Re-enable Windows CI

Cherry-picked from #10282 so CI will pass.

.github/workflows/validate.yml

@@ -72,6 +72,10 @@ jobs:
         # support, so the PR *must* have a changelog entry.
         ghc: ['9.10.1', '9.8.2', '9.6.4', '9.4.8', '9.2.8', '9.0.2', '8.10.7', '8.8.4', '8.6.5']
         exclude:
+          # Throws fatal "cabal-tests.exe: fd:8: hGetLine: end of file" exception
+          # even with --io-manager=native
+          - os: windows-latest
+            ghc: '9.0.2'
           # corrupts GHA cache or the fabric of reality itself, see https://github.com/haskell/cabal/issues/8356
           - os: windows-latest
             ghc: '8.10.7'

cabal-testsuite/PackageTests/Backpack/Includes3/cabal-external.test.hs

@@ -2,7 +2,6 @@ import Test.Cabal.Prelude
 
 main = cabalTest $ do
     skipUnlessGhcVersion ">= 8.1"
-    skipIfWindows -- TODO: https://github.com/haskell/cabal/issues/6271
     withProjectFile "cabal.external.project" $ do
         cabal "v2-build" ["exe"]
         withPlan $ do

validate.sh

@@ -324,6 +324,9 @@ CABAL_TESTSUITE_BDIR="$(pwd)/$BUILDDIR/build/$ARCH/$BASEHC/cabal-testsuite-3"
 CABALNEWBUILD="${CABAL} build $JOBS -w $HC --builddir=$BUILDDIR --project-file=$PROJECTFILE"
 CABALLISTBIN="${CABAL} list-bin --builddir=$BUILDDIR --project-file=$PROJECTFILE"
 
+# See https://github.com/haskell/cabal/issues/9571 for why we set this for Windows
+RTSOPTS="$([ $ARCH = "x86_64-windows" ] && [ "$($HC --numeric-version)" != "9.0.2" ] && [ "$(echo -e "$(ghc --numeric-version)\n9.0.2" | sort -V | head -n1)" = "9.0.2" ] && echo "+RTS --io-manager=native" || echo "")"
+
 # header
 #######################################################################