Add support for signUrl when default credentials are used

- Add a method to convert ApplicationDefaultCredentials to ServiceAccountAuthCredentials
- Add type check and conversion to Storage.signUrl

Author: mziccard

gcloud-java-core/src/main/java/com/google/gcloud/AuthCredentials.java

@@ -28,6 +28,7 @@
 import com.google.api.client.json.jackson.JacksonFactory;
 import com.google.auth.http.HttpCredentialsAdapter;
 import com.google.auth.oauth2.GoogleCredentials;
+import com.google.auth.oauth2.ServiceAccountCredentials;
 
 import java.io.IOException;
 import java.io.InputStream;
@@ -212,7 +213,7 @@ public RestorableState<AuthCredentials> capture() {
     }
   }
 
-  private static class ApplicationDefaultAuthCredentials extends AuthCredentials {
+  public static class ApplicationDefaultAuthCredentials extends AuthCredentials {
 
     private GoogleCredentials googleCredentials;
 
@@ -255,6 +256,15 @@ protected HttpRequestInitializer httpRequestInitializer(HttpTransport transport,
       return new HttpCredentialsAdapter(googleCredentials.createScoped(scopes));
     }
 
+    public ServiceAccountAuthCredentials toServiceAccountCredentials() {
+      if (googleCredentials instanceof ServiceAccountCredentials) {
+        ServiceAccountCredentials credentials = (ServiceAccountCredentials) googleCredentials;
+        return new ServiceAccountAuthCredentials(credentials.getClientEmail(),
+            credentials.getPrivateKey());
+      }
+      return null;
+    }
+
     @Override
     public RestorableState<AuthCredentials> capture() {
       return STATE;

gcloud-java-storage/src/main/java/com/google/gcloud/storage/StorageImpl.java

@@ -43,6 +43,8 @@
 import com.google.common.hash.Hashing;
 import com.google.common.io.BaseEncoding;
 import com.google.common.primitives.Ints;
+import com.google.gcloud.AuthCredentials;
+import com.google.gcloud.AuthCredentials.ApplicationDefaultAuthCredentials;
 import com.google.gcloud.AuthCredentials.ServiceAccountAuthCredentials;
 import com.google.gcloud.PageImpl;
 import com.google.gcloud.BaseService;
@@ -584,9 +586,15 @@ public URL signUrl(BlobInfo blobInfo, long duration, TimeUnit unit, SignUrlOptio
     ServiceAccountAuthCredentials cred =
         (ServiceAccountAuthCredentials) optionMap.get(SignUrlOption.Option.SERVICE_ACCOUNT_CRED);
     if (cred == null) {
-      checkArgument(options().authCredentials() instanceof ServiceAccountAuthCredentials,
-          "Signing key was not provided and could not be derived");
-      cred = (ServiceAccountAuthCredentials) this.options().authCredentials();
+      AuthCredentials serviceCred = this.options().authCredentials();
+      if (serviceCred instanceof ServiceAccountAuthCredentials) {
+        cred = (ServiceAccountAuthCredentials) serviceCred;
+      } else {
+        if (serviceCred instanceof ApplicationDefaultAuthCredentials) {
+          cred = ((ApplicationDefaultAuthCredentials) serviceCred).toServiceAccountCredentials();
+        }
+      }
+      checkArgument(cred != null, "Signing key was not provided and could not be derived");
     }
     // construct signature - see https://cloud.google.com/storage/docs/access-control#Signed-URLs
     StringBuilder stBuilder = new StringBuilder();