Objective-C thread safe code to prevent a crash - objective-c

Question

I got an iPhone crash report with a SIGSEGV and I think I've narrowed down the possible cause and a solution.
Since crashes caused by threads are hard to debug I can't repro this problem, but could use some help with my hypothesis - is it sound?
My code uses ASIHttpRequest to download a set of files using an ASINetWorkQueue. Here is a simplified sample
//initialize download queue and do this code block in a loop for each file
NSURL *fileURL = [NSURL URLWithString:...
__block ASIHTTPRequest *fileRequest = [ASIHTTPRequest requestWithURL:fileURL];
[fileRequest setCompletionBlock:^{
//do some stuff
}];
[fileRequest setFailedBlock:^{
NSString *someError = [NSString stringWithFormat:...
[self someErrorMethod:someError];
}];
[downloadQueue addOperation:...
-(void)someErrorMethod(NSString *errorMessage) {
DDLogWarn(errorMessage);
if ([self downloadQueue]) {
for (ASIHTTPRequest *request in [[self downloadQueue] operations]) {
[request clearDelegatesAndCancel];
}
[[self downloadQueue] reset];
}
}
The top 2 lines of the crash report are
libobjc.A.dylib 0x31846fbc objc_msgSend + 15
MyApp 0x0002cab5 -[Myapp someErrorMethod:] (MyApp.m:)
My thinking for why this happened
A file download fails and the failed block is called
It goes through every request and clears delegates and cancels them and then resets the queue
However, while it is till running, another file download fails and enters the failed block callback
However, since it has now been cancelled, its failed block has been released
When the code tries to log the error message, its memory has been released and unpredictable results follow
Does this make sense? Since I am newish to Objective-C, is my analysis correct or am I missing something obvious?
I am thinking of using a lock to make the errorMethod thread safe with the hope that it will fix this issue. Does that sound like the right solution based on the code above?
Thanks

Answer 1

This doesn't sound likely. ASIHttpRequest likely performs all of its callbacks on the same thread (I'm fairly certain on this one).
If I had to guess, your error is more likely in this line:
DDLogWarn(errorMessage);
The first parameter to DDLogWarn is a format, not a string. This will likely crash in any case that errorMessage includes a %. What you meant is:
DDLogWarn(#"%#", errorMessage);
Since DDLogWarn() is a varags method, it will start substituting the (random) values it finds on the stack for any % substitutions in the string. It will read the stack until you run out of % substitutions. If any of the % substitutions are pointer-based (like %s or %#), then it will follow the pointer to a random location.
SEG_ACCERR means that you've requested a piece of memory you don't own. SEG_MAPERR means you've requested a piece of memory that is not mapped. Either is an expected result of following a totally random pointer.