gdk: Add gdk_parallel_task_run()

This is just the API. Users will come later.

I considered putting it into gdkmemoryformat.c because it's likely gonna
be the only user and this one function is so little code, but it didn't
fit at all.
So now it's a new file.

gdk/gdkparalleltask.c

@@ -0,0 +1,86 @@
+/*
+ * Copyright © 2024 Benjamin Otte
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library. If not, see <http://www.gnu.org/licenses/>.
+ *
+ * Authors: Benjamin Otte <otte@gnome.org>
+ */
+
+#include "config.h"
+
+#include "gdkparalleltaskprivate.h"
+
+typedef struct _TaskData TaskData;
+
+struct _TaskData
+{
+  GdkTaskFunc task_func;
+  gpointer task_data;
+  int n_running_tasks;
+};
+
+static void
+gdk_parallel_task_thread_func (gpointer data,
+                               gpointer unused)
+{
+  TaskData *task = data;
+
+  task->task_func (task->task_data);
+
+  g_atomic_int_add (&task->n_running_tasks, -1);
+}
+
+/**
+ * gdk_parallel_task_run:
+ * @task_func: the function to spawn
+ * @task_data: data to pass to the function
+ *
+ * Spawns the given function in many threads.
+ * Once all functions have exited, this function returns.
+ **/
+void
+gdk_parallel_task_run (GdkTaskFunc task_func,
+                       gpointer    task_data)
+{
+  static GThreadPool *pool;
+  TaskData task = {
+    .task_func = task_func,
+    .task_data = task_data,
+  };
+  int i, n_tasks;
+
+  if (g_once_init_enter (&pool))
+    {
+      GThreadPool *the_pool = g_thread_pool_new (gdk_parallel_task_thread_func,
+                              NULL,
+                              MAX (2, g_get_num_processors ()) - 1,
+                              FALSE,
+                              NULL);
+      g_once_init_leave (&pool, the_pool);
+    }
+
+  n_tasks = g_get_num_processors ();
+  task.n_running_tasks = n_tasks;
+  /* Start with 1 because we run 1 task ourselves */
+  for (i = 1; i < n_tasks; i++)
+    {
+      g_thread_pool_push (pool, &task, NULL);
+    }
+
+  gdk_parallel_task_thread_func (&task, NULL);
+
+  while (g_atomic_int_get (&task.n_running_tasks) > 0)
+    g_thread_yield ();
+}
+

gdk/gdkparalleltaskprivate.h

@@ -0,0 +1,32 @@
+/*
+ * Copyright © 2024 Benjamin Otte
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library. If not, see <http://www.gnu.org/licenses/>.
+ *
+ * Authors: Benjamin Otte <otte@gnome.org>
+ */
+
+#pragma once
+
+#include <glib.h>
+
+G_BEGIN_DECLS
+
+typedef void (* GdkTaskFunc) (gpointer user_data);
+
+void                    gdk_parallel_task_run               (GdkTaskFunc                 task_func,
+                                                             gpointer                    task_data);
+
+G_END_DECLS
+

gdk/meson.build

@@ -52,6 +52,7 @@ gdk_public_sources = files([
   'gdkmonitor.c',
   'gdkpaintable.c',
   'gdkpango.c',
+  'gdkparalleltask.c',
   'gdkpipeiostream.c',
   'gdkpopup.c',
   'gdkpopuplayout.c',