Block callback crashes with EXC_BAD_ACCESS

Question

I have a custom NSOperation subclass that I use for making HTTP requests. It accepts a block-based callback that is executed when the NSOperation completes. Everything works accordingly, but I'm experiencing a strange, intermittent crash when trying to execute my completion callback. I've read a lot of block-based EXEC_BAD_ACCESS issues are caused by not correctly copying a block when passing it to additional methods. I believe my issue relates to how I'm making use of blocks. I'll include a standard use case for my application below. The root of my issue is likely down to an ownership misunderstanding where blocks are concerned. ``` // Perform a HTTP request to a specified endpoint and declare a callback block [self performRequestToEndpoint:@"endpoint" completion:^(HTTPResponse *response) { NSLog(@"Completed with response: %@", response); }]; // A helper function to avoid having to pass around too many parameters - (void)performRequestWithEndpoint:(NSString *)endpoint completion:(voi…

Accepted Answer

This is a shot in the dark. You have two completion blocks, only one of which you explicitly copy (using the property). My mental model says that the `completionBlock` passed to `performRequestWithEndpoint:completion:` should be captured in the scope of the block you pass on. But I know some paranoid people who might try this:

```
- (void)performRequestWithEndpoint:(NSString *)endpoint 
                        completion:(void (^)(HTTPResponse *response))completionBlock
{
    void (^copiedBlock)(HTTPResponse *response) = [completionBlock copy];

[self requestWithMethod:@"GET" path:endpoint completion:^(HTTPResponse *response) {
        if(![response error] && copiedBlock) {
            copiedBlock(response);
        }
    ];
}    
```

Block callback crashes with EXC_BAD_ACCESS

Solution

Problem