Adding token fetching implementing for Compute Engine.

google/google.go

@@ -63,6 +63,11 @@
 package google
 
 import (
+	"encoding/json"
+	"net/http"
+	"path"
+	"time"
+
 	"github.com/rakyll/oauth2"
 )
 
@@ -72,9 +77,17 @@ const (
 	uriGoogleToken = "https://accounts.google.com/o/oauth2/token"
 )
 
+type metaTokenRespBody struct {
+	AccessToken string        `json:"access_token"`
+	ExpiresIn   time.Duration `json:"expires_in"`
+	TokenType   string        `json:"token_type"`
+}
+
 // ComputeEngineConfig represents a OAuth 2.0 consumer client
 // running on Google Compute Engine.
-type ComputeEngineConfig struct{}
+type ComputeEngineConfig struct {
+	account string
+}
 
 // NewConfig creates a new OAuth2 config that uses Google
 // endpoints.
@@ -89,10 +102,10 @@ func NewServiceAccountConfig(opts *oauth2.JWTOptions) (*oauth2.JWTConfig, error)
 }
 
 // NewComputeEngineConfig creates a new config that can fetch tokens
-// from Google Compute Engine instance's metaserver.
-func NewComputeEngineConfig() (*ComputeEngineConfig, error) {
-	// Should fetch an access token from the meta server.
-	return &ComputeEngineConfig{}, nil
+// from Google Compute Engine instance's metaserver. If no account is
+// provided, default is used.
+func NewComputeEngineConfig(account string) (*ComputeEngineConfig, error) {
+	return &ComputeEngineConfig{account: account}, nil
 }
 
 // NewTransport creates an authorized transport.
@@ -101,6 +114,31 @@ func (c *ComputeEngineConfig) NewTransport() (oauth2.Transport, error) {
 }
 
 // FetchToken retrieves a new access token via metadata server.
-func (c *ComputeEngineConfig) FetchToken(existing *oauth2.Token) (*oauth2.Token, error) {
-	panic("not yet implemented")
+func (c *ComputeEngineConfig) FetchToken(existing *oauth2.Token) (token *oauth2.Token, err error) {
+	account := "default"
+	if c.account != "" {
+		account = c.account
+	}
+	u := "http://" + path.Join("metadata/computeMetadata/v1/instance/service-accounts", account, "token")
+	req, err := http.NewRequest("GET", u, nil)
+	if err != nil {
+		return
+	}
+	req.Header.Add("X-Google-Metadata-Request", "True")
+	resp, err := (&http.Client{Transport: oauth2.DefaultTransport}).Do(req)
+	if err != nil {
+		return
+	}
+	defer resp.Body.Close()
+	var tokenResp metaTokenRespBody
+	err = json.NewDecoder(resp.Body).Decode(&tokenResp)
+	if err != nil {
+		return
+	}
+	token = &oauth2.Token{
+		AccessToken: tokenResp.AccessToken,
+		TokenType:   tokenResp.TokenType,
+		Expiry:      time.Now().Add(tokenResp.ExpiresIn * time.Second),
+	}
+	return
 }