NSData的奇怪内存泄漏了什么

Question

使用NSData时，我注意到一些奇怪的内存泄漏。 只需创建一个简单的项目并在其中添加我的代码。 我使用了一些大的JPEG文件F8F32F0F-EC04-414D-9E67-518F8B1F598D.jpg。它大约是4.5 Mb。这是一个代码：

#define DISPATCH_QUEUE_NAME "com.example.syncqueue"

@implementation MySimpleViewController {
    dispatch_queue_t _backgroundQueue;
}

- (void)viewDidLoad
{
    [super viewDidLoad];

    _backgroundQueue = dispatch_queue_create(DISPATCH_QUEUE_NAME, NULL);
    dispatch_async(_backgroundQueue, ^(void) {

        while (true) {
            NSString *path = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES)[0];
            path = [path stringByAppendingPathComponent:@"F8F32F0F-EC04-414D-9E67-518F8B1F598D.jpg"];

            [self uploadDocumentWithPath:path];

            [NSThread sleepForTimeInterval:5];
        }

    });
}

- (void)uploadDocumentWithPath:(NSString*)path
{
    NSLog(@"> uploadDocumentWithPath: %@", path);

    NSData* httpBody = [self createBodyWithPaths:@[path]];
    httpBody = nil;

    NSLog(@"< uploadDocumentWithPath");

    return nil;
}

- (NSData*)createBodyWithPaths:(NSArray *)paths
{
    NSMutableData *httpBody = [[NSMutableData alloc] init];

    NSLog(@"paths counts: %d", [paths count]);

    for (NSString *path in paths) {
        NSData   *data      = [NSData dataWithContentsOfFile:path];

        NSLog(@"data size: %lu", (unsigned long)[data length]);

        [httpBody appendData:[@"some data 1" dataUsingEncoding:NSUTF8StringEncoding]]; <---!!!
        [httpBody appendData:data];
        [httpBody appendData:[@"some data 2" dataUsingEncoding:NSUTF8StringEncoding]];

        data = nil;
    }

    return httpBody;
}

它在我的两款设备上都可以在iOS 7.1和iOS 7的iPhone 5上再现

以下是我从问题研究中发现的一个有趣的通知：

如果删除以下行，问题就会消失：

[httpBody appendData:[@"some data 1" dataUsingEncoding:NSUTF8StringEncoding]];

如果我从for ()方法删除createBodyWithPaths子句，也不会重现：

- (NSData*)createBodyWithPaths:(NSArray *)paths
{
    NSMutableData *httpBody = [[NSMutableData alloc] init];

    NSLog(@"paths counts: %d", [paths count]);

    //for (NSString *path in paths) {
        NSData   *data      = [NSData dataWithContentsOfFile:paths[0]];

        NSLog(@"data size: %lu", (unsigned long)[data length]);

        [httpBody appendData:[@"some data 1" dataUsingEncoding:NSUTF8StringEncoding]]; <---!!!
        [httpBody appendData:data];
        [httpBody appendData:[@"some data 2" dataUsingEncoding:NSUTF8StringEncoding]];

        data = nil;
    //}

    return httpBody;
}

下图中图表上的每一步都是循环迭代。

我知道如何解决我的项目中的问题，我将删除for子句，因为我只需要附加一个文件。但这个问题的性质很有趣。