Fix concurrency issue for Single definitions

[Sloy]

The bug

We were having some crashes in our production app when injecting a Segment SDK (analytics tracker). This SDK has the particularity that only one instance can be created with the same tag. (Source for reference)

We are constructing this SDK inside a Koin's single definition. In theory, single only creates one instance of the bean, even in multi-threaded environments. But in our millions of daily sessions we get a few hundred crashes where this assumption fails.

We managed to reproduce the issue consistently in a unit test by simulating concurrent injections a few thousands of times. 🤯
I'm submitting in the PR the most simple version of the test that I could write (our initial test was way more complicated 😂 ). The number of repetitions and threads come from trial and error, with lower numbers the test would sometime give a false positive. I'm not sure if you want to keep the test, or if this is the right place to put it. Let me know.

The cause

After some research, I came to the conclusion that the issue lies in the concurrency code from SingleInstanceFactory.
The synchronization code in the create() function works fine, but there's one little operation that happens outside the synchronized block: the value = assignment.
The bug is so hard to reproduce because the affected code is such a tiny part!

The fix

I surrounded the code in get() with another synchronized, and that seems to fix the issue. 🎉
I'm no concurrency expert myself, but I can't think of any problems with this solution.

[arnaudgiuliani]

Good catch.

Then we don't need to keep it on create:

 override fun create(context: InstanceContext): T {
        return if (value == null) {
            super.create(context)
        } else value ?: error("Single instance created couldn't return value")
    }

    @Suppress("UNCHECKED_CAST")
    override fun get(context: InstanceContext): T {
        return synchronized(this) {
            if (!isCreated()) {
                value = create(context)
            }
            value ?: error("Single instance created couldn't return value")
        }
    }

[arnaudgiuliani]

Remove sycnhonized from create 👍

[Sloy]

Done!

[arnaudgiuliani]

Great work 👍