Add -fkeep-cafs

Summary:
I noticed while playing around with
https://github.com/fbsamples/ghc-hotswap/ that the main binary needs to
have a custom main function to set `config.keep_cafs = true` when
initialising the runtime. This is pretty annoying, it means an extra
C file with some cryptic incantations in it, and a `-no-hs-main` flag.

So I've replaced this with a link-time flag to GHC, `-fkeep-cafs` that
does the same thing.

Test Plan:
New unit test that tests for the RTS's GC'd CAFs assertion, and also
the -keep-cafs flag.

Reviewers: bgamari, osa1, erikd, noamz

Reviewed By: osa1

Subscribers: rwbarton, carter

Differential Revision: https://phabricator.haskell.org/D5183

1 files changed, 25 insertions, 0 deletions

diff --git a/testsuite/tests/rts/KeepCafsMain.hs b/testsuite/tests/rts/KeepCafsMain.hs
new file mode 100644
index 0000000000..2f6ad5a4f9
--- /dev/null
+++ b/testsuite/tests/rts/KeepCafsMain.hs
@@ -0,0 +1,25 @@
+module Main (main) where
+
+import Foreign
+import GHCi.ObjLink
+import System.Mem
+import System.Exit
+
+foreign import ccall "dynamic"
+  callGetX :: FunPtr (IO Int) -> IO Int
+
+main :: IO ()
+main = do
+  initObjLinker DontRetainCAFs
+  let
+    loadAndCall obj = do
+      loadObj obj
+      resolveObjs
+      r <- lookupSymbol "getX"
+      case r of
+        Nothing -> die "cannot find getX"
+        Just ptr -> callGetX (castPtrToFunPtr ptr) >>= print
+      unloadObj obj
+      performGC
+  loadAndCall "KeepCafs1.o"
+  loadAndCall "KeepCafs2.o"