Objective-C sync HTTP GET JSON with timeout

#import <Foundation/Foundation.h>

static id FetchJSON(NSURL *url, NSTimeInterval timeout, NSError **error) {
    NSMutableURLRequest *req = [NSMutableURLRequest requestWithURL:url cachePolicy:NSURLRequestUseProtocolCachePolicy timeoutInterval:timeout];
    dispatch_semaphore_t sem = dispatch_semaphore_create(0);
    __block NSData *body = nil;
    __block NSError *failure = nil;
    NSURLSessionDataTask *task = [[NSURLSession sharedSession] dataTaskWithRequest:req completionHandler:^(NSData *data, NSURLResponse *resp, NSError *err) {
        body = data;
        failure = err;
        dispatch_semaphore_signal(sem);
    }];
    [task resume];
    if (dispatch_semaphore_wait(sem, dispatch_time(DISPATCH_TIME_NOW, (int64_t)(timeout * NSEC_PER_SEC))) != 0) {
        [task cancel];
        if (error) *error = [NSError errorWithDomain:@"HTTPError" code:1 userInfo:@{NSLocalizedDescriptionKey: @"request timed out"}];
        return nil;
    }
    if (failure) { if (error) *error = failure; return nil; }
    return [NSJSONSerialization JSONObjectWithData:body options:0 error:error];
}