plan: add FIXME comments for topology handling and unused port parameter

Author: nikagra

connectionpool.go

@@ -272,7 +272,7 @@ func (h *hostConnPool) String() string {
 		h.filling, h.closed, size, h.size, h.host)
 }
 
-func newHostConnPool(session *Session, host *HostInfo, port, size int,
+func newHostConnPool(session *Session, host *HostInfo, port, size int, // FIXME: Remove unused port parameter
 	keyspace string) *hostConnPool {
 
 	pool := &hostConnPool{

scylla.go

@@ -395,6 +395,9 @@ outer:
 		idx = p.shardOf(mmt)
 	}
 
+	// FIXME: Add bounds check: shardOf() result may exceed len(p.conns) during topology transitions
+	// FIXME: Prevent connection leaks when shard count decreases
+
 	if c := p.conns[idx]; c != nil {
 		// We have this shard's connection
 		// so let's give it to the caller.
@@ -462,10 +465,16 @@ func (p *scyllaConnPicker) Put(conn *Conn) error {
 		return errors.New("server reported that it has no shards")
 	}
 
-	if nrShards != len(p.conns) {
-		if nrShards != p.nrShards {
-			return fmt.Errorf("server %s reported that number of shard changed from %d to %d", p.address, p.nrShards, nrShards)
+	if nrShards != p.nrShards {
+		if gocqlDebug {
+			p.logger.Printf("scylla: %s shard count changed from %d to %d, rebuilding connection pool",
+				p.address, p.nrShards, nrShards)
 		}
+
+		p.handleShardTopologyChange(conn, nrShards)
+	}
+
+	if nrShards != len(p.conns) {
 		conns := p.conns
 		p.conns = make([]*Conn, nrShards, nrShards)
 		copy(p.conns, conns)
@@ -513,6 +522,9 @@ func (p *scyllaConnPicker) Put(conn *Conn) error {
 	return nil
 }
 
+func (p *scyllaConnPicker) handleShardTopologyChange(newConn *Conn, newShardCount int) {
+}
+
 func (p *scyllaConnPicker) shouldCloseExcessConns() bool {
 	if p.nrConns >= p.nrShards {
 		return true
@@ -626,6 +638,10 @@ func (p *scyllaConnPicker) NextShard() (shardID, nrShards int) {
 	// to consider the next shard after the previously attempted one
 	for i := 1; i <= p.nrShards; i++ {
 		shardID := (p.lastAttemptedShard + i) % p.nrShards
+
+		// FIXME: Replace p.nrShards with len(p.conns) in loop bounds to prevent index out of bounds
+		// FIXME: Handle topology changes where cached p.nrShards != actual len(p.conns)
+
 		if p.conns == nil || p.conns[shardID] == nil {
 			p.lastAttemptedShard = shardID
 			return shardID, p.nrShards