Windows testsuite failures

[Ben Gamari]

Simon Peyton Jones via ghc-devs <ghc-devs at haskell.org> writes:

> Both Tamar and Omer are right.
>
>   * Not doing CI on Windows is bad. It means that bugs get introduced,
>     and not discovered until later. This is Tamer’s point, and it is
>     valid.
>   * Holding up MRs because a failures in Windows CI that is unrelated
>     to the MR is also bad. It a frustrating waste of time, and
>     discourages all authors. (In contrast, holding up an MR because it
>     introduces a bug on Windows is fine, indeed desirable.) This is
>     Omer’s point, and it is valid.
>
> The obvious solution is: let’s fix Windows CI, so that it doesn’t fail
> except when the MR genuinely introduces a bug.
>
> How hard would it be to do that? Do we even know what the problem is?
>
This latest issue was quite tricky since the root cause was in an
unexpected place (it seems that some of the Windows GitLab runners
somehow no longer had symlink permission, perhaps due to an operating
system update; I had expected the problem to be in the testsuite driver
due to previous issues in that area [1]). Given the relative scarcity of
Windows CI capacity and the difficulty of hitting the issue to begin
with, it took quite a while to realized the problem.

However, this morning I identified the issue and, as a workaround,
temporarily disabled forced usage of symlinks on Windows CI. I have also
opened #17706, which should allow us to use symlinks without fear of
this potential breakage.

Cheers,

- Ben


[1] https://gitlab.haskell.org/ghc/ghc/commit/e35fe8d58f18bd179efdc848c617dc9eddf4478b
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