owner: add some comments

cdc/owner.go

@@ -86,6 +86,10 @@ func (o *Owner) getMinGCSafePointCache(ctx context.Context) model.Ts {
 		}
 		o.minGCSafePointCache.ts = oracle.ComposeTS(physicalTs-(o.gcTTL*1000), logicalTs)
 
+		// o.pdGCSafePoint pd is the smallest gcSafePoint across all services.
+		// If tikv_gc_life_time > gcTTL, means that tikv_gc_safe_point < o.minGCSafePointCache.ts here.
+		// It also means that pd.pdGCSafePoint < o.minGCSafePointCache.ts here, we should use its value as the min value.
+		// This ensures that when tikv_gc_life_time > gcTTL , cdc will not advance the gcSafePoint.
 		if o.pdGCSafePoint < o.minGCSafePointCache.ts {
 			o.minGCSafePointCache.ts = o.pdGCSafePoint
 		}
@@ -132,7 +136,7 @@ type Owner struct {
 	gcSafepointLastUpdate time.Time
 	// stores the ts obtained from PD and is updated every MinGCSafePointCacheUpdateInterval.
 	minGCSafePointCache minGCSafePointCacheEntry
-	// stores the actual gcSafePoint store in pd
+	// stores the actual gcSafePoint stored in pd
 	pdGCSafePoint model.Ts
 	// record last time that flushes all changefeeds' replication status
 	lastFlushChangefeeds    time.Time
@@ -765,9 +769,12 @@ func (o *Owner) flushChangeFeedInfos(ctx context.Context) error {
 			// A changefeed will not enter the map twice, because in run(),
 			// handleAdminJob() will always be executed before flushChangeFeedInfos(),
 			// ensuring that the previous changefeed in staleChangeFeeds has been stopped and removed from o.changeFeeds.
-			// 2. We need the <= check here is because when a changefeed is stagnant, its checkpointTs will be updated to pd,
-			// and it would be the minimum gcSafePoint of cdc. In order to avoid circular dependence, it is necessary to check for equality.
-			// Checking whether they are equal will make the loop assignment to happen only once, and then it will be skipped in next time.
+			// 2. We need the `<=` check here is because when a changefeed is stagnant, its checkpointTs will be updated to pd,
+			// and it would be the minimum gcSafePoint across all services.
+			// So as described above(line 92) minGCSafePoint = gcSafePoint = CheckpointTs would happens.
+			// In this case, if we check `<` here , this changefeed will not be put into staleChangeFeeds, and its checkpoints will be updated to pd again and again.
+			// This will cause the cdc's gcSafePoint never advance.
+			// If we check `<=` here, when we encounter the changefeed again, we will put it into staleChangeFeeds.
 			if changefeed.status.CheckpointTs <= minGCSafePoint {
 				staleChangeFeeds[id] = changefeed.status
 			}