Don't call DEAD_WEAK finalizer again on shutdown (#7170)

Summary:
There's a race condition like this:

  # A foreign pointer gets promoted to the last generation
  # It has its finalizer called manually
  # We start shutting down the runtime in `hs_exit_` from the main
    thread
  # A minor GC starts running (`scheduleDoGC`) on one of the threads
  # The minor GC notices that we're in `SCHED_INTERRUPTING` state and
    advances to `SCHED_SHUTTING_DOWN`
  # The main thread tries to do major GC (with `scheduleDoGC`), but it
    exits early because we're in `SCHED_SHUTTING_DOWN` state
  # We end up with a `DEAD_WEAK` left on the list of weak pointers of
    the last generation, because it relied on major GC removing it from
    that list

This change:
  * Ignores DEAD_WEAK finalizers when shutting down
  * Makes the major GC on shutdown more likely
  * Fixes a bogus assert

Test Plan:
before this diff https://ghc.haskell.org/trac/ghc/ticket/7170#comment:5
reproduced and after it doesn't

Reviewers: ezyang, austin, simonmar

Reviewed By: simonmar

Subscribers: bgamari, thomie

Differential Revision: https://phabricator.haskell.org/D921

GHC Trac Issues: #7170

1 files changed, 2 insertions, 1 deletions

diff --git a/rts/sm/MarkWeak.c b/rts/sm/MarkWeak.c
index c5a107c480..60ac53f4b0 100644
--- a/rts/sm/MarkWeak.c
+++ b/rts/sm/MarkWeak.c
@@ -348,7 +348,8 @@ static void checkWeakPtrSanity(StgWeak *hd, StgWeak *tl)
 {
     StgWeak *w, *prev;
     for (w = hd; w != NULL; prev = w, w = w->link) {
-        ASSERT(INFO_PTR_TO_STRUCT(UNTAG_CLOSURE((StgClosure*)w)->header.info)->type == WEAK);
+        ASSERT(INFO_PTR_TO_STRUCT(UNTAG_CLOSURE((StgClosure*)w)->header.info)->type == WEAK
+            || UNTAG_CLOSURE((StgClosure*)w)->header.info == &stg_DEAD_WEAK_info);
         checkClosure((StgClosure*)w);
     }
     if (tl != NULL) {