Convert OCE to empty value on certain VS operations

[0101]

Description

There are OperationCancelledExceptions in telemetry originating from FSharp project options not found. in AddSemanticClassificationsAsync and GetBlockStructureAsync operations.

There's not much we can do without options but maybe we can just return empty results instead of propagating OCE further up.

One reason we don't have options is that when project is loading and an F# file is already open VS asks as for semantic classification, block structure (and some other things) using a MiscellaneousFiles workspace. There could potentially be other cases.

This proposed change converts all OCEs in those operations to empty results, unless cancellation has been requested by the caller through the CT they passed us.

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[T-Gro]

We should only do that if the token causing the OCE is a different one from the one our cancellable uses.
Or, maybe a lot more explicit - out code failing on project options would throw any other exception, which will be explicitly caught here.

[0101]

We should only do that if the token causing the OCE is a different one from the one our cancellable uses.

What token is used when we throw from our cancellable? Also how can we tell who caused it?

Or, maybe a lot more explicit - out code failing on project options would throw any other exception, which will be explicitly caught here.

The downside of that is that we would have to start catching it in a lot of places.

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Also, what actually happens when this is canceled for another reason? Since the OCE from here goes to Watson no one is relying on it in any way. Assuming the caller handles it somehow when they initiate the cancellation, they shouldn't have a problem with us returning an empty result (which will in that case probably just be thrown away).

[0101]

Ok, we could check that the (CancellationToken we get called with).IsCancellationRequested = false which seems to be the case when we just throw an OCE.

[T-Gro]

OCE exception has a field with the token that caused it. Comparison of the two tokens should be possible.

[T-Gro]

Btw. the fact that we are (sometimes) reporting OCE to Watson is a mistake caused by general catch handle on our end, we should not be doing that.

[0101]

Btw. the fact that we are (sometimes) reporting OCE to Watson is a mistake caused by general catch handle on our end, we should not be doing that.

How is that? These get called through MEF. We're not catching anything. Or are we hooking into this mechanism somewhere indirectly?

[T-Gro]

At some places, we have a universal try-with catching all exceptions, and doing our own telemetry reporting.

[0101]

Yeah we should deal with those. But it's not the case here.