3 files changed, 51 insertions, 1 deletions
diff --git a/internal/thread.h b/internal/thread.h
index a2926febc3..e079ebb22b 100644
--- a/internal/thread.h
+++ b/internal/thread.h
@@ -60,6 +60,7 @@ int rb_thread_wait_for_single_fd(int fd, int events, struct timeval * timeout);
 struct rb_io_close_wait_list {
     struct ccan_list_head pending_fd_users;
     VALUE closing_thread;
+    VALUE closing_fiber;
     VALUE wakeup_mutex;
 };
 int rb_notify_fd_close(int fd, struct rb_io_close_wait_list *busy);
diff --git a/test/fiber/test_io.rb b/test/fiber/test_io.rb
index 0e3e086d5a..7973399acb 100644
--- a/test/fiber/test_io.rb
+++ b/test/fiber/test_io.rb
@@ -234,4 +234,47 @@ class TestFiberIO < Test::Unit::TestCase
 
     assert_equal "ok\n", result
   end
+
+  # Tests for https://bugs.ruby-lang.org/issues/20723 which would
+  # otherwise deadlock this test.
+  def test_close_while_reading_on_thread
+    # Windows has UNIXSocket, but only with VS 2019+
+    omit "UNIXSocket is not defined!" unless defined?(UNIXSocket)
+
+    i, o = Socket.pair(:UNIX, :STREAM)
+    if RUBY_PLATFORM=~/mswin|mingw/
+      i.nonblock = true
+      o.nonblock = true
+    end
+
+    message = nil
+
+    reading_thread = Thread.new do
+      Thread.current.report_on_exception = false
+      i.wait_readable
+    end
+
+    fs_thread = Thread.new do
+      # Wait until the reading thread is blocked on read:
+      Thread.pass until reading_thread.status == "sleep"
+
+      scheduler = Scheduler.new
+      Fiber.set_scheduler scheduler
+      Fiber.schedule do
+        i.close
+      end
+    end
+
+    assert_raise(IOError) { reading_thread.join }
+    refute_nil fs_thread.join(5), "expected thread to terminate within 5 seconds"
+
+    assert_predicate(i, :closed?)
+  ensure
+    fs_thread&.kill
+    fs_thread&.join rescue nil
+    reading_thread&.kill
+    reading_thread&.join rescue nil
+    i&.close
+    o&.close
+  end
 end
diff --git a/thread.c b/thread.c
index 5a79200785..2a937ca278 100644
--- a/thread.c
+++ b/thread.c
@@ -1698,7 +1698,12 @@ thread_io_wake_pending_closer(struct waiting_fd *wfd)
     RB_VM_LOCK_LEAVE();
 
     if (has_waiter) {
-        rb_thread_wakeup(wfd->busy->closing_thread);
+        rb_thread_t *th = rb_thread_ptr(wfd->busy->closing_thread);
+        if (th->scheduler != Qnil) {
+            rb_fiber_scheduler_unblock(th->scheduler, wfd->busy->closing_thread, wfd->busy->closing_fiber);
+        } else {
+            rb_thread_wakeup(wfd->busy->closing_thread);
+        }
         rb_mutex_unlock(wfd->busy->wakeup_mutex);
     }
 }
@@ -2625,6 +2630,7 @@ rb_notify_fd_close(int fd, struct rb_io_close_wait_list *busy)
 
     has_any = !ccan_list_empty(&busy->pending_fd_users);
     busy->closing_thread = rb_thread_current();
+    busy->closing_fiber = rb_fiber_current();
     wakeup_mutex = Qnil;
     if (has_any) {
         wakeup_mutex = rb_mutex_new();