How to avoid reference count underflow in _NSCFURLProtocolBridge in custom NSURLProtocol in GC environment

Question

The basics are I have a custom NSURLProtocol. In startLoading, [self client] is of type:

<_NSCFURLProtocolBridge> {NSURLProtocol, CFURLProtocol}

The problem is running this in a garbage-collected environment. Because I'm writing a screensaver, I'm forced to make it garbage-collected. However, the _NSCFURLProtocolBridge protocol seems to always throw:

malloc: reference count underflow for (memory_id_here), break on auto_refcount_underflow_error to debug

An example dump to the debug console is:

ScreenSaverEngine[1678:6807] client is <_NSCFURLProtocolBridge 0x20025ab00> {NSURLProtocol 0x200258ec0, CFURLProtocol 0x20029c400} ScreenSaverEngine(1678,0x102eda000) malloc: reference count underflow for 0x20025ab00, break on auto_refcount_underflow_error to debug.

You can see that the underflow occurs for <_NSCFURLProtocolBridge 0x20025ab00>.

When I break on auto_refcount_underflow_error, it seems to stack-trace back up to URLProtocolDidFinishLoading: in:

id client = [self client];
...
[client URLProtocolDidFinishLoading:self];

This problem seems to have existed for a while, but there seems to be no answer at all online:

http://lists.apple.com/archives/cocoa-dev/2008/May/msg01272.html http://www.cocoabuilder.com/archive/message/cocoa/2007/12/17/195056

The bug only shows itself in garbage-collected environments for these listed bugs as well. Any thoughts on how I can work around this without causing memory issues? I'm assuming this probably has something to do with the CF type underneath NSURLProtocol being released improperly?

Answer 1

Last WWDC we confirmed this bug with a webkit engineer, he could see the bug right there in the code so hopefully they'll fix it. The workaround is to CFRetain the client in the initWithRequest method.

- (id)initWithRequest:(NSURLRequest *)request cachedResponse:(NSCachedURLResponse *)cachedResponse client:(id <NSURLProtocolClient>)client
{
    // work around for NSURLProtocol bug
    // note that this leaks!
    CFRetain(client);

    if (self = [super initWithRequest:request cachedResponse:cachedResponse client:client])
    {
    }

    return self;
}