`Ractor::Selector#empty?`

It returns the waiting set is empty or not.

Also add Ractor::Selector's tests.

bootstraptest/test_ractor.rb

@@ -1617,4 +1617,100 @@ assert_match /\Atest_ractor\.rb:1:\s+warning:\s+Ractor is experimental/, %q{
   eval("Ractor.new{}.take", nil, "test_ractor.rb", 1)
 }
 
+## Ractor::Selector
+
+# Selector#empty? returns true
+assert_equal 'true', %q{
+  s = Ractor::Selector.new
+  s.empty?
+}
+
+# Selector#empty? returns false if there is target ractors
+assert_equal 'false', %q{
+  s = Ractor::Selector.new
+  s.add Ractor.new{}
+  s.empty?
+}
+
+# Selector#clear removes all ractors from the waiting list
+assert_equal 'true', %q{
+  s = Ractor::Selector.new
+  s.add Ractor.new{10}
+  s.add Ractor.new{20}
+  s.clear
+  s.empty?
+}
+
+# Selector#wait can wait multiple ractors
+assert_equal '[10, 20, true]', %q{
+  s = Ractor::Selector.new
+  s.add Ractor.new{10}
+  s.add Ractor.new{20}
+  r, v = s.wait
+  vs = []
+  vs << v
+  r, v = s.wait
+  vs << v
+  [*vs.sort, s.empty?]
+}
+
+# Selector#wait can wait multiple ractors with receiving.
+assert_equal '30', %q{
+  RN = 30
+  rs = RN.times.map{
+    Ractor.new{ :v }
+  }
+  s = Ractor::Selector.new(*rs)
+
+  results = []
+  until s.empty?
+    results << s.wait
+
+    # Note that s.wait can raise an exception because other Ractors/Threads
+    # can take from the same ractors in the waiting set.
+    # In this case there is no other takers so `s.wait` doesn't raise an error.
+  end
+
+  results.size
+}
+
+# Selector#wait can support dynamic addition
+assert_equal '600', %q{
+  RN = 100
+  s = Ractor::Selector.new
+  rs = RN.times.map{
+    Ractor.new{
+      Ractor.main << Ractor.new{ Ractor.yield :v3; :v4 }
+      Ractor.main << Ractor.new{ Ractor.yield :v5; :v6 }
+      Ractor.yield :v1
+      :v2
+    }
+  }
+
+  rs.each{|r| s.add(r)}
+  h = {v1: 0, v2: 0, v3: 0, v4: 0, v5: 0, v6: 0}
+
+  loop do
+    case s.wait receive: true
+    in :receive, r
+      s.add r
+    in r, v
+      h[v] += 1
+      break if h.all?{|k, v| v == RN}
+    end
+  end
+
+  h.sum{|k, v| v}
+}
+
+# Selector should be GCed (free'ed) withtou trouble
+assert_equal 'ok', %q{
+  RN = 30
+  rs = RN.times.map{
+    Ractor.new{ :v }
+  }
+  s = Ractor::Selector.new(*rs)
+  :ok
+}
+
 end # if !ENV['GITHUB_WORKFLOW']

ractor.c

@@ -979,7 +979,7 @@ ractor_take_will_lock(rb_ractor_t *r, struct rb_ractor_basket *b)
 
     RACTOR_LOCK(r);
     {
-        taken =ractor_take_will(r, b);
+        taken = ractor_take_will(r, b);
     }
     RACTOR_UNLOCK(r);
 
@@ -1009,6 +1009,8 @@ ractor_register_take(rb_ractor_t *cr, rb_ractor_t *r, struct rb_ractor_basket *t
         }
         else if (!is_take && ractor_take_has_will(r)) {
             RUBY_DEBUG_LOG("has_will");
+            VM_ASSERT(config != NULL);
+            config->closed = true;
         }
         else if (r->sync.outgoing_port_closed) {
             closed = true;
@@ -1515,6 +1517,13 @@ ractor_selector_clear(rb_execution_context_t *ec, VALUE selv)
     return selv;
 }
 
+static VALUE
+ractor_selector_empty_p(rb_execution_context_t *ec, VALUE selv)
+{
+    struct rb_ractor_selector *s = RACTOR_SELECTOR_PTR(selv);
+    return s->take_ractors->num_entries == 0 ? Qtrue : Qfalse;
+}
+
 static int
 ractor_selector_wait_i(st_data_t key, st_data_t val, st_data_t dat)
 {
@@ -1687,6 +1696,10 @@ ractor_selector_wait(rb_execution_context_t *ec, VALUE selv, VALUE do_receivev,
           }
           break;
       }
+      case basket_type_will:
+        // no more messages
+        ractor_selector_remove(ec, selv, taken_basket.sender);
+        break;
       default:
         break;
     }

ractor.rb

@@ -428,6 +428,18 @@ class Ractor
       __builtin_ractor_selector_clear
     end
 
+    # call-seq:
+    #   selector.empty?
+    #
+    # Returns true if the number of ractors in the waiting set at the current time is zero.
+    #
+    # Note that even if <tt>#empty?</tt> returns false, the subsequent <tt>#wait</tt>
+    # may raise an exception because other ractors may close the target ractors.
+    #
+    def empty?
+      __builtin_ractor_selector_empty_p
+    end
+
     # call-seq:
     #   selector.wait(receive: false, yield_value: yield_value, move: false) -> [ractor or symbol, value]
     #