Make driver print warnings returned by server

cluster.go

@@ -109,6 +109,8 @@ type ClusterConfig struct {
 	// Default: nil
 	Authenticator Authenticator
 
+	WarningsHandler WarningHandlerBuilder
+
 	// An Authenticator factory. Can be used to create alternative authenticators.
 	// Default: nil
 	AuthProvider func(h *HostInfo) (Authenticator, error)
@@ -322,6 +324,7 @@ func NewCluster(hosts ...string) *ClusterConfig {
 		WriteCoalesceWaitTime:        200 * time.Microsecond,
 		MetadataSchemaRequestTimeout: 60 * time.Second,
 		DisableSkipMetadata:          true,
+		WarningsHandler:              DefaultWarningHandler{},
 	}
 
 	return cfg

conn.go

@@ -66,6 +66,14 @@ type Authenticator interface {
 	Success(data []byte) error
 }
 
+type WarningHandlerBuilder interface {
+	New(session *Session) WarningHandler
+}
+
+type WarningHandler interface {
+	Warnings(qry ExecutableQueryInterface, host *HostInfo, warnings []string)
+}
+
 type PasswordAuthenticator struct {
 	Username              string
 	Password              string
@@ -1404,7 +1412,16 @@ func marshalQueryValue(typ TypeInfo, value interface{}, dst *queryValues) error
 	return nil
 }
 
-func (c *Conn) executeQuery(ctx context.Context, qry *Query) *Iter {
+func (c *Conn) executeQuery(ctx context.Context, qry *Query) (iter *Iter) {
+	defer func() {
+		if iter == nil || c.session == nil {
+			return
+		}
+		warnings := iter.Warnings()
+		if len(warnings) > 0 && c.session.warningHandler != nil {
+			c.session.warningHandler.Warnings(qry, iter.host, warnings)
+		}
+	}()
 	params := queryParams{
 		consistency: qry.cons,
 	}
@@ -1670,7 +1687,17 @@ func (c *Conn) UseKeyspace(keyspace string) error {
 	return nil
 }
 
-func (c *Conn) executeBatch(ctx context.Context, batch *Batch) *Iter {
+func (c *Conn) executeBatch(ctx context.Context, batch *Batch) (iter *Iter) {
+	defer func() {
+		if iter == nil || c.session == nil {
+			return
+		}
+		warnings := iter.Warnings()
+		if len(warnings) > 0 && c.session.warningHandler != nil {
+			c.session.warningHandler.Warnings(batch, iter.host, warnings)
+		}
+	}()
+
 	if c.version == protoVersion1 {
 		return &Iter{err: ErrUnsupported}
 	}

control.go

@@ -66,9 +66,9 @@ func (c *controlConn) getSession() *Session {
 func createControlConn(session *Session) *controlConn {
 
 	control := &controlConn{
-		session:            session,
-		quit:               make(chan struct{}),
-		retry:              &SimpleRetryPolicy{NumRetries: 3},
+		session: session,
+		quit:    make(chan struct{}),
+		retry:   &SimpleRetryPolicy{NumRetries: 3},
 	}
 
 	control.conn.Store((*connHost)(nil))

query_executor.go

@@ -7,27 +7,29 @@ import (
 	"time"
 )
 
-type ExecutableQuery interface {
-	borrowForExecution()    // Used to ensure that the query stays alive for lifetime of a particular execution goroutine.
-	releaseAfterExecution() // Used when a goroutine finishes its execution attempts, either with ok result or an error.
-	execute(ctx context.Context, conn *Conn) *Iter
-	attempt(keyspace string, end, start time.Time, iter *Iter, host *HostInfo)
-	retryPolicy() RetryPolicy
-	speculativeExecutionPolicy() SpeculativeExecutionPolicy
+type ExecutableQueryInterface interface {
 	GetRoutingKey() ([]byte, error)
 	Keyspace() string
 	Table() string
 	IsIdempotent() bool
 	IsLWT() bool
 	GetCustomPartitioner() Partitioner
-
-	withContext(context.Context) ExecutableQuery
-
 	RetryableQuery
-
 	GetSession() *Session
 }
 
+type ExecutableQuery interface {
+	ExecutableQueryInterface
+
+	borrowForExecution()    // Used to ensure that the query stays alive for lifetime of a particular execution goroutine.
+	releaseAfterExecution() // Used when a goroutine finishes its execution attempts, either with ok result or an error.
+	execute(ctx context.Context, conn *Conn) *Iter
+	attempt(keyspace string, end, start time.Time, iter *Iter, host *HostInfo)
+	retryPolicy() RetryPolicy
+	speculativeExecutionPolicy() SpeculativeExecutionPolicy
+	withContext(context.Context) ExecutableQuery
+}
+
 type queryExecutor struct {
 	pool   *policyConnPool
 	policy HostSelectionPolicy

session.go

@@ -87,6 +87,7 @@ type Session struct {
 	tabletsRoutingV1 bool
 
 	usingTimeoutClause string
+	warningHandler     WarningHandler
 }
 
 var queryPool = &sync.Pool{
@@ -182,7 +183,9 @@ func newSessionCommon(cfg ClusterConfig) (*Session, error) {
 		return nil, fmt.Errorf("gocql: unable to create session: %v", err)
 	}
 	s.connCfg = connCfg
-
+	if cfg.WarningsHandler != nil {
+		s.warningHandler = cfg.WarningsHandler.New(s)
+	}
 	return s, nil
 }
 

warning_handler.go

@@ -0,0 +1,23 @@
+package gocql
+
+type DefaultWarningHandler struct {
+	logger StdLogger
+}
+
+var NoopWarningHandler WarningHandler = nil
+
+func (d DefaultWarningHandler) New(session *Session) WarningHandler {
+	return DefaultWarningHandler{
+		logger: session.logger,
+	}
+}
+
+var _ WarningHandler = DefaultWarningHandler{}
+
+func (d DefaultWarningHandler) Warnings(qry ExecutableQueryInterface, host *HostInfo, warnings []string) {
+	if host != nil && len(host.hostId) > 0 {
+		d.logger.Printf("[%s] warnings: %v", host.hostId, warnings)
+	} else {
+		d.logger.Printf("Cluster warnings: %v", warnings)
+	}
+}