api: disable http.Transport.ForceAttemptHTTP2 by default

Revert soap.Client http.Transport back to manual construction, rather than Clone() (see 313aa85)

Disable ForceAttemptHTTP2 by default, as we currently see degraded transfer rates with large file uploads.

Closes #3564

vim25/soap/client.go

@@ -135,20 +135,31 @@ func ParseURL(s string) (*url.URL, error) {
 	return u, nil
 }
 
+// Go's ForceAttemptHTTP2 default is true, we disable by default.
+// This undocumented env var can be used to enable.
+var http2 = os.Getenv("GOVMOMI_HTTP2") == "true"
+
 func NewClient(u *url.URL, insecure bool) *Client {
 	var t *http.Transport
 
 	if d, ok := http.DefaultTransport.(*http.Transport); ok {
-		t = d.Clone()
+		// Inherit the same defaults explicitly set in http.DefaultTransport,
+		// unless otherwise noted.
+		t = &http.Transport{
+			Proxy:                 d.Proxy,
+			DialContext:           d.DialContext,
+			ForceAttemptHTTP2:     http2, // false by default in govmomi
+			MaxIdleConns:          d.MaxIdleConns,
+			IdleConnTimeout:       d.IdleConnTimeout,
+			TLSHandshakeTimeout:   d.TLSHandshakeTimeout,
+			ExpectContinueTimeout: d.ExpectContinueTimeout,
+		}
 	} else {
 		t = new(http.Transport)
 	}
 
-	if insecure {
-		if t.TLSClientConfig == nil {
-			t.TLSClientConfig = new(tls.Config)
-		}
-		t.TLSClientConfig.InsecureSkipVerify = insecure
+	t.TLSClientConfig = &tls.Config{
+		InsecureSkipVerify: insecure,
 	}
 
 	c := newClientWithTransport(u, insecure, t)