glib-aux: add nm_g_source_sentinel_get() util

This helper is useful to get a dummy GSource instance that can be
refed, unrefed and destroyed. It can act as a replacement for
a timeout source with infinite timeout.

src/libnm-glib-aux/nm-shared-utils.c

@@ -4851,6 +4851,33 @@ nm_utils_parse_debug_string(const char *string, const GDebugKey *keys, guint nke
 
 /*****************************************************************************/
 
+GSource *_nm_g_source_sentinel[] = {
+    NULL,
+};
+
+GSource *
+_nm_g_source_sentinel_get_init(GSource **p_source)
+{
+    static const GSourceFuncs source_funcs = {
+        NULL,
+    };
+    GSource *source;
+
+again:
+    source = g_source_new((GSourceFuncs *) &source_funcs, sizeof(GSource));
+    g_source_set_priority(source, G_PRIORITY_DEFAULT_IDLE);
+    g_source_set_name(source, "nm_g_source_sentinel");
+
+    if (!g_atomic_pointer_compare_and_exchange(p_source, NULL, source)) {
+        g_source_unref(source);
+        goto again;
+    }
+
+    return source;
+}
+
+/*****************************************************************************/
+
 GSource *
 nm_g_idle_source_new(int            priority,
                      GSourceFunc    func,

src/libnm-glib-aux/nm-shared-utils.h

@@ -1594,6 +1594,31 @@ nm_source_func_unref_gobject(gpointer user_data)
     return G_SOURCE_REMOVE;
 }
 
+extern GSource *_nm_g_source_sentinel[1];
+
+GSource *_nm_g_source_sentinel_get_init(GSource **p_source);
+
+/* Get a GSource sentinel (dummy instance). This instance should never be
+ * attached to a GMainContext. The only currently known purpose is to use it
+ * as dummy value instead of an infinity timeout. That is, if we configurably
+ * want to schedule a timeout that might be infinity, we might set the GSource
+ * instance to nm_g_source_sentinel_get(). On this instance, we still may
+ * call g_source_ref(), g_source_unref() and g_source_destroy(). But nothing
+ * else. */
+#define nm_g_source_sentinel_get(idx)                                         \
+    ({                                                                        \
+        GSource *_s;                                                          \
+                                                                              \
+        G_STATIC_ASSERT((idx) >= 0);                                          \
+        G_STATIC_ASSERT((idx) < G_N_ELEMENTS(_nm_g_source_sentinel));         \
+                                                                              \
+        _s = g_atomic_pointer_get(&_nm_g_source_sentinel[idx]);               \
+        if (G_UNLIKELY(!_s))                                                  \
+            _s = _nm_g_source_sentinel_get_init(&_nm_g_source_sentinel[idx]); \
+                                                                              \
+        _s;                                                                   \
+    })
+
 GSource *nm_g_idle_source_new(int            priority,
                               GSourceFunc    func,
                               gpointer       user_data,

src/libnm-glib-aux/tests/test-shared-general.c

@@ -1293,6 +1293,58 @@ again:
 
 /*****************************************************************************/
 
+static void
+test_nm_g_source_sentinel(void)
+{
+    GSource *s1;
+    GSource *s2;
+    int      n;
+    int      i;
+    int      refs;
+
+    s1 = nm_g_source_sentinel_get(0);
+    g_assert_nonnull(s1);
+    g_assert_cmpint(g_atomic_int_get(&s1->ref_count), ==, 1);
+
+    s2 = nm_g_source_sentinel_get(0);
+    g_assert_nonnull(s2);
+    g_assert(s2 == s1);
+    g_assert_cmpint(g_atomic_int_get(&s1->ref_count), ==, 1);
+
+    n = nmtst_get_rand_uint32() % 7;
+    for (refs = 0, i = 0; i < n; i++) {
+        if (nmtst_get_rand_bool()) {
+            refs++;
+            g_source_ref(s1);
+        }
+        if (nmtst_get_rand_bool())
+            g_source_destroy(s1);
+        if (refs > 0 && nmtst_get_rand_bool()) {
+            refs--;
+            g_source_unref(s1);
+        }
+
+        if (nmtst_get_rand_bool()) {
+            s2 = nm_g_source_sentinel_get(0);
+            g_assert(s2 == s1);
+            g_assert_cmpint(g_atomic_int_get(&s1->ref_count), >=, 1);
+        }
+    }
+
+    for (; refs > 0;) {
+        if (nmtst_get_rand_bool())
+            g_source_destroy(s1);
+        if (nmtst_get_rand_bool()) {
+            refs--;
+            g_source_unref(s1);
+        }
+    }
+
+    g_assert_cmpint(g_atomic_int_get(&s1->ref_count), ==, 1);
+}
+
+/*****************************************************************************/
+
 NMTST_DEFINE();
 
 int
@@ -1323,6 +1375,7 @@ main(int argc, char **argv)
     g_test_add_func("/general/test_is_specific_hostname", test_is_specific_hostname);
     g_test_add_func("/general/test_strv_dup_packed", test_strv_dup_packed);
     g_test_add_func("/general/test_utils_hashtable_cmp", test_utils_hashtable_cmp);
+    g_test_add_func("/general/test_nm_g_source_sentinel", test_nm_g_source_sentinel);
 
     return g_test_run();
 }