Proposed workaround for NSURLCachedResponse leak as described at https://github.com/steipete/SDURLCache/issues/7

NSCachedURLResponse+CueCore.m

#import "NSCachedURLResponse+MemoryLeakFix.h"
#import <objc/runtime.h>

/**
 * In iOS 5 onward, there is a significant memory leak in NSCachedURLResponse.
 * Discussion: https://github.com/steipete/SDURLCache/issues/7#issuecomment-5066300
 */

@implementation NSCachedURLResponse (MemoryLeakFix)

/**
 * We will attempt to reproduce the leak with a single-byte data object.
 *
 * The memory leak won't present itself if NSCachedURLResponse segments the data internally.
 * The safest way to prevent that is to use a single byte object.
 */
static BOOL needsWorkaround()
{
    NSData *data1, *data2;
    @autoreleasepool {
        data1 = [[NSData alloc] initWithBytes:"a" length:1];
        NSURL *url = [NSURL URLWithString:@"http://www.google.com"];
        NSURLResponse *response =
        [[NSURLResponse alloc] initWithURL:url
                                  MIMEType:@"text/html"
                     expectedContentLength:0
                          textEncodingName:@"utf-8"];
        NSCachedURLResponse *cached =
        [[NSCachedURLResponse alloc] initWithResponse:response data:data1];
        [response release];
        data2 = cached.data;
        [cached release];
    }
    if (data1 == data2) {
        if (data1.retainCount == 2) {
            NSLog(@"Memory leak detected, activating workaround");
            [data1 release];
            [data1 release];
            return YES;
        } else {
            NSLog(@"No memory leak detected, not activating workaround");
            [data1 release];
            return NO;
        }
    } else {
        NSLog(@"Inconclusive, not activating memory leak workaround");
        [data1 release];
        return NO;
    }
}

/**
 * We call the original accessor twice. If the returned pointers are equal with increased retain count, we know
 * we've hit the bad code path, and must manually counterbalance each retained call with a release.
 *
 * NOTE: Logically, we should only have to check for pointer equality,
 * but we did see double-release crashes until we added the manual retainCount test.
 */
- (NSData *)cueSwizzledData;
{
    @synchronized(self) {
        NSData *retval = [self cueSwizzledData];
        @synchronized (retval) {
            NSInteger count = [retval retainCount];
            if (retval == [self cueSwizzledData]) {
                if ([retval retainCount] > count) {
                    [retval release];
                    [retval release];
                }
            }
            
        }
        return retval;
    }
}

+ (void)cueInstallDataFixIfNeeded;
{
    if (needsWorkaround()) {
        Method oldMethod = class_getInstanceMethod([NSCachedURLResponse class], @selector(data));
        Method newMethod = class_getInstanceMethod([NSCachedURLResponse class], @selector(cueSwizzledData));
        method_exchangeImplementations(oldMethod, newMethod);        
    }
}

@end