Drop the MEET packet if the link node is in handshake state (#1436)

After #1307 got merged, we notice there is a assert happen in setClusterNodeToInboundClusterLink:
```
=== ASSERTION FAILED ===
==> '!link->node' is not true
```

In #778, we will call setClusterNodeToInboundClusterLink to attach the node to the link
during the MEET processing, so if we receive a another MEET packet in a short time, the
node is still in handshake state, we will meet this assert and crash the server.

If the link is bound to a node and the node is in the handshake state, and we receive
a MEET packet, it may be that the sender sent multiple MEET packets so in here we are
dropping the MEET to avoid the assert in setClusterNodeToInboundClusterLink. The assert
will happen if the other sends a MEET packet because it detects that there is no inbound
link, this node creates a new node in HANDSHAKE state (with a random node name), and
respond with a PONG. The other node receives the PONG and removes the CLUSTER_NODE_MEET
flag. This node is supposed to open an outbound connection to the other node in the next
cron cycle, but before this happens, the other node re-sends a MEET on the same link
because it still detects no inbound connection.

Note that in getNodeFromLinkAndMsg, the node in the handshake state has a random name
and not truly "known", so we don't know the sender. Dropping the MEET packet can prevent
us from creating a random node, avoid incorrect link binding, and avoid duplicate MEET
packet eliminate the handshake state.

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

src/cluster_legacy.c

@@ -3003,7 +3003,8 @@ int clusterIsValidPacket(clusterLink *link) {
     }
 
     if (type == server.cluster_drop_packet_filter || server.cluster_drop_packet_filter == -2) {
-        serverLog(LL_WARNING, "Dropping packet that matches debug drop filter");
+        serverLog(LL_WARNING, "Dropping packet of type %s that matches debug drop filter",
+                  clusterGetMessageTypeString(type));
         return 0;
     }
 
@@ -3094,7 +3095,7 @@ int clusterProcessPacket(clusterLink *link) {
         if (server.debug_cluster_close_link_on_packet_drop &&
             (type == server.cluster_drop_packet_filter || server.cluster_drop_packet_filter == -2)) {
             freeClusterLink(link);
-            serverLog(LL_WARNING, "Closing link for matching packet type %hu", type);
+            serverLog(LL_WARNING, "Closing link for matching packet type %s", clusterGetMessageTypeString(type));
             return 0;
         }
         return 1;
@@ -3110,8 +3111,8 @@ int clusterProcessPacket(clusterLink *link) {
             freeClusterLink(link);
             serverLog(
                 LL_NOTICE,
-                "Closing link for node that sent a lightweight message of type %hu as its first message on the link",
-                type);
+                "Closing link for node that sent a lightweight message of type %s as its first message on the link",
+                clusterGetMessageTypeString(type));
             return 0;
         }
         clusterNode *sender = link->node;
@@ -3120,6 +3121,27 @@ int clusterProcessPacket(clusterLink *link) {
         return 1;
     }
 
+    if (type == CLUSTERMSG_TYPE_MEET && link->node && nodeInHandshake(link->node)) {
+        /* If the link is bound to a node and the node is in the handshake state, and we receive
+         * a MEET packet, it may be that the sender sent multiple MEET packets so in here we are
+         * dropping the MEET to avoid the assert in setClusterNodeToInboundClusterLink. The assert
+         * will happen if the other sends a MEET packet because it detects that there is no inbound
+         * link, this node creates a new node in HANDSHAKE state (with a random node name), and
+         * respond with a PONG. The other node receives the PONG and removes the CLUSTER_NODE_MEET
+         * flag. This node is supposed to open an outbound connection to the other node in the next
+         * cron cycle, but before this happens, the other node re-sends a MEET on the same link
+         * because it still detects no inbound connection. We improved the re-send logic of MEET in
+         * #1441, now we will only re-send MEET packet once every handshake timeout period.
+         *
+         * Note that in getNodeFromLinkAndMsg, the node in the handshake state has a random name
+         * and not truly "known", so we don't know the sender. Dropping the MEET packet can prevent
+         * us from creating a random node, avoid incorrect link binding, and avoid duplicate MEET
+         * packet eliminate the handshake state. */
+        serverLog(LL_NOTICE, "Dropping MEET packet from node %.40s because the node is already in handshake state",
+                  link->node->name);
+        return 1;
+    }
+
     uint16_t flags = ntohs(hdr->flags);
     uint64_t sender_claimed_current_epoch = 0, sender_claimed_config_epoch = 0;
     clusterNode *sender = getNodeFromLinkAndMsg(link, hdr);