connection creation race condition

doq/client.go

@@ -15,10 +15,11 @@ import (
 // Client encapsulates and provides logic for querying DNS servers over QUIC.
 // The client should be thread-safe. The client reuses single QUIC connection to the server, while creating multiple parallel QUIC streams.
 type Client struct {
-	lock           sync.Mutex
+	connLock sync.RWMutex
+	conn     quic.Connection
+
 	addr           string
 	tlsconfig      *tls.Config
-	conn           quic.Connection
 	writeTimeout   time.Duration
 	readTimeout    time.Duration
 	connectTimeout time.Duration
@@ -55,8 +56,8 @@ func NewClient(addr string, options Options) *Client {
 }
 
 func (c *Client) dial(ctx context.Context) error {
-	c.lock.Lock()
-	defer c.lock.Unlock()
+	c.connLock.Lock()
+	defer c.connLock.Unlock()
 	if c.conn != nil {
 		c.conn.ConnectionState()
 		if err := c.conn.Context().Err(); err == nil {
@@ -100,17 +101,8 @@ func (c *Client) dial(ctx context.Context) error {
 
 // Send sends DNS request using DNS over QUIC.
 func (c *Client) Send(ctx context.Context, msg *dns.Msg) (*dns.Msg, error) {
-	if c.conn == nil {
-		// connection not yet created, create one
-		if err := c.dial(ctx); err != nil {
-			return nil, err
-		}
-	}
-	if err := c.conn.Context().Err(); err != nil {
-		// connection is not healthy, try to dial a new one
-		if err := c.dial(ctx); err != nil {
-			return nil, err
-		}
+	if err := c.dialIfNeeded(ctx); err != nil {
+		return nil, err
 	}
 
 	stream, err := c.conn.OpenStreamSync(ctx)
@@ -137,6 +129,31 @@ func (c *Client) Send(ctx context.Context, msg *dns.Msg) (*dns.Msg, error) {
 	return readMsg(readCtx, stream)
 }
 
+func (c *Client) dialIfNeeded(ctx context.Context) error {
+	c.connLock.RLock()
+	connNotCreated := c.conn == nil
+	c.connLock.RUnlock()
+
+	if connNotCreated {
+		// connection not yet created, create one
+		if err := c.dial(ctx); err != nil {
+			return err
+		}
+	}
+
+	c.connLock.RLock()
+	connFailed := c.conn.Context().Err() != nil
+	c.connLock.RUnlock()
+
+	if connFailed {
+		// connection is not healthy, try to dial a new one
+		if err := c.dial(ctx); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
 func writeMsg(ctx context.Context, stream quic.Stream, msg *dns.Msg) error {
 	pack, err := msg.Pack()
 	if err != nil {