logpolicy: split out DialContext into a func

Updates tailscale/corp#10030

Signed-off-by: Maisem Ali <maisem@tailscale.com>

ipn/ipnserver/proxyconnect.go

@@ -37,8 +37,7 @@ func (s *Server) handleProxyConnectConn(w http.ResponseWriter, r *http.Request)
 		return
 	}
 
-	tr := logpolicy.NewLogtailTransport(logHost)
-	back, err := tr.DialContext(ctx, "tcp", hostPort)
+	back, err := logpolicy.DialContext(ctx, "tcp", hostPort)
 	if err != nil {
 		s.logf("error CONNECT dialing %v: %v", hostPort, err)
 		http.Error(w, "Connect failure", http.StatusBadGateway)

logpolicy/logpolicy.go

@@ -667,25 +667,12 @@ func (p *Policy) Shutdown(ctx context.Context) error {
 	return nil
 }
 
-// NewLogtailTransport returns an HTTP Transport particularly suited to uploading
-// logs to the given host name. This includes:
-//   - If DNS lookup fails, consult the bootstrap DNS list of Tailscale hostnames.
+// DialContext is a net.Dialer.DialContext specialized for use by logtail.
+// It does the following:
+//   - If DNS lookup fails, consults the bootstrap DNS list of Tailscale hostnames.
 //   - If TLS connection fails, try again using LetsEncrypt's built-in root certificate,
 //     for the benefit of older OS platforms which might not include it.
-func NewLogtailTransport(host string) *http.Transport {
-	// Start with a copy of http.DefaultTransport and tweak it a bit.
-	tr := http.DefaultTransport.(*http.Transport).Clone()
-
-	tr.Proxy = tshttpproxy.ProxyFromEnvironment
-	tshttpproxy.SetTransportGetProxyConnectHeader(tr)
-
-	// We do our own zstd compression on uploads, and responses never contain any payload,
-	// so don't send "Accept-Encoding: gzip" to save a few bytes on the wire, since there
-	// will never be any body to decompress:
-	tr.DisableCompression = true
-
-	// Log whenever we dial:
-	tr.DialContext = func(ctx context.Context, netw, addr string) (net.Conn, error) {
+func DialContext(ctx context.Context, netw, addr string) (net.Conn, error) {
 	nd := netns.FromDialer(log.Printf, &net.Dialer{
 		Timeout:   30 * time.Second,
 		KeepAlive: netknob.PlatformTCPKeepAlive(),
@@ -731,6 +718,23 @@ func NewLogtailTransport(host string) *http.Transport {
 	return c, err
 }
 
+// NewLogtailTransport returns an HTTP Transport particularly suited to uploading
+// logs to the given host name. See DialContext for details on how it works.
+func NewLogtailTransport(host string) *http.Transport {
+	// Start with a copy of http.DefaultTransport and tweak it a bit.
+	tr := http.DefaultTransport.(*http.Transport).Clone()
+
+	tr.Proxy = tshttpproxy.ProxyFromEnvironment
+	tshttpproxy.SetTransportGetProxyConnectHeader(tr)
+
+	// We do our own zstd compression on uploads, and responses never contain any payload,
+	// so don't send "Accept-Encoding: gzip" to save a few bytes on the wire, since there
+	// will never be any body to decompress:
+	tr.DisableCompression = true
+
+	// Log whenever we dial:
+	tr.DialContext = DialContext
+
 	// We're contacting exactly 1 hostname, so the default's 100
 	// max idle conns is very high for our needs. Even 2 is
 	// probably double what we need:
@@ -762,7 +766,7 @@ func goVersion() string {
 type noopPretendSuccessTransport struct{}
 
 func (noopPretendSuccessTransport) RoundTrip(req *http.Request) (*http.Response, error) {
-	io.ReadAll(req.Body)
+	io.Copy(io.Discard, req.Body)
 	req.Body.Close()
 	return &http.Response{
 		StatusCode: 200,