Login to deployed App fails with java.io.FileNotFoundException (due to mishandled HTTP 500 error)

[GoogleCodeExporter]

What steps will reproduce the problem?
1. Deploy app to App Engine.
2. Attempt to login using following code:
    LoginUtils.setLoginUrl("https://<my app>.appspot.com");
    LoginUtils.loginAppEngine( this, cookie_manager_avail_listener, chosen_account ) ;

Notes:
1. I have used this URL with a token generated with code similar to the 
implementation at 
http://blog.notdot.net/2010/05/Authenticating-against-App-Engine-from-an-Android
-app, using chosen_account - seems to work.
2. I've replaced my app's name with <my app> just in this issue entry.
3. "this" above is my Activity subclass.
4. Deployed App Engine server works fine (no HTTP 500s) when used with the GWT 
generated web client.

What is the expected output? What do you see instead?
Expected a successful login, got a thrown exception:

E/AndroidRuntime(13546): java.lang.RuntimeException: Failure in attempt to get 
cookie: 
E/AndroidRuntime(13546):    at 
com.gdevelop.gwt.syncrpc.android.GetCookieRunnable.run(GetCookieRunnable.java:12
1)
E/AndroidRuntime(13546):    at java.lang.Thread.run(Thread.java:841)
E/AndroidRuntime(13546): Caused by: java.io.FileNotFoundException: https://<my 
app>.appspot.com/_ah/login
E/AndroidRuntime(13546):    at 
com.android.okhttp.internal.http.HttpURLConnectionImpl.getInputStream(HttpURLCon
nectionImpl.java:186)
E/AndroidRuntime(13546):    at 
com.android.okhttp.internal.http.HttpsURLConnectionImpl.getInputStream(HttpsURLC
onnectionImpl.java:246)
E/AndroidRuntime(13546):    at 
com.gdevelop.gwt.syncrpc.Utils.getResposeText(Utils.java:98)
E/AndroidRuntime(13546):    at 
com.gdevelop.gwt.syncrpc.android.GetCookieRunnable.run(GetCookieRunnable.java:98
)
E/AndroidRuntime(13546):    ... 1 more


What version of the product are you using? On what operating system?
SyncProxyAndroid-0.4.2.zip - GWT 2.6.0 - Released 2014-03-11

I am using the Google plugin in Eclipse on Ubuntu Linux.


Please provide any additional information below.

I have traced the error to here: 
https://code.google.com/p/gwt-syncproxy/source/browse/trunk/SPALibrary/src/com/g
develop/gwt/syncrpc/android/GetCookieRunnable.java#98

From what I can tell the server is returning HTTP 500, which means that the 
server crashed out, probably without generating output.  Since there is no 
input to open, we get an exception.

Original issue reported on code.google.com by rmar...@gmail.com on 1 Dec 2014 at 9:40