[gnome-photos/wip/max-threads] gegl: Don't exceed arbitrary maximum number of threads

[Simon McVittie <smcv src gnome org>]

commit 4b1d7f31446683a0dfd98895cc81800d5b99d219
Author: Simon McVittie <smcv debian org>
Date:   Wed Dec 9 12:46:04 2020 +0000

    gegl: Don't exceed arbitrary maximum number of threads
    
    gegl has an arbitrary maximum value for GeglConfig:threads, currently 64,
    and exceeding it causes critical warnings. This can make the build-time
    tests fail on massively parallel hardware. For example, on a dual
    POWER8 system used to build Debian ppc64el packages (2 CPUs, 10 cores
    per CPU and 8 threads per core, for a total of 160 threads), we ask
    gegl to use 80 threads, which is more than 64.
    
    The simple approach to this would be to clamp to the range
    [1, GEGL_MAX_THREADS], but this would not be robust against possible
    future changes to the value of GEGL_MAX_THREADS, so this commit
    introspects the property to find out the true maximum at runtime.
    
    Bug-Debian: https://bugs.debian.org/976932
    Signed-off-by: Simon McVittie <smcv debian org>

 src/photos-gegl.c | 26 +++++++++++++++++++++-----
 1 file changed, 21 insertions(+), 5 deletions(-)
---
diff --git a/src/photos-gegl.c b/src/photos-gegl.c
index c2e12af3..a2424400 100644
--- a/src/photos-gegl.c
+++ b/src/photos-gegl.c
@@ -680,23 +680,39 @@ void
 photos_gegl_init (void)
 {
   GeglConfig *config;
-  gint threads;
+  GParamSpec *threads_property;
+  GParamSpecInt *threads_property_int;
+  guint threads;
+  guint max_threads;
   guint n_processors;
 
   n_processors = g_get_num_processors ();
-  g_return_if_fail (n_processors > 0);
+  config = gegl_config ();
+
+  /* gegl has an arbitrary maximum for the number of threads. The maximum
+   * for the version of gegl we were compiled against is given by the
+   * GEGL_MAX_THREADS macro, but there's no guarantee that the version we
+   * are eventually run against will be the same, so introspect the property
+   * to get the current maximum. */
+  threads_property = g_object_class_find_property (G_OBJECT_GET_CLASS (config),
+                                                   "threads");
+  g_return_if_fail (threads_property != NULL);
+  g_return_if_fail (G_IS_PARAM_SPEC_INT (threads_property));
+  threads_property_int = G_PARAM_SPEC_INT (threads_property);
+  g_return_if_fail (threads_property_int->maximum >= 1);
+  max_threads = threads_property_int->maximum;
 
   /* The number of threads should match the number of physical CPU
    * cores, not the number of virtual hyper-threading cores. In the
    * absence of an API to get the number of physical CPU cores, we
    * assume that a number higher than one is indicative of
    * hyper-threading, and hence divide by two.
+   * Make sure it's in the allowed range for the threads property.
    */
-  threads = (gint) (n_processors > 1 ? n_processors / 2 : n_processors);
+  threads = CLAMP (n_processors / 2, 1U, max_threads);
 
-  config = gegl_config ();
   g_object_set (config, "application-license", "GPL3", NULL);
-  g_object_set (config, "threads", threads, NULL);
+  g_object_set (config, "threads", (gint) threads, NULL);
   g_object_set (config, "use-opencl", FALSE, NULL);
 
   gegl_init (NULL, NULL);