Wake up join list within thread EC context. (#4471)

* Wake up join list within thread EC context.

* Consume items from join list so that they are not re-executed.

If `rb_fiber_scheduler_unblock` raises an exception, it can result in a
segfault if `rb_threadptr_join_list_wakeup` is not within a valid EC. This
change moves `rb_threadptr_join_list_wakeup` into the thread's top level EC
which initially caused an infinite loop because on exception will retry. We
explicitly remove items from the thread's join list to avoid this situation.

* Verify the required scheduler interface.

* Test several scheduler hooks methods with broken `unblock` implementation.

1 files changed, 24 insertions, 0 deletions

diff --git a/scheduler.c b/scheduler.c
index 5937d61007..064a517309 100644
--- a/scheduler.c
+++ b/scheduler.c
@@ -55,12 +55,36 @@ rb_fiber_scheduler_get(void)
     return thread->scheduler;
 }
 
+static void
+verify_interface(VALUE scheduler)
+{
+    if (!rb_respond_to(scheduler, id_block)) {
+        rb_raise(rb_eArgError, "Scheduler must implement #block!");
+    }
+
+    if (!rb_respond_to(scheduler, id_unblock)) {
+        rb_raise(rb_eArgError, "Scheduler must implement #unblock!");
+    }
+
+    if (!rb_respond_to(scheduler, id_kernel_sleep)) {
+        rb_raise(rb_eArgError, "Scheduler must implement #kernel_sleep!");
+    }
+
+    if (!rb_respond_to(scheduler, id_io_wait)) {
+        rb_raise(rb_eArgError, "Scheduler must implement #io_wait!");
+    }
+}
+
 VALUE
 rb_fiber_scheduler_set(VALUE scheduler)
 {
     rb_thread_t *thread = GET_THREAD();
     VM_ASSERT(thread);
 
+    if (scheduler != Qnil) {
+        verify_interface(scheduler);
+    }
+
     // We invoke Scheduler#close when setting it to something else, to ensure the previous scheduler runs to completion before changing the scheduler. That way, we do not need to consider interactions, e.g., of a Fiber from the previous scheduler with the new scheduler.
     if (thread->scheduler != Qnil) {
         rb_fiber_scheduler_close(thread->scheduler);