Fix: start_response() should only re-raise exc_info if headers have been written

2 files changed, 16 insertions, 2 deletions

diff --git a/waitress/task.py b/waitress/task.py
index a4c8f2e..ab3282f 100644
--- a/waitress/task.py
+++ b/waitress/task.py
@@ -346,7 +346,7 @@ class WSGITask(Task):
                                      "without providing exc_info.")
             if exc_info:
                 try:
-                    if self.complete:
+                    if self.wrote_header:
                         # higher levels will catch and handle raised exception:
                         # 1. "service" method in task.py
                         # 2. "service" method in channel.py
diff --git a/waitress/tests/test_task.py b/waitress/tests/test_task.py
index 8cae8dc..5bdd270 100644
--- a/waitress/tests/test_task.py
+++ b/waitress/tests/test_task.py
@@ -356,10 +356,14 @@ class TestWSGITask(unittest.TestCase):
     def test_execute_app_calls_start_response_w_exc_info_complete(self):
         def app(environ, start_response):
             start_response('200 OK', [], [ValueError, ValueError(), None])
+            return [b'a']
         inst = self._makeOne()
         inst.complete = True
         inst.channel.server.application = app
-        self.assertRaises(ValueError, inst.execute)
+        inst.execute()
+        self.assertTrue(inst.complete)
+        self.assertEqual(inst.status, '200 OK')
+        self.assertTrue(inst.channel.written)
 
     def test_execute_app_calls_start_response_w_exc_info_incomplete(self):
         def app(environ, start_response):
@@ -373,6 +377,16 @@ class TestWSGITask(unittest.TestCase):
         self.assertEqual(inst.status, '200 OK')
         self.assertTrue(inst.channel.written)
 
+    def test_execute_app_calls_start_response_w_header_written(self):
+        def app(environ, start_response):
+            start_response('200 OK', [], [ValueError, ValueError(), None])
+            return [b'a']
+        inst = self._makeOne()
+        inst.complete = True
+        inst.wrote_header = True
+        inst.channel.server.application = app
+        self.assertRaises(ValueError, inst.execute)
+
     def test_execute_bad_header_key(self):
         def app(environ, start_response):
             start_response('200 OK', [(None, 'a')])