Objective-C retry helper with capped exponential backoff

#import <Foundation/Foundation.h>
#import <math.h>
#import <stdlib.h>

static void RetryWithBackoff(NSInteger maxRetries, BOOL (^operation)(NSError **)) {
    __block NSInteger attempt = 0;
    __block void (^run)(void);
    run = ^{
        NSError *error = nil;
        if (operation(&error) || attempt >= maxRetries) {
            if (error) NSLog(@"retry gave up: %@", error.localizedDescription);
            return;
        }
        double delay = MIN(0.25 * pow(2.0, attempt), 8.0);
        delay += (double)arc4random_uniform(250) / 1000.0; // jitter below 250 ms
        attempt += 1;
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(delay * NSEC_PER_SEC)),
                       dispatch_get_global_queue(QOS_CLASS_DEFAULT, 0), run);
    };
    run();
}