Resolve bounds checks on cluster_legacy.c (#1463)

We are getting a number of errors like:
```
array subscript ‘clusterMsg[0]’ is partly outside array bounds of ‘unsigned char[2272]’
```

Which is basically GCC telling us that we have an object which is longer
than the underlying storage of the allocation. We actually do this a
lot, but GCC is generally not aware of how big the underlying allocation
is, so it doesn't throw this error. We are specifically getting this
error because the msgBlock can be of variable length depending on the
type of message, but GCC assumes it's the longest one possible. The
solution I went with here was make the message type optional, so that it
wasn't included in the size. I think this also makes some sense, since
it's really just a helper for us to easily cast the object around.

I considered disabling this error, but it is generally pretty useful
since it can catch real issues. Another solution would be to
over-allocate to the largest possible object, which could hurt
performance as we initialize it to zero.

Results:
https://github.com/madolson/valkey/actions/runs/12423414811/job/34686899884

This is a slightly cleaned up version of
#1439. I thought I had another
strategy but alas, it didn't work out.

Signed-off-by: Madelyn Olson <[email protected]>

src/cluster_legacy.c

@@ -424,9 +424,19 @@ typedef struct {
     union {
         clusterMsg msg;
         clusterMsgLight msg_light;
-    };
+    } data[];
 } clusterMsgSendBlock;
 
+/* Helper function to extract a normal message from a send block. */
+static clusterMsgLight *getLightMessageFromSendBlock(clusterMsgSendBlock *msgblock) {
+    return &msgblock->data[0].msg_light;
+}
+
+/* Helper function to extract a light message from a send block. */
+static clusterMsg *getMessageFromSendBlock(clusterMsgSendBlock *msgblock) {
+    return &msgblock->data[0].msg;
+}
+
 /* -----------------------------------------------------------------------------
  * Initialization
  * -------------------------------------------------------------------------- */
@@ -1288,15 +1298,15 @@ void clusterReset(int hard) {
  * CLUSTER communication link
  * -------------------------------------------------------------------------- */
 clusterMsgSendBlock *createClusterMsgSendBlock(int type, uint32_t msglen) {
-    uint32_t blocklen = msglen + offsetof(clusterMsgSendBlock, msg);
+    uint32_t blocklen = msglen + offsetof(clusterMsgSendBlock, data);
     clusterMsgSendBlock *msgblock = zcalloc(blocklen);
     msgblock->refcount = 1;
     msgblock->totlen = blocklen;
     server.stat_cluster_links_memory += blocklen;
     if (IS_LIGHT_MESSAGE(type)) {
-        clusterBuildMessageHdrLight(&msgblock->msg_light, type, msglen);
+        clusterBuildMessageHdrLight(getLightMessageFromSendBlock(msgblock), type, msglen);
     } else {
-        clusterBuildMessageHdr(&msgblock->msg, type, msglen);
+        clusterBuildMessageHdr(getMessageFromSendBlock(msgblock), type, msglen);
     }
     return msgblock;
 }
@@ -3668,7 +3678,7 @@ void clusterWriteHandler(connection *conn) {
     while (totwritten < NET_MAX_WRITES_PER_EVENT && listLength(link->send_msg_queue) > 0) {
         listNode *head = listFirst(link->send_msg_queue);
         clusterMsgSendBlock *msgblock = (clusterMsgSendBlock *)head->value;
-        clusterMsg *msg = &msgblock->msg;
+        clusterMsg *msg = getMessageFromSendBlock(msgblock);
         size_t msg_offset = link->head_msg_send_offset;
         size_t msg_len = ntohl(msg->totlen);
 
@@ -3853,7 +3863,7 @@ void clusterSendMessage(clusterLink *link, clusterMsgSendBlock *msgblock) {
     if (!link) {
         return;
     }
-    if (listLength(link->send_msg_queue) == 0 && msgblock->msg.totlen != 0)
+    if (listLength(link->send_msg_queue) == 0 && getMessageFromSendBlock(msgblock)->totlen != 0)
         connSetWriteHandlerWithBarrier(link->conn, clusterWriteHandler, 1);
 
     listAddNodeTail(link->send_msg_queue, msgblock);
@@ -3864,7 +3874,7 @@ void clusterSendMessage(clusterLink *link, clusterMsgSendBlock *msgblock) {
     server.stat_cluster_links_memory += sizeof(listNode);
 
     /* Populate sent messages stats. */
-    uint16_t type = ntohs(msgblock->msg.type);
+    uint16_t type = ntohs(getMessageFromSendBlock(msgblock)->type);
     if (type < CLUSTERMSG_TYPE_COUNT) server.cluster->stats_bus_messages_sent[type]++;
 }
 
@@ -4050,7 +4060,7 @@ void clusterSendPing(clusterLink *link, int type) {
      * sizeof(clusterMsg) or more. */
     if (estlen < (int)sizeof(clusterMsg)) estlen = sizeof(clusterMsg);
     clusterMsgSendBlock *msgblock = createClusterMsgSendBlock(type, estlen);
-    clusterMsg *hdr = &msgblock->msg;
+    clusterMsg *hdr = getMessageFromSendBlock(msgblock);
 
     if (!link->inbound && type == CLUSTERMSG_TYPE_PING) link->node->ping_sent = mstime();
 
@@ -4195,10 +4205,10 @@ clusterMsgSendBlock *clusterCreatePublishMsgBlock(robj *channel, robj *message,
     clusterMsgSendBlock *msgblock = createClusterMsgSendBlock(type, msglen);
     clusterMsgDataPublish *hdr_data_msg;
     if (is_light) {
-        clusterMsgLight *hdr_light = &msgblock->msg_light;
+        clusterMsgLight *hdr_light = getLightMessageFromSendBlock(msgblock);
         hdr_data_msg = &hdr_light->data.publish.msg;
     } else {
-        clusterMsg *hdr = &msgblock->msg;
+        clusterMsg *hdr = getMessageFromSendBlock(msgblock);
         hdr_data_msg = &hdr->data.publish.msg;
     }
     hdr_data_msg->channel_len = htonl(channel_len);
@@ -4221,7 +4231,7 @@ void clusterSendFail(char *nodename) {
     uint32_t msglen = sizeof(clusterMsg) - sizeof(union clusterMsgData) + sizeof(clusterMsgDataFail);
     clusterMsgSendBlock *msgblock = createClusterMsgSendBlock(CLUSTERMSG_TYPE_FAIL, msglen);
 
-    clusterMsg *hdr = &msgblock->msg;
+    clusterMsg *hdr = getMessageFromSendBlock(msgblock);
     memcpy(hdr->data.fail.about.nodename, nodename, CLUSTER_NAMELEN);
 
     clusterBroadcastMessage(msgblock);
@@ -4237,7 +4247,7 @@ void clusterSendUpdate(clusterLink *link, clusterNode *node) {
     uint32_t msglen = sizeof(clusterMsg) - sizeof(union clusterMsgData) + sizeof(clusterMsgDataUpdate);
     clusterMsgSendBlock *msgblock = createClusterMsgSendBlock(CLUSTERMSG_TYPE_UPDATE, msglen);
 
-    clusterMsg *hdr = &msgblock->msg;
+    clusterMsg *hdr = getMessageFromSendBlock(msgblock);
     memcpy(hdr->data.update.nodecfg.nodename, node->name, CLUSTER_NAMELEN);
     hdr->data.update.nodecfg.configEpoch = htonu64(node->configEpoch);
     memcpy(hdr->data.update.nodecfg.slots, node->slots, sizeof(node->slots));
@@ -4259,7 +4269,7 @@ void clusterSendModule(clusterLink *link, uint64_t module_id, uint8_t type, cons
     msglen += sizeof(clusterMsgModule) - 3 + len;
     clusterMsgSendBlock *msgblock = createClusterMsgSendBlock(CLUSTERMSG_TYPE_MODULE, msglen);
 
-    clusterMsg *hdr = &msgblock->msg;
+    clusterMsg *hdr = getMessageFromSendBlock(msgblock);
     hdr->data.module.msg.module_id = module_id; /* Already endian adjusted. */
     hdr->data.module.msg.type = type;
     hdr->data.module.msg.len = htonl(len);
@@ -4348,11 +4358,10 @@ void clusterRequestFailoverAuth(void) {
     uint32_t msglen = sizeof(clusterMsg) - sizeof(union clusterMsgData);
     clusterMsgSendBlock *msgblock = createClusterMsgSendBlock(CLUSTERMSG_TYPE_FAILOVER_AUTH_REQUEST, msglen);
 
-    clusterMsg *hdr = &msgblock->msg;
     /* If this is a manual failover, set the CLUSTERMSG_FLAG0_FORCEACK bit
      * in the header to communicate the nodes receiving the message that
      * they should authorized the failover even if the primary is working. */
-    if (server.cluster->mf_end) hdr->mflags[0] |= CLUSTERMSG_FLAG0_FORCEACK;
+    if (server.cluster->mf_end) getMessageFromSendBlock(msgblock)->mflags[0] |= CLUSTERMSG_FLAG0_FORCEACK;
     clusterBroadcastMessage(msgblock);
     clusterMsgSendBlockDecrRefCount(msgblock);
 }