Add System.Reactive.Compatibility as a dependency to DurableTask.Core

[ConnorMcMahon]

The Distributed Tracing work required adding System.Reactive.Core as a
dependency. However, if the customer app was using a different major
version of System.Reactive, it could lead to ambiguous compilation
issues.

Adding this package (and upgrading to use v4 of System.Reactive.Core)
should resolve this issue.

Addresses Azure/azure-functions-durable-extension#1620

[davidmrdavid]

Looking at the CI failures, it would appear that the correlation tracing work depends on System.Reactive.Core, no?

[ConnorMcMahon]

Oddly it is building locally for me, but not in the CI, so I'm trying to understand why.

[cgillum]

Where is the dependency on System.Reactive coming from?

[davidmrdavid]

I can confirm that System.Reactive.Core was added in the Dist.Tracing PR: https://github.com/Azure/durabletask/pull/422/files

[ConnorMcMahon]

To clarify, it looks like CorrelationTraceClient uses some delegates that require types defined in System.Reactive (you can also get those types in System.Reactive.Core). It looks like we originally took on a dependency on System.Reactive.Core (v3) to address this.

The customer who encountered an issue was using v4 of System.Reactive, which leads to a compilation error. This is a well-known issue with using different major versions of System.Reactive.Core and System.Reactive.

Our options to make sure that customers don't run into this are:

1. Remove our dependency on System.Reactive.Core if possible.
2. Upgrade our System.Reactive.Core dependency to v4 (this would then break customers using v3 of System.Reactive)
3. Include a reference to the System.Reactive.Compatibility package, which should resolve this issue.

I think either option 1 or 3 are viable, but since I am running into build issues with option 1, I'll probably just go with option 3.

[davidmrdavid]

LGTM! Glad there already exists a compatibility layer :)