utils: Never return a group_id of 0, add GST_GROUP_ID_INVALID

Various plugins use special values (0 or G_MAXUINT32) as an
invalid/unset group_id, but nothing guarantees a groupid won't have
that value.

Instead define a value which group_id will never have and make
gst_group_id_next() always return a value different from that.

API: GST_GROUP_ID_INVALID

docs/gst/gstreamer-sections.txt

@@ -3571,6 +3571,7 @@ GFLOAT_FROM_LE
 GFLOAT_SWAP_LE_BE
 GFLOAT_TO_BE
 GFLOAT_TO_LE
+GST_GROUP_ID_INVALID
 
 gst_guint64_to_gdouble
 gst_gdouble_to_guint64

gst/gstutils.c

@@ -4177,14 +4177,22 @@ gst_pad_get_stream (GstPad * pad)
  * This function is used to generate a new group-id for the
  * stream-start event.
  *
+ * This function never returns %GST_GROUP_ID_INVALID (which is 0)
+ *
  * Returns: A constantly incrementing unsigned integer, which might
  * overflow back to 0 at some point.
  */
 guint
 gst_util_group_id_next (void)
 {
-  static gint counter = 0;
-  return g_atomic_int_add (&counter, 1);
+  static gint counter = 1;
+  gint ret = g_atomic_int_add (&counter, 1);
+
+  /* Make sure we don't return GST_GROUP_ID_INVALID */
+  if (G_UNLIKELY (ret == GST_GROUP_ID_INVALID))
+    ret = g_atomic_int_add (&counter, 1);
+
+  return ret;
 }
 
 /* Compute log2 of the passed 64-bit number by finding the highest set bit */

gst/gstutils.h

@@ -107,6 +107,18 @@ guint32         gst_util_seqnum_next            (void);
 GST_EXPORT
 gint32          gst_util_seqnum_compare         (guint32 s1, guint32 s2);
 
+/**
+ * GST_GROUP_ID_INVALID:
+ *
+ * A value which is guaranteed to never be returned by
+ * gst_util_group_id_next().
+ *
+ * Can be used as a default value in variables used to store group_id.
+ *
+ * Since: 1.14
+ */
+#define GST_GROUP_ID_INVALID (0)
+
 GST_EXPORT
 guint           gst_util_group_id_next          (void);