Error on reload when running in process

[CharliePoole]

I noticed this running the NUnit TestModel tests. There are two errors due to accessing an unloaded domain when reloading. This only occurs if the tests are run in process.

This needs a bit more research:

1. Is it also happening when running tests under the experimental GUI?
2. Does it happen in actual use of the GUI or only in the tests?
3. Is it caused by the GUI or by the engine?
4. If caused by the engine, is there a workaround?

[CharliePoole]

Re points 1 and 2: I have confirmed that this happens in both GUI's tests and that an exception is thrown if you try to reload any test assembly in either GUI. So it's either the GUI model or the engine.

[CharliePoole]

Also confirming that it happens with either the 3.8 or the 3.9 engine, so it's not a new engine issue.

[CharliePoole]

Problem seems to be in the engine.

When running a single assembly in process, the chain of runners is

MasterTestRunner => MultipleTestDoainRunner => TestDomainRunner

After the call to Runner.Unload that chain is left intact but the framework driver continues to hold a reference to the domain with which it was originally created. That leads to an error, since the domain has been unloaded. We only see this when running in process because unloading a process runner causes the entire agent process to be terminated and reloading creates and re-initializes a new process. Running in a local domain should work analogously but doesn't.

I'm going to try to work around this but I'll also create an issue for the engine.

[CharliePoole]

Created issue nunit/nunit-console#522

[CharliePoole]

Problem remains when running in process. Possible regression when some unrelated PRs were merged.