Simplify isNodeAvailable function and add comments

Use getNodeReplicationOffset to replace the original logic
and add the corresponding annotations.

Signed-off-by: Binbin <[email protected]>

src/cluster.c

@@ -1332,16 +1332,18 @@ void addNodeToNodeReply(client *c, clusterNode *node) {
  * not finished their initial sync, in failed state, or are
  * otherwise considered not available to serve read commands. */
 int isNodeAvailable(clusterNode *node) {
+    /* We don't consider PFAIL here because it's not a reliable indicator
+     * for node available and we don't want clients to use it. */
     if (clusterNodeIsFailing(node)) {
         return 0;
     }
-    long long repl_offset = clusterNodeReplOffset(node);
-    if (clusterNodeIsMyself(node)) {
-        /* Nodes do not update their own information
-         * in the cluster node list. */
-        repl_offset = getNodeReplicationOffset(node);
+
+    /* Hide empty replicas in here, from a data-path POV, an empty replica
+     * is not available. */
+    if (getNodeReplicationOffset(node) == 0) {
+        return 0;
     }
-    return (repl_offset != 0);
+    return 1;
 }
 
 void addNodeReplyForClusterSlot(client *c, clusterNode *node, int start_slot, int end_slot) {

src/cluster.h

@@ -103,7 +103,6 @@ clusterNode *getNodeBySlot(int slot);
 int clusterNodeClientPort(clusterNode *n, int use_tls);
 char *clusterNodeHostname(clusterNode *node);
 const char *clusterNodePreferredEndpoint(clusterNode *n, client *c);
-long long clusterNodeReplOffset(clusterNode *node);
 clusterNode *clusterLookupNode(const char *name, int length);
 int detectAndUpdateCachedNodeHealth(void);
 client *createCachedResponseClient(int resp);

src/cluster_legacy.c

@@ -5797,6 +5797,10 @@ void clusterUpdateSlots(client *c, unsigned char *slots, int del) {
     }
 }
 
+/* Get the replication offset of a node.
+ *
+ * Nodes do not update their own information in the cluster node list,
+ * so for myself node, we cannot use node->repl_offset directly. */
 long long getNodeReplicationOffset(clusterNode *node) {
     if (node->flags & CLUSTER_NODE_MYSELF) {
         return nodeIsReplica(node) ? replicationGetReplicaOffset() : server.primary_repl_offset;
@@ -6793,10 +6797,6 @@ char *clusterNodeHostname(clusterNode *node) {
     return node->hostname;
 }
 
-long long clusterNodeReplOffset(clusterNode *node) {
-    return node->repl_offset;
-}
-
 const char *clusterNodePreferredEndpoint(clusterNode *n, client *c) {
     char *hostname = clusterNodeHostname(n);
     switch (server.cluster_preferred_endpoint_type) {