Fix Fiber#transfer

Fiber#transfer previously made it impossible to resume the fiber
if it was transferred to (no resuming the target of Fiber#transfer).
However, the documentation specifies that you cannot resume a fiber
that has transferred to another fiber (no resuming the source of
Fiber#transfer), unless control is transferred back.

Fix the code by setting the transferred flag on the current/source
fiber, and unsetting the transferred flag on the target fiber.

Fixes [Bug #9664]
Fixes [Bug #12555]

1 files changed, 2 insertions, 1 deletions

diff --git a/cont.c b/cont.c
index 61f42b386f..8e1dc6db87 100644
--- a/cont.c
+++ b/cont.c
@@ -2226,7 +2226,8 @@ static VALUE
 rb_fiber_m_transfer(int argc, VALUE *argv, VALUE fiber_value)
 {
     rb_fiber_t *fiber = fiber_ptr(fiber_value);
-    fiber->transferred = 1;
+    fiber_current()->transferred = 1;
+    fiber->transferred = 0;
     return fiber_switch(fiber, argc, argv, 0, PASS_KW_SPLAT);
 }