Fix possible crash or deadlock arising from calling notify() from multiple queues concurrently

[kattrali]

Goal

Its possible that when notify() is called concurrently from multiple queues, that a crash occurs when caching/resetting crash state. This changeset solves the underlying issues by not sharing state between calls to notify() and isolating calls to suspend threads (for backtrace collection).

Reproduction case

dispatch_queue_t queue1 = dispatch_queue_create("Queue 1", DISPATCH_QUEUE_CONCURRENT);
dispatch_queue_t queue2 = dispatch_queue_create("Queue 2", DISPATCH_QUEUE_CONCURRENT);

for (int i = 0; i < 4; i++) {
    NSString *message = [NSString stringWithFormat:@"Err %ld", (long)i];
    NSError *error = [FooError errorWithDomain:@"com.example"
                                          code:340
                                      userInfo:nil];
    dispatch_async(queue1, ^{
        [Bugsnag notifyError:error];
    });
    dispatch_async(queue2, ^{
        [Bugsnag notifyError:error];
    });
}

Fixes #399

Changeset

The changes can be divided into four parts:

1. Adding a (failing on master) test which calls notify() several times from different concurrent queues
2. Refactoring crash reporting internals to allow providing separate crash contexts for each invocation of notify()
3. Implementing separate crash contexts for notify() calls
4. Adding locking around suspending threads and writing crash state.

The easiest way to view the changeset is one commit at a time, since each chunk can be evaluated independently.

Tests

• Tested manually on a couple different iOS/tvOS devices running iOS 11 & 12
• Added automated tests replicating the failing scenario

[tomlongridge]

LGTM 👍

One suggestion for a unit test to cover new function (if feasible).

[tomlongridge]

Can we include a test for bsg_kscrash_generate_report_path?

[kattrali]

Yeah I should - it turned out to be a bit tricky to stub the file store but I've added a task to improve coverage here.

[nkavian]

When will a new release be ready? Thanks.

[kattrali]

Hi @nkavian! Doing a release now, should be available soon.

[nkavian]

Thanks, confirming the latest release fixed my issue!