Do not assume that http.DefaultClient and http.DefaultTransport is always available.

google/appengine.go

@@ -12,31 +12,32 @@ import (
 // AppEngineConfig represents a configuration for an
 // App Engine application's Google service account.
 type AppEngineConfig struct {
-	// Default transport to be used while constructing
+	// Transport represents the default transport to be used while constructing
 	// oauth2.Transport instances from this configuration.
 	Transport *urlfetch.Transport
-	context   appengine.Context
-	scopes    []string
+
+	context appengine.Context
+	scopes  []string
 }
 
 // NewAppEngineConfig creates a new AppEngineConfig for the
 // provided auth scopes.
 func NewAppEngineConfig(context appengine.Context, scopes []string) *AppEngineConfig {
-	return &AppEngineConfig{context: context, scopes: scopes}
+	return &AppEngineConfig{
+		Transport: &urlfetch.Transport{
+			Context:                       context,
+			Deadline:                      0,
+			AllowInvalidServerCertificate: false,
+		},
+		context: context,
+		scopes:  scopes,
+	}
 }
 
 // NewTransport returns a transport that authorizes
 // the requests with the application's service account.
 func (c *AppEngineConfig) NewTransport() *oauth2.Transport {
-	if c.Transport != nil {
-		return oauth2.NewTransport(c.Transport, c, nil)
-	}
-	transport := &urlfetch.Transport{
-		Context:                       c.context,
-		Deadline:                      0,
-		AllowInvalidServerCertificate: false,
-	}
-	return oauth2.NewTransport(transport, c, nil)
+	return oauth2.NewTransport(c.Transport, c, nil)
 }
 
 // FetchToken fetches a new access token for the provided scopes.

google/appenginevm.go

@@ -3,29 +3,40 @@
 package google
 
 import (
-	"net/http"
-
 	"github.com/golang/oauth2"
 	"google.golang.org/appengine"
+	"google.golang.org/appengine/urlfetch"
 )
 
 // AppEngineConfig represents a configuration for an
 // App Engine application's Google service account.
 type AppEngineConfig struct {
-	context appengine.Context
-	scopes  []string
+	// Transport represents the default transport to be used while constructing
+	// oauth2.Transport instances from this configuration.
+	Transport *urlfetch.Transport
+	context   appengine.Context
+
+	scopes []string
 }
 
 // NewAppEngineConfig creates a new AppEngineConfig for the
 // provided auth scopes.
 func NewAppEngineConfig(context appengine.Context, scopes []string) *AppEngineConfig {
-	return &AppEngineConfig{context: context, scopes: scopes}
+	return &AppEngineConfig{
+		Transport: &urlfetch.Transport{
+			Context:                       context,
+			Deadline:                      0,
+			AllowInvalidServerCertificate: false,
+		},
+		context: context,
+		scopes:  scopes,
+	}
 }
 
 // NewTransport returns a transport that authorizes
 // the requests with the application's service account.
 func (c *AppEngineConfig) NewTransport() *oauth2.Transport {
-	return oauth2.NewTransport(http.DefaultTransport, c, nil)
+	return oauth2.NewTransport(c.Transport, c, nil)
 }
 
 // FetchToken fetches a new access token for the provided scopes.

google/google.go

@@ -37,6 +37,15 @@ type metaTokenRespBody struct {
 // ComputeEngineConfig represents a OAuth 2.0 consumer client
 // running on Google Compute Engine.
 type ComputeEngineConfig struct {
+	// Client is the default HTTP client to be used while retrieving
+	// tokens from the OAuth 2.0 provider.
+	Client *http.Client
+
+	// Transport represents the default round tripper to be used
+	// while constructing new oauth2.Transport instances from
+	// this configuration.
+	Transport http.RoundTripper
+
 	account string
 }
 
@@ -56,12 +65,16 @@ func NewServiceAccountConfig(opts *oauth2.JWTOptions) (*oauth2.JWTConfig, error)
 // from Google Compute Engine instance's metaserver. If no account is
 // provided, default is used.
 func NewComputeEngineConfig(account string) *ComputeEngineConfig {
-	return &ComputeEngineConfig{account: account}
+	return &ComputeEngineConfig{
+		Client:    http.DefaultClient,
+		Transport: http.DefaultTransport,
+		account:   account,
+	}
 }
 
 // NewTransport creates an authorized transport.
 func (c *ComputeEngineConfig) NewTransport() *oauth2.Transport {
-	return oauth2.NewTransport(http.DefaultTransport, c, nil)
+	return oauth2.NewTransport(c.Transport, c, nil)
 }
 
 // FetchToken retrieves a new access token via metadata server.
@@ -76,7 +89,7 @@ func (c *ComputeEngineConfig) FetchToken(existing *oauth2.Token) (token *oauth2.
 		return
 	}
 	req.Header.Add("X-Google-Metadata-Request", "True")
-	resp, err := http.DefaultClient.Do(req)
+	resp, err := c.Client.Do(req)
 	if err != nil {
 		return
 	}