Fix GC hole with collectible assemblies and tailcalls #51315
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We must take special care to keep the tailcall dispatcher targets alive while tailcalls are in-flight. In particular, given the following callstack: [B]M2() [SPC]DispatchTailCalls [A]M() it could happen that [B]M2() queued a tail call to a function [B]M3(). Since there is a live dispatcher on the call stack, this would result in [B]M2() storing a function pointer pointing to [B]M3() and returning to this dispatcher to let it take care of the tailcall. If B was loaded in a collectible ALC, it would then be possible for there to be nothing keeping this ALC alive, and for the assembly to be unloaded before the dispatcher invoked the function pointer. I was unable to come up with a test case where this happened without making changes to the dispatcher; the window otherwise seems to be too small. To reproduce the problem I thus had to add a Thread.Sleep(50) into the dispatcher, which quickly resulted in an AccessViolationException in the scenario above. With the changes in this commit I was then no logner able to reproduce the problem. Fix dotnet#41314
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We must take special care to keep the tailcall dispatcher targets alive
while tailcalls are in-flight. In particular, given the following
callstack:
[B]M2()
[SPC]DispatchTailCalls
[A]M()
it could happen that [B]M2() queued a tail call to a function [B]M3().
Since there is a live dispatcher on the call stack, this would result in
[B]M2() storing a function pointer pointing to [B]M3() and returning to
this dispatcher to let it take care of the tailcall.
If B was loaded in a collectible ALC, it would then be possible for
there to be nothing keeping this ALC alive, and for the assembly to be
unloaded before the dispatcher invoked the function pointer.
I was unable to come up with a test case where this happened without
making changes to the dispatcher; the window otherwise seems to be too
small. To reproduce the problem I thus had to add a Thread.Sleep(50)
into the dispatcher, which quickly resulted in an
AccessViolationException in the scenario above. With the changes in this
commit I was then no logner able to reproduce the problem.
Fix #41314