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.

This reverts commit 6505c77501.

test/fiber/scheduler.rb

@@ -112,8 +112,10 @@ class Scheduler
 
     self.run
   ensure
-    @urgent.each(&:close)
-    @urgent = nil
+    if @urgent
+      @urgent.each(&:close)
+      @urgent = nil
+    end
 
     @closed = true
 
@@ -240,3 +242,13 @@ class BrokenUnblockScheduler < Scheduler
     raise "Broken unblock!"
   end
 end
+
+class SleepingUnblockScheduler < Scheduler
+  # This method is invoked when the thread is exiting.
+  def unblock(blocker, fiber)
+    super
+
+    # This changes the current thread state to `THREAD_RUNNING` which causes `thread_join_sleep` to hang.
+    sleep(0.1)
+  end
+end

test/fiber/test_thread.rb

@@ -20,6 +20,31 @@ class TestFiberThread < Test::Unit::TestCase
     assert_equal :done, thread.value
   end
 
+  def test_thread_join_implicit
+    sleeping = false
+    finished = false
+
+    thread = Thread.new do
+      scheduler = Scheduler.new
+      Fiber.set_scheduler scheduler
+
+      Fiber.schedule do
+        sleeping = true
+        sleep(0.1)
+        finished = true
+      end
+
+      :done
+    end
+
+    Thread.pass until sleeping
+
+    thread.join
+
+    assert_equal :done, thread.value
+    assert finished, "Scheduler thread's task should be finished!"
+  end
+
   def test_thread_join_blocking
     thread = Thread.new do
       scheduler = Scheduler.new
@@ -66,4 +91,18 @@ class TestFiberThread < Test::Unit::TestCase
       thread.join
     end
   end
+
+  def test_thread_join_hang
+    thread = Thread.new do
+      scheduler = SleepingUnblockScheduler.new
+
+      Fiber.set_scheduler scheduler
+
+      Fiber.schedule do
+        Thread.new{sleep(0.01)}.value
+      end
+    end
+
+    thread.join
+  end
 end

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;
 
@@ -688,7 +689,7 @@ rb_vm_proc_local_ep(VALUE proc)
 VALUE rb_vm_invoke_proc_with_self(rb_execution_context_t *ec, rb_proc_t *proc, VALUE self,
                                   int argc, const VALUE *argv, int kw_splat, VALUE passed_block_handler);
 
-static void
+static VALUE
 thread_do_start_proc(rb_thread_t *th)
 {
     VALUE args = th->invoke_arg.proc.args;
@@ -702,7 +703,6 @@ thread_do_start_proc(rb_thread_t *th)
     th->ec->root_lep = rb_vm_proc_local_ep(procval);
     th->ec->root_svar = Qfalse;
 
-    EXEC_EVENT_HOOK(th->ec, RUBY_EVENT_THREAD_BEGIN, th->self, 0, 0, 0, Qundef);
     vm_check_ints_blocking(th->ec);
 
     if (th->invoke_type == thread_invoke_type_ractor_proc) {
@@ -713,11 +713,12 @@ thread_do_start_proc(rb_thread_t *th)
         rb_ractor_receive_parameters(th->ec, th->ractor, args_len, (VALUE *)args_ptr);
         vm_check_ints_blocking(th->ec);
 
-        // kick thread
-        th->value = rb_vm_invoke_proc_with_self(th->ec, proc, self,
-                                                args_len, args_ptr,
-                                                th->invoke_arg.proc.kw_splat,
-                                                VM_BLOCK_HANDLER_NONE);
+        return rb_vm_invoke_proc_with_self(
+            th->ec, proc, self,
+            args_len, args_ptr,
+            th->invoke_arg.proc.kw_splat,
+            VM_BLOCK_HANDLER_NONE
+        );
     }
     else {
         args_len = RARRAY_LENINT(args);
@@ -733,17 +734,12 @@ thread_do_start_proc(rb_thread_t *th)
 
         vm_check_ints_blocking(th->ec);
 
-        // kick thread
-        th->value = rb_vm_invoke_proc(th->ec, proc,
-                                      args_len, args_ptr,
-                                      th->invoke_arg.proc.kw_splat,
-                                      VM_BLOCK_HANDLER_NONE);
-    }
-
-    EXEC_EVENT_HOOK(th->ec, RUBY_EVENT_THREAD_END, th->self, 0, 0, 0, Qundef);
-
-    if (th->invoke_type == thread_invoke_type_ractor_proc) {
-        rb_ractor_atexit(th->ec, th->value);
+        return rb_vm_invoke_proc(
+            th->ec, proc,
+            args_len, args_ptr,
+            th->invoke_arg.proc.kw_splat,
+            VM_BLOCK_HANDLER_NONE
+        );
     }
 }
 
@@ -751,20 +747,33 @@ static void
 thread_do_start(rb_thread_t *th)
 {
     native_set_thread_name(th);
+    VALUE result = Qundef;
+
+    EXEC_EVENT_HOOK(th->ec, RUBY_EVENT_THREAD_BEGIN, th->self, 0, 0, 0, Qundef);
 
     switch (th->invoke_type) {
       case thread_invoke_type_proc:
+        result = thread_do_start_proc(th);
+        break;
+
       case thread_invoke_type_ractor_proc:
-        thread_do_start_proc(th);
+        result = thread_do_start_proc(th);
+        rb_ractor_atexit(th->ec, result);
         break;
+
       case thread_invoke_type_func:
-        th->value = (*th->invoke_arg.func.func)(th->invoke_arg.func.arg);
+        result = (*th->invoke_arg.func.func)(th->invoke_arg.func.arg);
         break;
+
       case thread_invoke_type_none:
         rb_bug("unreachable");
     }
 
     rb_fiber_scheduler_set(Qnil);
+
+    th->value = result;
+
+    EXEC_EVENT_HOOK(th->ec, RUBY_EVENT_THREAD_END, th->self, 0, 0, 0, Qundef);
 }
 
 void rb_ec_clear_current_thread_trace_func(const rb_execution_context_t *ec);
@@ -817,6 +826,9 @@ 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 +869,12 @@ 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 +892,6 @@ 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 +1168,12 @@ 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 +1200,7 @@ 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) {
@@ -3319,11 +3340,11 @@ rb_thread_status(VALUE thread)
 static VALUE
 rb_thread_alive_p(VALUE thread)
 {
-    if (rb_threadptr_dead(rb_thread_ptr(thread))) {
-	return Qfalse;
+    if (thread_finished(rb_thread_ptr(thread))) {
+        return Qfalse;
     }
     else {
-	return Qtrue;
+        return Qtrue;
     }
 }
 

vm.c

@@ -3075,6 +3075,8 @@ th_init(rb_thread_t *th, VALUE self)
     th->thread_id_string[0] = '\0';
 #endif
 
+    th->value = Qundef;
+
 #if OPT_CALL_THREADED_CODE
     th->retval = Qundef;
 #endif
@@ -3087,16 +3089,17 @@ static VALUE
 ruby_thread_init(VALUE self)
 {
     rb_thread_t *th = GET_THREAD();
-    rb_thread_t *targe_th = rb_thread_ptr(self);
+    rb_thread_t *target_th = rb_thread_ptr(self);
     rb_vm_t *vm = th->vm;
 
-    targe_th->vm = vm;
-    th_init(targe_th, self);
+    target_th->vm = vm;
+    th_init(target_th, self);
+
+    target_th->top_wrapper = 0;
+    target_th->top_self = rb_vm_top_self();
+    target_th->ec->root_svar = Qfalse;
+    target_th->ractor = th->ractor;
 
-    targe_th->top_wrapper = 0;
-    targe_th->top_self = rb_vm_top_self();
-    targe_th->ec->root_svar = Qfalse;
-    targe_th->ractor = th->ractor;
     return self;
 }