Add flag to workaround GHC heisenbug on CI (#2444)

In very specific tests in GitLab CI we are affected by GHC bug #19421.
We can work around the issue by passing `-with-rtsopts=-xm20000000` when
compiling an affected binary. This is a stopgap measure until the real
bug is fixed.

We have seen the bug:
- In `clash-testsuite` in `clashLibTest`s
- In `ffi:example` in the `clash` binary itself
- In `prelude:doctests`, probably in the `doctests` binary itself,
  although this is not certain.

and then only in GHC 9.0.2, although the bug should be in other versions
of GHC as well.

This workaround was applied only to GHC 9.0.2 on CI and only to those
cases that were observed to go wrong, although as a consequence now the
`clash` binary is always built with the RTS option.

The `ffi:example` test should no longer be affected by the GHC bug, but
it is a fairly useless test in its current state as it does not fail
when there are issues, and the tested code is actually faulty. It is
disabled to be fixed later.

.ci/cabal.project.local

@@ -10,7 +10,7 @@ package *
 
 package clash-prelude
   ghc-options: -Werror
-  flags: +doctests
+  flags: +doctests -workaround-ghc-mmap-crash
   tests: True
   benchmarks: True
 
@@ -24,6 +24,7 @@ package clash-lib
 
 package clash-ghc
   ghc-options: -Werror
+  flags: -workaround-ghc-mmap-crash
 
 package clash-cosim
   ghc-options: -Werror
@@ -47,7 +48,7 @@ package clash-lib-hedgehog
 package clash-testsuite
   ghc-options: -Werror
   -- enable cosim
-  flags: +cosim
+  flags: +cosim -workaround-ghc-mmap-crash
 
 package clash-benchmark
   ghc-options: -Werror

.ci/setup.sh

@@ -78,6 +78,10 @@ if [ ! -f cabal.project.local ]; then
   fi
 
   set +u
+  if [[ "$WORKAROUND_GHC_MMAP_CRASH" == "yes" ]]; then
+    sed -i 's/-workaround-ghc-mmap-crash/+workaround-ghc-mmap-crash/g' cabal.project.local
+  fi
+
   if [[ "$GHC_HEAD" == "yes" ]]; then
     cat .ci/cabal.project.local.append-HEAD >> cabal.project.local
   fi

.gitlab-ci.yml

@@ -33,7 +33,9 @@ tests:
     strategy: depend
   parallel:
     matrix:
-      - GHC_VERSION: [8.8.4, 8.10.7, 9.0.2]
+      - GHC_VERSION: [8.8.4, 8.10.7]
+      - GHC_VERSION: 9.0.2
+        WORKAROUND_GHC_MMAP_CRASH: "yes"
       - GHC_VERSION: 8.6.5
         MULTIPLE_HIDDEN: "no"
 

clash-ghc/clash-ghc.cabal

@@ -65,12 +65,24 @@ flag use-ghc-paths
   default: False
   manual: True
 
+flag workaround-ghc-mmap-crash
+  description:
+    Only use this flag when hit by GHC bug #19421. See clash-compiler PR #2444.
+  default: False
+  manual: True
+
 executable clash
   Main-Is:            src-ghc/Batch.hs
   Build-Depends:      base, clash-ghc
-  GHC-Options:        -Wall -Wcompat -threaded -rtsopts -with-rtsopts=-A128m
+  GHC-Options:        -Wall -Wcompat -threaded -rtsopts
   if flag(dynamic)
     GHC-Options: -dynamic
+  -- Note that multiple -with-rtsopts are not cumulative, so we can't add the
+  -- common RTS options in the unconditional GHC-Options
+  if flag(workaround-ghc-mmap-crash)
+    GHC-Options: "-with-rtsopts=-A128m -xm20000000"
+  else
+    GHC-Options: -with-rtsopts=-A128m
   extra-libraries:    pthread
   default-language:   Haskell2010
 

clash-prelude/clash-prelude.cabal

@@ -117,6 +117,12 @@ flag benchmarks
   default: True
   manual: True
 
+flag workaround-ghc-mmap-crash
+  description:
+    Only use this flag when hit by GHC bug #19421. See clash-compiler PR #2444.
+  default: False
+  manual: True
+
 common common-options
   default-language:   Haskell2010
   default-extensions: BangPatterns
@@ -373,6 +379,10 @@ test-suite doctests
       doctest-parallel >= 0.2 && < 0.4,
       filepath
 
+  if flag(workaround-ghc-mmap-crash)
+    ghc-options:    -with-rtsopts=-xm20000000
+
+
 test-suite unittests
   import:           common-options
   type:             exitcode-stdio-1.0

tests/clash-testsuite.cabal

@@ -28,6 +28,12 @@ flag multiple-hidden
   default: False
   manual: True
 
+flag workaround-ghc-mmap-crash
+  description:
+    Only use this flag when hit by GHC bug #19421. See clash-compiler PR #2444.
+  default: False
+  manual: True
+
 common basic-config
   default-language: Haskell2010
   ghc-options: -Wall -Wcompat
@@ -76,6 +82,9 @@ common basic-config
   if flag(multiple-hidden)
     cpp-options:       -DCLASH_MULTIPLE_HIDDEN
 
+  if flag(workaround-ghc-mmap-crash)
+    cpp-options:       -DCLASH_WORKAROUND_GHC_MMAP_CRASH
+
 library
   import: basic-config
   hs-source-dirs:

tests/src/Test/Tasty/Clash.hs

@@ -1,3 +1,4 @@
+{-# LANGUAGE CPP #-}
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE KindSignatures #-}
@@ -564,7 +565,11 @@ clashLibTest' modName target extraGhcArgs path =
   clashBuild workDir = ("clash (exec)", singleTest "clash (exec)" (ClashBinaryTest {
       cbBuildTarget=target
     , cbSourceDirectory=sourceDir
-    , cbExtraBuildArgs="-DCLASHLIBTEST" : extraGhcArgs
+    , cbExtraBuildArgs="-DCLASHLIBTEST" :
+#ifdef CLASH_WORKAROUND_GHC_MMAP_CRASH
+        "-with-rtsopts=-xm20000000" :
+#endif
+        extraGhcArgs
     , cbExtraExecArgs=[]
     , cbModName=modName
     , cbOutputDirectory=workDir