ptp: Don't switch domain's master clock if it is equivalent to the previous one

Otherwise it can happen that we regularly switch back and forth between
clocks under certain circumstances for no good reason.

Also remove redundant comparison when comparing the steps removed between two
clocks.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/5520>

subprojects/gstreamer/libs/gst/net/gstptpclock.c

@@ -696,21 +696,18 @@ parse_ptp_message (PtpMessage * msg, const guint8 * data, gsize size)
 
 static gint
 compare_announce_message (const PtpAnnounceMessage * a,
-    const PtpAnnounceMessage * b)
+    const PtpAnnounceMessage * b, gboolean skip_tiebreakers)
 {
   /* IEEE 1588 Figure 27 */
   if (a->grandmaster_identity == b->grandmaster_identity) {
-    if (a->steps_removed + 1 < b->steps_removed)
-      return -1;
-    else if (a->steps_removed > b->steps_removed + 1)
-      return 1;
-
-    /* Error cases are filtered out earlier */
     if (a->steps_removed < b->steps_removed)
       return -1;
     else if (a->steps_removed > b->steps_removed)
       return 1;
 
+    if (skip_tiebreakers)
+      return 0;
+
     /* Error cases are filtered out earlier */
     if (a->master_clock_identity.clock_identity <
         b->master_clock_identity.clock_identity)
@@ -819,9 +816,56 @@ select_best_master_clock (PtpDomainData * domain, GstClockTime now)
   for (l = qualified_messages; l; l = l->next) {
     PtpAnnounceMessage *msg = l->data;
 
-    if (!best || compare_announce_message (msg, best) < 0)
+    if (!best || compare_announce_message (msg, best, FALSE) < 0)
       best = msg;
   }
+
+  GST_DEBUG ("Found master clock for domain %u: 0x%016" G_GINT64_MODIFIER
+      "x %u on interface %u with grandmaster clock 0x%016" G_GINT64_MODIFIER
+      "x", domain->domain, best->master_clock_identity.clock_identity,
+      best->master_clock_identity.port_number, best->iface_idx,
+      best->grandmaster_identity);
+
+  // Check if the newly selected best clock and the previous one are
+  // equivalent except for tiebreakers. In that case, don't actually switch
+  // to avoid switching regularly between clocks.
+  if (domain->have_master_clock &&
+      (compare_clock_identity (&domain->master_clock_identity,
+              &best->master_clock_identity) != 0
+          || domain->iface_idx != best->iface_idx)
+      ) {
+    // Find announce sender for the currently selected clock
+    for (l = domain->announce_senders; l; l = l->next) {
+      PtpAnnounceSender *sender = l->data;
+
+      if (compare_clock_identity (&domain->master_clock_identity,
+              &sender->master_clock_identity) == 0
+          && domain->iface_idx == sender->iface_idx) {
+
+        // Find qualified message for it (if there is none then it timed out)
+        for (m = qualified_messages; m; m = m->next) {
+          PtpAnnounceMessage *msg = m->data;
+
+          if (compare_clock_identity (&sender->master_clock_identity,
+                  &msg->master_clock_identity) == 0
+              && sender->iface_idx == msg->iface_idx) {
+
+            if (compare_announce_message (msg, best, TRUE) == 0) {
+              GST_DEBUG
+                  ("Currently selected master clock for domain %u is equivalent",
+                  domain->domain);
+              best = msg;
+            }
+
+            break;
+          }
+        }
+
+        break;
+      }
+    }
+  }
+
   g_clear_pointer (&qualified_messages, g_list_free);
 
   if (domain->have_master_clock
@@ -830,9 +874,10 @@ select_best_master_clock (PtpDomainData * domain, GstClockTime now)
       && domain->iface_idx == best->iface_idx) {
     GST_DEBUG ("Master clock in domain %u did not change", domain->domain);
   } else {
-    GST_DEBUG ("Selected master clock for domain %u: 0x%016" G_GINT64_MODIFIER
+    GST_DEBUG ("Selected new master clock for domain %u: 0x%016"
-        "x %u on interface %u with grandmaster clock 0x%016" G_GINT64_MODIFIER
+        G_GINT64_MODIFIER "x %u on interface %u with grandmaster clock 0x%016"
-        "x", domain->domain, best->master_clock_identity.clock_identity,
+        G_GINT64_MODIFIER "x", domain->domain,
+        best->master_clock_identity.clock_identity,
         best->master_clock_identity.port_number, best->iface_idx,
         best->grandmaster_identity);