func defaultBaseTransport(ctx context.Context, clientCertSource cert.Source) http.RoundTripper {
	if appengineUrlfetchHook != nil {
		return appengineUrlfetchHook(ctx)
	}
	trans := clonedTransport(http.DefaultTransport)
	if trans == nil {
		trans = fallbackBaseTransport()
	}
	trans.MaxIdleConnsPerHost = 100
	if clientCertSource != nil {
		trans.TLSClientConfig = &tls.Config{
			GetClientCertificate: clientCertSource,
		}
	}
	configureHTTP2(trans)
	return trans
}
defaultBaseTransport returns the base HTTP transport.
On App Engine, this is urlfetch.Transport.
Otherwise, use a default transport, taking most defaults from
http.DefaultTransport.
If TLSCertificate is available, set TLSClientConfig as well.
package http
import (
	"context"
	"crypto/tls"
	"errors"
	"net"
	"net/http"
	"time"
	"go.opencensus.io/plugin/ochttp"
	"golang.org/x/oauth2"
	"google.golang.org/api/googleapi/transport"
	"google.golang.org/api/internal"
	"google.golang.org/api/option"
	"google.golang.org/api/transport/cert"
	"google.golang.org/api/transport/http/internal/propagation"
	"google.golang.org/api/transport/internal/dca"
)
func NewClient(ctx context.Context, opts ...option.ClientOption) (*http.Client, string, error) {
	settings, err := newSettings(opts)
	if err != nil {
		return nil, "", err
	}
	clientCertSource, endpoint, err := dca.GetClientCertificateSourceAndEndpoint(settings)
	if err != nil {
		return nil, "", err
	}
	if settings.HTTPClient != nil {
		return settings.HTTPClient, endpoint, nil
	}
	trans, err := newTransport(ctx, defaultBaseTransport(ctx, clientCertSource), settings)
	if err != nil {
		return nil, "", err
	}
	return &http.Client{Transport: trans}, endpoint, nil
}
func NewTransport(ctx context.Context, base http.RoundTripper, opts ...option.ClientOption) (http.RoundTripper, error) {
	settings, err := newSettings(opts)
	if err != nil {
		return nil, err
	}
	if settings.HTTPClient != nil {
		return nil, errors.New("transport/http: WithHTTPClient passed to NewTransport")
	}
	return newTransport(ctx, base, settings)
}
func newTransport(ctx context.Context, base http.RoundTripper, settings *internal.DialSettings) (http.RoundTripper, error) {
	paramTransport := &parameterTransport{
		base:          base,
		userAgent:     settings.UserAgent,
		quotaProject:  settings.QuotaProject,
		requestReason: settings.RequestReason,
	}
	var trans http.RoundTripper = paramTransport
	trans = addOCTransport(trans, settings)
	switch {
	case settings.NoAuth:
	case settings.APIKey != "":
		trans = &transport.APIKey{
			Transport: trans,
			Key:       settings.APIKey,
		}
	default:
		creds, err := internal.Creds(ctx, settings)
		if err != nil {
			return nil, err
		}
		if paramTransport.quotaProject == "" {
			paramTransport.quotaProject = internal.QuotaProjectFromCreds(creds)
		}
		ts := creds.TokenSource
		if settings.ImpersonationConfig == nil && settings.TokenSource != nil {
			ts = settings.TokenSource
		}
		trans = &oauth2.Transport{
			Base:   trans,
			Source: ts,
		}
	}
	return trans, nil
}
func newSettings(opts []option.ClientOption) (*internal.DialSettings, error) {
	var o internal.DialSettings
	for _, opt := range opts {
		opt.Apply(&o)
	}
	if err := o.Validate(); err != nil {
		return nil, err
	}
	if o.GRPCConn != nil {
		return nil, errors.New("unsupported gRPC connection specified")
	}
	return &o, nil
}
type parameterTransport struct {
	userAgent     string
	quotaProject  string
	requestReason string
	base http.RoundTripper
}
func (t *parameterTransport) RoundTrip(req *http.Request) (*http.Response, error) {
	rt := t.base
	if rt == nil {
		return nil, errors.New("transport: no Transport specified")
	}
	newReq := *req
	newReq.Header = make(http.Header)
	for k, vv := range req.Header {
		newReq.Header[k] = vv
	}
	if t.userAgent != "" {
		newReq.Header.Set("User-Agent", t.userAgent)
	}
	if t.quotaProject != "" {
		newReq.Header.Set("X-Goog-User-Project", t.quotaProject)
	}
	if t.requestReason != "" {
		newReq.Header.Set("X-Goog-Request-Reason", t.requestReason)
	}
	return rt.RoundTrip(&newReq)
}
var appengineUrlfetchHook func(context.Context) http.RoundTripper