upstream: remove sync.Once

upstream/upstream_doh.go

@@ -20,15 +20,16 @@ const DoHMaxConnsPerHost = 1
 type dnsOverHTTPS struct {
 	boot *bootstrapper
 
+	// mu exists for lazy initialization purposes and protects client from
+	// data race during lazy initialization.  It provides the exchange with
+	// invalid upstream possibility, which is needed for now. Should be
+	// refactored further.
+	mu sync.Mutex
+
 	// The Client's Transport typically has internal state (cached TCP
 	// connections), so Clients should be reused instead of created as
 	// needed. Clients are safe for concurrent use by multiple goroutines.
 	client *http.Client
-
-	// For lazy initialization purposes.
-	sync.Once
-	// For lazy initialization purposes.
-	sync.Mutex
 }
 
 func (p *dnsOverHTTPS) Address() string { return p.boot.address }
@@ -94,8 +95,8 @@ func (p *dnsOverHTTPS) exchangeHTTPSClient(m *dns.Msg, client *http.Client) (*dn
 func (p *dnsOverHTTPS) getClient() (c *http.Client, err error) {
 	startTime := time.Now()
 
-	p.Lock()
-	defer p.Unlock()
+	p.mu.Lock()
+	defer p.mu.Unlock()
 	if p.client != nil {
 		return p.client, nil
 	}
@@ -107,12 +108,9 @@ func (p *dnsOverHTTPS) getClient() (c *http.Client, err error) {
 		return nil, fmt.Errorf("timeout exceeded: %d ms", int(elapsed/time.Millisecond))
 	}
 
-	// TODO(e.burkov): get rid of lazy initialization.
-	p.Do(func() {
-		c, err = p.createClient()
-	})
+	p.client, err = p.createClient()
 
-	return c, err
+	return p.client, err
 }
 
 func (p *dnsOverHTTPS) createClient() (*http.Client, error) {