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, 10 insertions, 1 deletions

diff --git a/rts/Weak.c b/rts/Weak.c
index f8faa4e1f5..92f1bdbd02 100644
--- a/rts/Weak.c
+++ b/rts/Weak.c
@@ -43,7 +43,16 @@ runAllCFinalizers(StgWeak *list)
     }
 
     for (w = list; w; w = w->link) {
-        runCFinalizers((StgCFinalizerList *)w->cfinalizers);
+        // We need to filter out DEAD_WEAK objects, because it's not guaranteed
+        // that the list will not have them when shutting down.
+        // They only get filtered out during GC for the generation they
+        // belong to.
+        // If there's no major GC between the time that the finalizer for the
+        // object from the oldest generation is manually called and shutdown
+        // we end up running the same finalizer twice. See #7170.
+        if (w->header.info != &stg_DEAD_WEAK_info) {
+            runCFinalizers((StgCFinalizerList *)w->cfinalizers);
+        }
     }
 
     if (task != NULL) {