Make sure policies are not reused between sessions

Sharing a policy between sessions is not supported because the policy
receives state updates from the session.

Let's update the documentation and add a panic in TokenAwareHostPolicy
constructor. It is better to panic early than to have races and
undefined behavior later.

See also discussion in scylladb#94

cluster.go

@@ -17,6 +17,8 @@ import (
 type PoolConfig struct {
 	// HostSelectionPolicy sets the policy for selecting which host to use for a
 	// given query (default: RoundRobinHostPolicy())
+	// It is not supported to use a single HostSelectionPolicy in multiple sessions
+	// (even if you close the old session before using in a new session).
 	HostSelectionPolicy HostSelectionPolicy
 }
 

policies.go

@@ -262,6 +262,7 @@ type KeyspaceUpdateEvent struct {
 
 // HostSelectionPolicy is an interface for selecting
 // the most appropriate host to execute a given query.
+// HostSelectionPolicy instances cannot be shared between sessions.
 type HostSelectionPolicy interface {
 	HostStateNotifier
 	SetPartitioner
@@ -388,6 +389,13 @@ type tokenAwareHostPolicy struct {
 }
 
 func (t *tokenAwareHostPolicy) Init(s *Session) {
+	t.mu.Lock()
+	defer t.mu.Unlock()
+	if t.getKeyspaceMetadata != nil {
+		// Init was already called.
+		// See https://github.com/scylladb/gocql/issues/94.
+		panic("sharing token aware host selection policy between sessions is not supported")
+	}
 	t.getKeyspaceMetadata = s.KeyspaceMetadata
 	t.getKeyspaceName = func() string { return s.cfg.Keyspace }
 	t.logger = s.logger