Thread#value: handle threads killed by a fork

[Bug #18902]

When a thread is killed because we forked, the `value` if left
to `Qundef`. Returning it woudl crash the VM.

1 files changed, 4 insertions, 0 deletions

diff --git a/thread.c b/thread.c
index 620a56d9a0..5fd7a7f67a 100644
--- a/thread.c
+++ b/thread.c
@@ -1214,6 +1214,10 @@ thread_value(VALUE self)
 {
     rb_thread_t *th = rb_thread_ptr(self);
     thread_join(th, Qnil, 0);
+    if (th->value == Qundef) {
+        // If the thread is dead because we forked th->value is still Qundef.
+        return Qnil;
+    }
     return th->value;
 }