Fix potential hang when joining threads.

If the thread termination invokes user code after `th->status` becomes
`THREAD_KILLED`, and the user unblock function causes that `th->status` to
become something else (e.g. `THREAD_RUNNING`), threads waiting in
`thread_join_sleep` will hang forever. We move the unblock function call
to before the thread status is updated, and allow threads to join as soon
as `th->value` becomes defined.

1 files changed, 17 insertions, 4 deletions

diff --git a/thread.c b/thread.c
index 0f6838e141..ec67bba2d5 100644
--- a/thread.c
+++ b/thread.c
@@ -632,6 +632,7 @@ thread_cleanup_func_before_exec(void *th_ptr)
 {
     rb_thread_t *th = th_ptr;
     th->status = THREAD_KILLED;
+
     // The thread stack doesn't exist in the forked process:
     th->ec->machine.stack_start = th->ec->machine.stack_end = NULL;
 
@@ -817,6 +818,9 @@ thread_start_func_2(rb_thread_t *th, VALUE *stack_start)
 
     thread_debug("thread start (get lock): %p\n", (void *)th);
 
+    // Ensure that we are not joinable.
+    VM_ASSERT(th->value == Qundef);
+
     EC_PUSH_TAG(th->ec);
 
     if ((state = EC_EXEC_TAG()) == TAG_NONE) {
@@ -857,6 +861,12 @@ thread_start_func_2(rb_thread_t *th, VALUE *stack_start)
         th->value = Qnil;
     }
 
+    // The thread is effectively finished and can be joined.
+    VM_ASSERT(th->value != Qundef);
+
+    rb_threadptr_join_list_wakeup(th);
+    rb_threadptr_unlock_all_locking_mutexes(th);
+
     if (th->invoke_type == thread_invoke_type_ractor_proc) {
         rb_thread_terminate_all(th);
         rb_ractor_teardown(th->ec);
@@ -874,9 +884,6 @@ thread_start_func_2(rb_thread_t *th, VALUE *stack_start)
         rb_threadptr_raise(ractor_main_th, 1, &errinfo);
     }
 
-    rb_threadptr_join_list_wakeup(th);
-    rb_threadptr_unlock_all_locking_mutexes(th);
-
     EC_POP_TAG();
 
     rb_ec_clear_current_thread_trace_func(th->ec);
@@ -1153,6 +1160,12 @@ remove_from_join_list(VALUE arg)
 
 static rb_hrtime_t *double2hrtime(rb_hrtime_t *, double);
 
+static int
+thread_finished(rb_thread_t *th)
+{
+    return th->status == THREAD_KILLED || th->value != Qundef;
+}
+
 static VALUE
 thread_join_sleep(VALUE arg)
 {
@@ -1179,7 +1192,7 @@ thread_join_sleep(VALUE arg)
         end = rb_hrtime_add(*limit, rb_hrtime_now());
     }
 
-    while (target_th->status != THREAD_KILLED) {
+    while (!thread_finished(target_th)) {
         VALUE scheduler = rb_fiber_scheduler_current();
 
         if (scheduler != Qnil) {