Interpreter thread in ghc-8.10.6 in macOS

[Domínguez, Facundo]

Dear devs,

I found recently that ghc-8.10.6 and ghc-8.10.7 might be behaving
differently than previous compilers when running Template Haskell code.

When upgrading HaskellR to use a newer ghc I found that it would work fine
with ghc-8.10.4, but not ghc-8.10.6 or ghc-8.10.7. HaskellR, and the R
runtime in particular, is sensitive to local state in the threads that are
used to execute the runtime. HaskellR is arranging to run the R runtime
when running TemplateHaskell splices, and this stopped working in MacOS
with ghc-8.10.6.

The error I got is not new. It comes from R and we would get it in ghci if
we try to use HaskellR without passing the command line flag
-fno-ghci-sandbox. But this is the first time we get the error when using
TemplateHaskell.

Does anyone know if there have been changes to the threads in which the TH
splices are run? And were there any changes specific to TH in MacOS?

Thanks in advance,
Facundo

[1] https://github.com/tweag/HaskellR/pull/368#issuecomment-968864170
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