prevent connection write collisions (#52)

* prevent connection write collisions

When multiple calls are made on the same connection, it's possible for
the writes to collide with each other. This adds a write mutex when
communicating with the libvirt daemon.

libvirt.go

@@ -39,6 +39,7 @@ type Libvirt struct {
 	conn net.Conn
 	r    *bufio.Reader
 	w    *bufio.Writer
+	mu   *sync.Mutex
 
 	// method callbacks
 	cm        sync.Mutex
@@ -809,6 +810,7 @@ func New(conn net.Conn) *Libvirt {
 		s:         0,
 		r:         bufio.NewReader(conn),
 		w:         bufio.NewWriter(conn),
+		mu:        &sync.Mutex{},
 		callbacks: make(map[uint32]chan response),
 		events:    make(map[uint32]chan *DomainEvent),
 	}

libvirt_integration_test.go

@@ -19,6 +19,7 @@ package libvirt
 import (
 	"encoding/xml"
 	"net"
+	"sync"
 	"testing"
 	"time"
 
@@ -251,6 +252,49 @@ func TestXMLIntegration(t *testing.T) {
 	}
 }
 
+// verify we're able to concurrently communicate with libvirtd.
+// see: https://github.com/digitalocean/go-libvirt/pull/52
+func Test_concurrentWrite(t *testing.T) {
+	l := New(testConn(t))
+
+	if err := l.Connect(); err != nil {
+		t.Error(err)
+	}
+	defer l.Disconnect()
+
+	count := 10
+	wg := sync.WaitGroup{}
+	wg.Add(count)
+
+	start := make(chan struct{})
+	done := make(chan struct{})
+
+	go func() {
+		wg.Wait()
+		close(done)
+	}()
+
+	for i := 0; i < count; i++ {
+		go func() {
+			defer wg.Done()
+			<-start
+
+			_, err := l.Domains()
+			if err != nil {
+				t.Fatal(err)
+			}
+		}()
+	}
+
+	close(start)
+
+	select {
+	case <-done:
+	case <-time.After(10 * time.Second):
+		t.Fatal("timed out waiting for execution to complete")
+	}
+}
+
 func testConn(t *testing.T) net.Conn {
 	conn, err := net.DialTimeout("tcp", testAddr, time.Second*2)
 	if err != nil {

rpc.go

@@ -338,6 +338,8 @@ func (l *Libvirt) request(proc uint32, program uint32, payload *bytes.Buffer) (<
 	}
 
 	// write header
+	l.mu.Lock()
+	defer l.mu.Unlock()
 	err := binary.Write(l.w, binary.BigEndian, p)
 	if err != nil {
 		return nil, err