Fixes docker driver that would panic when closed. (#3709)

This is happening because it can still received log, but the client might be already closed.

Fixes #3705

Signed-off-by: Cyril Tovena <cyril.tovena@gmail.com>

clients/cmd/docker-driver/loki.go

@@ -2,10 +2,12 @@ package main
 
 import (
 	"bytes"
+	"sync"
 
 	"github.com/docker/docker/daemon/logger"
 	"github.com/go-kit/kit/log"
 	"github.com/go-kit/kit/log/level"
+	"github.com/pkg/errors"
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/common/model"
 
@@ -24,6 +26,9 @@ type loki struct {
 	labels  model.LabelSet
 	logger  log.Logger
 
+	closed bool
+	mutex  sync.RWMutex
+
 	stop func()
 }
 
@@ -59,6 +64,13 @@ func New(logCtx logger.Info, logger log.Logger) (logger.Logger, error) {
 
 // Log implements `logger.Logger`
 func (l *loki) Log(m *logger.Message) error {
+	l.mutex.RLock()
+	defer l.mutex.RUnlock()
+
+	if l.closed {
+		return errors.New("client closed")
+	}
+
 	if len(bytes.Fields(m.Line)) == 0 {
 		level.Debug(l.logger).Log("msg", "ignoring empty line", "line", string(m.Line))
 		return nil
@@ -84,7 +96,10 @@ func (l *loki) Name() string {
 
 // Log implements `logger.Logger`
 func (l *loki) Close() error {
+	l.mutex.Lock()
+	defer l.mutex.Unlock()
 	l.stop()
 	l.client.StopNow()
+	l.closed = true
 	return nil
 }

clients/cmd/docker-driver/loki_test.go

@@ -0,0 +1,25 @@
+package main
+
+import (
+	"testing"
+	"time"
+
+	util_log "github.com/cortexproject/cortex/pkg/util/log"
+	"github.com/docker/docker/daemon/logger"
+	"github.com/stretchr/testify/require"
+)
+
+func Test_loki_LogWhenClosed(t *testing.T) {
+	l, err := New(logger.Info{
+		Config: map[string]string{
+			"loki-url": "http://localhost:3000",
+		},
+	}, util_log.Logger)
+	require.Nil(t, err)
+	msg := logger.NewMessage()
+	msg.Line = []byte(`foo`)
+	msg.Timestamp = time.Now()
+	require.Nil(t, l.Log(msg))
+	require.Nil(t, l.Close())
+	require.NotNil(t, l.Log(msg))
+}