[Mono] Unloadable ALCs.

[lateralusX]

#77399 setup basic support for unloadable ALCs in Mono. This issue tracks remaining work to complete unloadable ALCs on Mono runtime. Initial list of work items is extracted from https://github.com/dotnet/runtime/blob/main/docs/design/mono/unloadability.md#remaining-work:

Must

• Enable Unloading is currently disabled due to CI failures
• Managed frames. Frames belonging to code in the ALC should keep the ALC alive. This can be implemented by having all methods allocate a volatile local variable and store a reference to their LoaderAllocator object into it.
• Reflection pointers. Icalls which take a assembly/type etc. handle as parameter need to keep the ALC alive, otherwise there will be subtle races.
• TLS variables.
• Testing and leak detection.
• Enable/disable compiler flag.

Should

• Profiling, perf counters.
• Diagnostics support. What keeps an ALC alive.

Could

• Boehm GC support.

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

Tagging subscribers to this area:
See info in area-owners.md if you want to be subscribed.

Issue Details

---

#77399 setup basic support for unloadable ALCs in Mono. This issue tracks remaining work to complete unloadable ALCs on Mono runtime. Initial list of work items is extracted from https://github.com/dotnet/runtime/blob/main/docs/design/mono/unloadability.md#remaining-work:

Must

• Managed frames. Frames belonging to code in the ALC should keep the ALC alive. This can be implemented by having all methods allocate a volatile local variable and store a reference to their LoaderAllocator object into it.
• Reflection pointers. Icalls which take a assembly/type etc. handle as parameter need to keep the ALC alive, otherwise there will
  be subtle races.
• TLS variables.
• Testing and leak detection.
• Enable/disable compiler flag.

##Should

• Thread abort. Although its not part of current .NET APIs, it might be useful to have a way to abort threads executing code in an ALC, similarly to how domain unloads worked previously.
• Profiling, perf counters, etc.
• Diagnostics support, i.e. what keeps an ALC alive.

##Could

• Boehm GC support.

Author:	lateralusX
Assignees:	-
Labels:	untriaged, area-AssemblyLoader-mono
Milestone:	-

[teo-tsirpanis]

abort threads executing code in an ALC, similarly to how domain unloads worked previously.

Note that forceful unloading of ALCs is an explicit non-goal and generally frowned upon for the same reason Thread.Abort is bad. Are you sure you want it?

[teo-tsirpanis]

@steveisok what is the milestone for this? untriaged means without a milestone and gets automatically removed by the bot.

[lateralusX]

abort threads executing code in an ALC, similarly to how domain unloads worked previously.

Note that forceful unloading of ALCs is an explicit non-goal and generally frowned upon for the same reason Thread.Abort is bad. Are you sure you want it?

Fully aware on existing unload design and Thread.Abort impact and it has been discussed over last years in various forms. This issue might not be the right form to carry additional/new "ideas" and it's generally not something we would just implement, but since we have many customers that tries to mitigate the lack of app domains (Unity is one example) we also work on ideas going forward on how to simplify/assist developers with existing scenarios/solutions relying on aspects of app domains to move over to dotnet runtime. I removed the Thread.Abort from this issue since it falls into a wider discussion and not directly needed for Mono ALC support.

[srxqds]

abort threads executing code in an ALC, similarly to how domain unloads worked previously.

Note that forceful unloading of ALCs is an explicit non-goal and generally frowned upon for the same reason Thread.Abort is bad. Are you sure you want it?

@lateralusX @teo-tsirpanis this is most import for embed reload process, hope you can consider adding this.

[SamMonoRT]

@vargaz @lateralusX - moving this to 9.0.0 milestone.