fix concurrency issue

client.go

@@ -7,6 +7,7 @@ import (
 	"net"
 	"regexp"
 	"strings"
+	"sync"
 	"time"
 )
 
@@ -57,6 +58,7 @@ type Client struct {
 	notify        chan Notification
 	disconnect    chan struct{}
 	res           []string
+	mutex         sync.Mutex
 
 	Server *ServerMethods
 }
@@ -234,7 +236,15 @@ func (c *Client) ExecCmd(cmd *Cmd) ([]string, error) {
 		return nil, ErrNotConnected
 	}
 
-	c.work <- cmd.String()
+	c.mutex.Lock()
+	defer c.mutex.Unlock()
+
+	select {
+	case c.work <- cmd.String():
+		// continue
+	case <-time.After(c.timeout):
+		return nil, ErrTimeout
+	}
 
 	select {
 	case err := <-c.err:

client_test.go

@@ -230,3 +230,41 @@ func TestClientBadHeader(t *testing.T) {
 	// Should never get here
 	assert.NoError(t, c.Close())
 }
+
+func TestConcurrency(t *testing.T) {
+	s := newServer(t)
+	if s == nil {
+		return
+	}
+	defer func() {
+		assert.NoError(t, s.Close())
+	}()
+
+	c, err := NewClient(s.Addr, Timeout(time.Millisecond*100))
+	if !assert.NoError(t, err) {
+		return
+	}
+
+	const iterations = 10
+	errors := make(chan error)
+
+	go func() {
+		defer close(errors)
+
+		for i := 0; i <= iterations; i++ {
+			if _, err2 := c.Server.GroupList(); err2 != nil {
+				errors <- err2
+			}
+		}
+	}()
+
+	for i := 0; i <= iterations; i++ {
+		_, err = c.Server.GroupList()
+		assert.NoError(t, err)
+	}
+
+	// receive errors from go-routine and wait for completion
+	for err := range errors {
+		assert.NoError(t, err)
+	}
+}