Handle interrupted connections

src/allmydata/test/web/test_common.py

@@ -13,13 +13,22 @@ from testtools.matchers import (
     Equals,
     Contains,
     MatchesPredicate,
+    AfterPreprocessing,
 )
 from testtools.twistedsupport import (
+    failed,
     succeeded,
     has_no_result,
 )
 
+from twisted.python.failure import (
+    Failure,
+)
+from twisted.internet.error import (
+    ConnectionDone,
+)
 from twisted.internet.defer import (
+    Deferred,
     fail,
 )
 from twisted.web.server import (
@@ -52,9 +61,11 @@ class StaticResource(Resource, object):
     def __init__(self, response):
         Resource.__init__(self)
         self._response = response
+        self._request = None
 
     @render_exception
     def render(self, request):
+        self._request = request
         return self._response
 
 
@@ -214,3 +225,25 @@ class RenderExceptionTests(SyncTestCase):
                 Equals(b"Internal Server Error"),
             ),
         )
+
+    def test_disconnected(self):
+        """
+        If the transport is disconnected before the response is available, nothing
+        is written to the request.
+        """
+        result = Deferred()
+        resource = StaticResource(result)
+        d = render(resource, {})
+
+        resource._request.connectionLost(Failure(ConnectionDone()))
+        result.callback(b"Some result")
+
+        self.assertThat(
+            d,
+            failed(
+                AfterPreprocessing(
+                    lambda reason: reason.type,
+                    Equals(ConnectionDone),
+                ),
+            ),
+        )

src/allmydata/web/common.py

@@ -41,6 +41,7 @@ from twisted.python.failure import (
     Failure,
 )
 from twisted.internet.defer import (
+    CancelledError,
     maybeDeferred,
 )
 from twisted.web.resource import (
@@ -472,7 +473,17 @@ def render_exception(render):
             # Apply `_finish` all of our result handling logic to whatever it
             # returned.
             result.addBoth(_finish, bound_render, request)
-            result.addActionFinish()
+            d = result.addActionFinish()
+
+        # If the connection is lost then there's no point running our _finish
+        # logic because it has nowhere to send anything.  There may also be no
+        # point in finishing whatever operation was being performed because
+        # the client cannot be informed of its result.  Also, Twisted Web
+        # raises exceptions from some Request methods if they're used after
+        # the connection is lost.
+        request.notifyFinish().addErrback(
+            lambda ignored: d.cancel(),
+        )
         return NOT_DONE_YET
 
     return g
@@ -497,6 +508,8 @@ def _finish(result, render, request):
     :return: ``None``
     """
     if isinstance(result, Failure):
+        if result.check(CancelledError):
+            return
         Message.log(
             message_type=u"allmydata:web:common-render:failure",
             message=result.getErrorMessage(),

src/allmydata/web/filenode.py

@@ -480,7 +480,10 @@ class FileDownloader(Resource, object):
         d = self.filenode.read(req, first, size)
 
         def _error(f):
-            req._tahoe_request_had_error = f # for HTTP-style logging
+            if f.check(defer.CancelledError):
+                # The HTTP connection was lost and we no longer have anywhere
+                # to send our result.  Let this pass through.
+                return f
             if req.startedWriting:
                 # The content-type is already set, and the response code has
                 # already been sent, so we can't provide a clean error