diff --git a/tornado/ioloop.py b/tornado/ioloop.py
index cadb4116..d6183176 100644
--- a/tornado/ioloop.py
+++ b/tornado/ioloop.py
@@ -616,10 +616,14 @@ class IOLoop(Configurable):
                     # result, which should just be ignored.
                     pass
                 else:
-                    self.add_future(ret, lambda f: f.result())
+                    self.add_future(ret, self._discard_future_result)
         except Exception:
             self.handle_callback_exception(callback)
 
+    def _discard_future_result(self, future):
+        """Avoid unhandled-exception warnings from spawned coroutines."""
+        future.result()
+
     def handle_callback_exception(self, callback):
         """This method is called whenever a callback run by the `IOLoop`
         throws an exception.

Excessive CPU load when returning empty collection
The following causes my CPU to stay pinned at 100% usage

```python
from tornado.ioloop import IOLoop
from tornado import gen
from threading import Thread

loop = IOLoop.current()
t = Thread(target=loop.start, daemon=True)
t.start()

@gen.coroutine
def f():
    yield gen.sleep(0)
    return {}

loop.add_callback(f)

from time import sleep  # just to stick around in case you run this in a script
sleep(100)
```

Replacing the empty dict `{}` with an empty list `[]` has the same effect.  Replacing with `None` behaves as expected.

Running Python 3.5, Tornado 4.4.1, on Ubuntu 16.04
diff --git a/tornado/test/ioloop_test.py b/tornado/test/ioloop_test.py
index 8570e73f..1bb8ce08 100644
--- a/tornado/test/ioloop_test.py
+++ b/tornado/test/ioloop_test.py
@@ -9,6 +9,7 @@ import socket
 import sys
 import threading
 import time
+import types
 
 from tornado import gen
 from tornado.ioloop import IOLoop, TimeoutError, PollIOLoop, PeriodicCallback
@@ -61,6 +62,25 @@ class FakeTimeIOLoop(PollIOLoop):
 
 
 class TestIOLoop(AsyncTestCase):
+    def test_add_callback_return_sequence(self):
+        # A callback returning {} or [] shouldn't spin the CPU, see Issue #1803.
+        self.calls = 0
+
+        loop = self.io_loop
+        test = self
+        old_add_callback = loop.add_callback
+
+        def add_callback(self, callback, *args, **kwargs):
+            test.calls += 1
+            old_add_callback(callback, *args, **kwargs)
+
+        loop.add_callback = types.MethodType(add_callback, loop)
+        loop.add_callback(lambda: {})
+        loop.add_callback(lambda: [])
+        loop.add_timeout(datetime.timedelta(milliseconds=50), loop.stop)
+        loop.start()
+        self.assertLess(self.calls, 10)
+
     @skipOnTravis
     def test_add_callback_wakeup(self):
         # Make sure that add_callback from inside a running IOLoop

pip install -e .[dev]
parse_log_pytest
pytest
sphinx sphinx_rtd_theme pytest
apt-get update apt-get install -y gcc
3.6
pytest --no-header -rA --tb=line --color=no -p no:cacheprovider -W ignore::DeprecationWarning
alabaster==0.7.13
attrs @ file:///opt/conda/conda-bld/attrs_1642510447205/work
Babel==2.11.0
certifi==2021.5.30
charset-normalizer==2.0.12
docutils==0.18.1
idna==3.10
imagesize==1.4.1
importlib-metadata @ file:///tmp/build/80754af9/importlib-metadata_1631916693255/work
iniconfig @ file:///home/linux1/recipes/ci/iniconfig_1610983019677/work
Jinja2==3.0.3
MarkupSafe==2.0.1
more-itertools @ file:///tmp/build/80754af9/more-itertools_1637733554872/work
packaging @ file:///tmp/build/80754af9/packaging_1637314298585/work
pluggy @ file:///tmp/build/80754af9/pluggy_1615976315926/work
py @ file:///opt/conda/conda-bld/py_1644396412707/work
Pygments==2.14.0
pyparsing @ file:///tmp/build/80754af9/pyparsing_1635766073266/work
pytest==6.2.4
pytz==2025.2
requests==2.27.1
snowballstemmer==2.2.0
Sphinx==5.3.0
sphinx-rtd-theme==2.0.0
sphinxcontrib-applehelp==1.0.2
sphinxcontrib-devhelp==1.0.2
sphinxcontrib-htmlhelp==2.0.0
sphinxcontrib-jquery==4.1
sphinxcontrib-jsmath==1.0.1
sphinxcontrib-qthelp==1.0.3
sphinxcontrib-serializinghtml==1.1.5
toml @ file:///tmp/build/80754af9/toml_1616166611790/work
-e git+https://github.com/tornadoweb/tornado.git@bec14a9152438fbf1b9584dc3da4500137577308#egg=tornado
typing_extensions @ file:///opt/conda/conda-bld/typing_extensions_1647553014482/work
urllib3==1.26.20
zipp @ file:///tmp/build/80754af9/zipp_1633618647012/work

name: tornado
channels:
  - defaults
  - https://repo.anaconda.com/pkgs/main
  - https://repo.anaconda.com/pkgs/r
  - conda-forge
dependencies:
  - _libgcc_mutex=0.1=main
  - _openmp_mutex=5.1=1_gnu
  - attrs=21.4.0=pyhd3eb1b0_0
  - ca-certificates=2025.2.25=h06a4308_0
  - certifi=2021.5.30=py36h06a4308_0
  - importlib-metadata=4.8.1=py36h06a4308_0
  - importlib_metadata=4.8.1=hd3eb1b0_0
  - iniconfig=1.1.1=pyhd3eb1b0_0
  - ld_impl_linux-64=2.40=h12ee557_0
  - libffi=3.3=he6710b0_2
  - libgcc-ng=11.2.0=h1234567_1
  - libgomp=11.2.0=h1234567_1
  - libstdcxx-ng=11.2.0=h1234567_1
  - more-itertools=8.12.0=pyhd3eb1b0_0
  - ncurses=6.4=h6a678d5_0
  - openssl=1.1.1w=h7f8727e_0
  - packaging=21.3=pyhd3eb1b0_0
  - pip=21.2.2=py36h06a4308_0
  - pluggy=0.13.1=py36h06a4308_0
  - py=1.11.0=pyhd3eb1b0_0
  - pyparsing=3.0.4=pyhd3eb1b0_0
  - pytest=6.2.4=py36h06a4308_2
  - python=3.6.13=h12debd9_1
  - readline=8.2=h5eee18b_0
  - setuptools=58.0.4=py36h06a4308_0
  - sqlite=3.45.3=h5eee18b_0
  - tk=8.6.14=h39e8969_0
  - toml=0.10.2=pyhd3eb1b0_0
  - typing_extensions=4.1.1=pyh06a4308_0
  - wheel=0.37.1=pyhd3eb1b0_0
  - xz=5.6.4=h5eee18b_1
  - zipp=3.6.0=pyhd3eb1b0_0
  - zlib=1.2.13=h5eee18b_1
  - pip:
      - alabaster==0.7.13
      - babel==2.11.0
      - charset-normalizer==2.0.12
      - docutils==0.18.1
      - idna==3.10
      - imagesize==1.4.1
      - jinja2==3.0.3
      - markupsafe==2.0.1
      - pygments==2.14.0
      - pytz==2025.2
      - requests==2.27.1
      - snowballstemmer==2.2.0
      - sphinx==5.3.0
      - sphinx-rtd-theme==2.0.0
      - sphinxcontrib-applehelp==1.0.2
      - sphinxcontrib-devhelp==1.0.2
      - sphinxcontrib-htmlhelp==2.0.0
      - sphinxcontrib-jquery==4.1
      - sphinxcontrib-jsmath==1.0.1
      - sphinxcontrib-qthelp==1.0.3
      - sphinxcontrib-serializinghtml==1.1.5
      - urllib3==1.26.20
prefix: /opt/conda/envs/tornado

tornado/test/ioloop_test.py::TestIOLoop::test_add_callback_return_sequence
tornado/test/ioloop_test.py::TestIOLoop::test_add_callback_from_signal tornado/test/ioloop_test.py::TestIOLoop::test_add_callback_from_signal_other_thread tornado/test/ioloop_test.py::TestIOLoop::test_add_callback_wakeup tornado/test/ioloop_test.py::TestIOLoop::test_add_callback_wakeup_other_thread tornado/test/ioloop_test.py::TestIOLoop::test_add_callback_while_closing tornado/test/ioloop_test.py::TestIOLoop::test_add_timeout_return tornado/test/ioloop_test.py::TestIOLoop::test_add_timeout_timedelta tornado/test/ioloop_test.py::TestIOLoop::test_call_at_return tornado/test/ioloop_test.py::TestIOLoop::test_call_later_return tornado/test/ioloop_test.py::TestIOLoop::test_close_file_object tornado/test/ioloop_test.py::TestIOLoop::test_exception_logging tornado/test/ioloop_test.py::TestIOLoop::test_exception_logging_future tornado/test/ioloop_test.py::TestIOLoop::test_exception_logging_native_coro tornado/test/ioloop_test.py::TestIOLoop::test_handle_callback_exception tornado/test/ioloop_test.py::TestIOLoop::test_handler_callback_file_object tornado/test/ioloop_test.py::TestIOLoop::test_mixed_fd_fileobj tornado/test/ioloop_test.py::TestIOLoop::test_multiple_add tornado/test/ioloop_test.py::TestIOLoop::test_read_while_writeable tornado/test/ioloop_test.py::TestIOLoop::test_reentrant tornado/test/ioloop_test.py::TestIOLoop::test_remove_handler_from_handler tornado/test/ioloop_test.py::TestIOLoop::test_remove_timeout_after_fire tornado/test/ioloop_test.py::TestIOLoop::test_remove_timeout_cleanup tornado/test/ioloop_test.py::TestIOLoop::test_remove_timeout_from_timeout tornado/test/ioloop_test.py::TestIOLoop::test_remove_without_add tornado/test/ioloop_test.py::TestIOLoop::test_spawn_callback tornado/test/ioloop_test.py::TestIOLoop::test_timeout_with_arguments tornado/test/ioloop_test.py::TestIOLoopCurrent::test_default_current tornado/test/ioloop_test.py::TestIOLoopCurrent::test_force_current tornado/test/ioloop_test.py::TestIOLoopCurrent::test_non_current tornado/test/ioloop_test.py::TestIOLoopAddCallback::test_pre_wrap tornado/test/ioloop_test.py::TestIOLoopAddCallback::test_pre_wrap_with_args tornado/test/ioloop_test.py::TestIOLoopAddCallbackFromSignal::test_pre_wrap tornado/test/ioloop_test.py::TestIOLoopAddCallbackFromSignal::test_pre_wrap_with_args tornado/test/ioloop_test.py::TestIOLoopFutures::test_add_future_stack_context tornado/test/ioloop_test.py::TestIOLoopFutures::test_add_future_threads tornado/test/ioloop_test.py::TestIOLoopRunSync::test_async_exception tornado/test/ioloop_test.py::TestIOLoopRunSync::test_async_result tornado/test/ioloop_test.py::TestIOLoopRunSync::test_current tornado/test/ioloop_test.py::TestIOLoopRunSync::test_native_coroutine tornado/test/ioloop_test.py::TestIOLoopRunSync::test_sync_exception tornado/test/ioloop_test.py::TestIOLoopRunSync::test_sync_result tornado/test/ioloop_test.py::TestIOLoopRunSync::test_timeout tornado/test/ioloop_test.py::TestPeriodicCallback::test_basic tornado/test/ioloop_test.py::TestPeriodicCallback::test_overrun
tornado/ioloop.py
tornado/ioloop.py