google: refactor JWT parsing code internally

The ADC code and the JWT-parsing function operate on the same data
format, but were using separate code paths, each of which was missing
things from the other.

While this presents no change in API surface, JWTConfigFromJSON now
strictly checks the "type" field in the JSON file before building a
config.

Change-Id: I2f593a16bf4591059fbf9002bccea06e41e5e161
Reviewed-on: https://go-review.googlesource.com/32678
Reviewed-by: Jaana Burcu Dogan <jbd@google.com>

google/default.go

@@ -6,7 +6,6 @@ package google
 
 import (
 	"encoding/json"
-	"errors"
 	"fmt"
 	"io/ioutil"
 	"net/http"
@@ -17,7 +16,6 @@ import (
 	"cloud.google.com/go/compute/metadata"
 	"golang.org/x/net/context"
 	"golang.org/x/oauth2"
-	"golang.org/x/oauth2/jwt"
 )
 
 // DefaultClient returns an HTTP Client that uses the
@@ -112,44 +110,9 @@ func tokenSourceFromFile(ctx context.Context, filename string, scopes []string)
 	if err != nil {
 		return nil, err
 	}
-	var d struct {
-		// Common fields
-		Type     string
-		ClientID string `json:"client_id"`
-
-		// User Credential fields
-		ClientSecret string `json:"client_secret"`
-		RefreshToken string `json:"refresh_token"`
-
-		// Service Account fields
-		ClientEmail  string `json:"client_email"`
-		PrivateKeyID string `json:"private_key_id"`
-		PrivateKey   string `json:"private_key"`
-	}
-	if err := json.Unmarshal(b, &d); err != nil {
+	var f credentialsFile
+	if err := json.Unmarshal(b, &f); err != nil {
 		return nil, err
 	}
-	switch d.Type {
-	case "authorized_user":
-		cfg := &oauth2.Config{
-			ClientID:     d.ClientID,
-			ClientSecret: d.ClientSecret,
-			Scopes:       append([]string{}, scopes...), // copy
-			Endpoint:     Endpoint,
-		}
-		tok := &oauth2.Token{RefreshToken: d.RefreshToken}
-		return cfg.TokenSource(ctx, tok), nil
-	case "service_account":
-		cfg := &jwt.Config{
-			Email:      d.ClientEmail,
-			PrivateKey: []byte(d.PrivateKey),
-			Scopes:     append([]string{}, scopes...), // copy
-			TokenURL:   JWTTokenURL,
-		}
-		return cfg.TokenSource(ctx), nil
-	case "":
-		return nil, errors.New("missing 'type' field in credentials")
-	default:
-		return nil, fmt.Errorf("unknown credential type: %q", d.Type)
-	}
+	return f.tokenSource(ctx, scopes)
 }