[Bug]: "InvalidOperationException: Collection was modified" When using joins

[rcm-adrien-gervaise]

Describe the bug 🐞

This is the come-back of #364
Subscribing to a join is still not thread-safe, as GetInitialUpdates will enumerate the underlying Dictionary, which could be updated at the same time.

Step to reproduce

[Fact]
    public void LetsSeeWhetherWeCanRandomlyHitADifferentRaceCondition()
    {
        var leftCache = new SourceCache<Thing, long>(x => x.Id);
        var rightCache = new SourceCache<Thing, long>(x => x.Id);
        var joined = leftCache.Connect().InnerJoin(rightCache.Connect(), x => x.Id,
            (keys, leftThing, rightThing) =>
                new Thing { Id = keys.leftKey, Name = $"{leftThing.Name} x {rightThing.Name}" });

        IDisposable StartUpdating(ISourceCache<Thing, long> sourceCache) =>
            Observable.Range(1, 1000000, Scheduler.Default)
                .Subscribe(x => sourceCache.AddOrUpdate(new Thing { Id = x, Name = $"{x}" }));

        using var leftUpdater = StartUpdating(leftCache);
        using var rightUpdater = StartUpdating(rightCache);
        using var subscription = joined.Subscribe();
    }

Reproduction repository

https://github.com/rcm-adrien-gervaise/DynamicData

Expected behavior

No exception should be thrown

Screenshots 🖼️

No response

IDE

No response

Operating system

No response

Version

No response

Device

No response

DynamicData Version

No response

Additional information ℹ️

Test reproducing the issue and suggested fix on https://github.com/rcm-adrien-gervaise/DynamicData

[JakenVeina]

Is this coming out of yesterday's release?

[rcm-adrien-gervaise]

No, this has been around for a while. We have the issue using 7.4.9, but I think it predates that.

[glennawatson]

No, this has been around for a while. We have the issue using 7.4.9, but I think it predates that.

Just confirming it happens in the brand new release from yesterday?

[JakenVeina]

Just confirming it happens in the brand new release from yesterday?

Another good question. But probably so, I don't think anything in the release would have affected Join operators.

[RolandPheasant]

That's a good reproduction thanks:

[RolandPheasant]

This has been fixed and deployed in v 8.3.27

[rcm-adrien-gervaise]

Thanks for the quick fix !

[github-actions]

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