v4: Review "Available Frameworks" functionality in view of pluggable agents

[ChrisMaddock]

Currently the engine must know about all available frameworks, to expose a list via the RuntimeFrameworkService to a runner, and validate the RequestedRuntimeFramework option.

This is a source of technical debt and a maintenance requirement for a number of legacy runtimes, and new runtimes that become available. We need to decide what this functionality would look like in a world of dynamic runtime availability and pluggable agents. Should the agent be responsible for announcing what frameworks it can run on?

[CharliePoole]

In the GUI, I expose this as a menu, which gets uncomfortably long on my machine because so much stuff is installed - also because I list each possible version of CLR4 framework available.

I'm planning to change that to list the available agents, both built-in and pluggable.

Maybe that's what the engine should support externally either in addition to or instead of the available runtimes.

In fact, my experimentation with pluggable agents showed me that the agent needs to know what runtimes it can run on., so making this an agent responsibility is sensible. Also, if we implement agents that run on other machines, then only the agent will be able to fulfill that role.

All that said... it's really pretty hard to think about this without knowing what will happen with the proposed elimination of both processmodel and domainusage selection. If we do both of those things, then it gets easier.

[ChrisMaddock]

In fact, my experimentation with pluggable agents showed me that the agent needs to know what runtimes it can run on., so making this an agent responsibility is sensible. Also, if we implement agents that run on other machines, then only the agent will be able to fulfill that role.

This makes a lot of sense. So the engine should ask all installed agents which runtimes they can run on. That makes sense as it may be one to many (e.g. a .NET Core 2.1 agent may run on the 2.1.0/2.1.1/2.1.2 runtime, and could potentially be forced onto a specific version). Or alternatively, the exact runtime available may not be known until runtime (e.g. a CLR v4 agent can only run on the single version of the v4 CLR which is installed on a machine, so may run on netfx4.5 on one machine, and 4.8 on another)