ (- gc-time-taken
                             last-gc-time-taken)
-                         internal-time-units-per-second)))
-                (when (> time-since-last 0.1)
-                  (format (current-error-port)
-                          "after gc (additional time taken: ~f, times: ~d, heap-allocated-since-gc: ~d, heap size: ~d, heap free: ~d)\n"
-                          time-since-last
-                          (assq-ref stats 'gc-times)
-                          (assq-ref stats 'heap-allocated-since-gc)
-                          (assq-ref stats 'heap-size)
-                          (assq-ref stats 'heap-free-size)))
-                (set! last-gc-time-taken
-                      (assq-ref (gc-stats) 'gc-time-taken))))))
-         (while #t
-           (usleep 100000))))
+                         internal-time-units-per-second))
+                     (gcs
+                      (- gc-times
+                         last-gc-times)))
+                 (format (current-error-port)
+                         "~d gc's (time taken: ~f, heap-allocated-since-gc: ~d, heap size: ~d, heap free: ~d)\n"
+                         gcs
+                         (/ time-since-last
+                            gcs)
+                         (assq-ref stats 'heap-allocated-since-gc)
+                         (assq-ref stats 'heap-size)
+                         (assq-ref stats 'heap-free-size))))
+             (sleep 5)
+             (loop gc-times
+                   gc-time-taken)))))
 
       ;; Provide some visual space between the startup output and the
       ;; server starting