Non threadsafe code in QueryProviderService

[pardahlman]

The QueryProviderService resolves a list of IQueryProvider based on the supplied command. Each provider kicks of an task that performs the query and updates the same IList<IQueryResult>. Here's a code snippet

for (var i = target.Count - 1; i >= 0; i--)
{
    var current = target[i];
    if (!result.Contains(current))
    {
        target.Remove(target[i]);
    }
}

If two threads simultaneous enters this for loop they would both get the same initial value for i, which is a problem in the following scenario:

• One of the threads execute the entire code block and removes the element before the second thread tries to access target[i], and an index out of bounds exception will be thrown
• The threads execute in parallel but in the remove statement they both access target[i] and one of them is going to succeed and the other one would get an index out of bounds

The reason this is not a problem today is that Jarvis only has three query providers, and none of them are overlapping.

I think it would make sense to execute the query providers in parallel, but edit the list in once all providers have returned their results.