Add a Twisted Web-based renderer and start using it on Python 3

This could be the thing that eventually replaces the Nevow-based testing
renderer on Python 2 as well.

src/allmydata/test/common_nevow.py

@@ -0,0 +1,46 @@
+"""
+General helpers related to Nevow.
+"""
+
+from nevow.context import WebContext
+from nevow.testutil import FakeRequest
+from nevow.appserver import (
+    processingFailed,
+    DefaultExceptionHandler,
+)
+from nevow.inevow import (
+    ICanHandleException,
+    IRequest,
+    IResource as INevowResource,
+    IData,
+)
+
+def render(resource, query_args):
+    """
+    Render (in the manner of the Nevow appserver) a Nevow ``Page`` or a
+    Twisted ``Resource`` against a request with the given query arguments .
+
+    :param resource: The page or resource to render.
+
+    :param query_args: The query arguments to put into the request being
+        rendered.  A mapping from ``bytes`` to ``list`` of ``bytes``.
+
+    :return Deferred: A Deferred that fires with the rendered response body as
+        ``bytes``.
+    """
+    ctx = WebContext(tag=resource)
+    req = FakeRequest(args=query_args)
+    ctx.remember(DefaultExceptionHandler(), ICanHandleException)
+    ctx.remember(req, IRequest)
+    ctx.remember(None, IData)
+
+    def maybe_concat(res):
+        if isinstance(res, bytes):
+            return req.v + res
+        return req.v
+
+    resource = INevowResource(resource)
+    d = maybeDeferred(resource.renderHTTP, ctx)
+    d.addErrback(processingFailed, req, ctx)
+    d.addCallback(maybe_concat)
+    return d

src/allmydata/test/common_tweb.py

@@ -0,0 +1,54 @@
+from zope.interface import (
+    classImplements,
+)
+from twisted.python.reflect import (
+    fullyQualifiedName,
+)
+from twisted.internet.defer import (
+    succeed,
+)
+from twisted.web.test.requesthelper import (
+    DummyRequest,
+)
+from twisted.web.iweb import (
+    IRequest,
+)
+from twisted.web.server import (
+    NOT_DONE_YET,
+)
+
+classImplements(DummyRequest, IRequest)
+
+def render(resource, query_args):
+    """
+    Render (in the manner of the Twisted Web Site) a Twisted ``Resource``
+    against a request with the given query arguments .
+
+    :param resource: The page or resource to render.
+
+    :param query_args: The query arguments to put into the request being
+        rendered.  A mapping from ``bytes`` to ``list`` of ``bytes``.
+
+    :return Deferred: A Deferred that fires with the rendered response body as
+        ``bytes``.
+    """
+    request = DummyRequest([])
+    request.args = query_args
+    result = resource.render(request)
+    if isinstance(result, bytes):
+        request.write(result)
+        done = succeed(None)
+    elif result == NOT_DONE_YET:
+        if request.finished:
+            done = succeed(None)
+        else:
+            done = request.notifyFinish()
+    else:
+        raise ValueError(
+            "{!r} returned {!r}, required bytes or NOT_DONE_YET.".format(
+                fullyQualifiedName(resource.render),
+                result,
+            ),
+        )
+    done.addCallback(lambda ignored: b"".join(request.written))
+    return done

src/allmydata/test/common_web.py

@@ -1,3 +1,4 @@
+from future.utils import PY2
 
 import treq
 from twisted.internet.defer import (
@@ -7,19 +8,6 @@ from twisted.internet.defer import (
 )
 from twisted.web.error import Error
 
-from nevow.context import WebContext
-from nevow.testutil import FakeRequest
-from nevow.appserver import (
-    processingFailed,
-    DefaultExceptionHandler,
-)
-from nevow.inevow import (
-    ICanHandleException,
-    IRequest,
-    IResource as INevowResource,
-    IData,
-)
-
 @inlineCallbacks
 def do_http(method, url, **kwargs):
     response = yield treq.request(method, url, persistent=False, **kwargs)
@@ -31,32 +19,16 @@ def do_http(method, url, **kwargs):
     returnValue(body)
 
 
-def render(resource, query_args):
-    """
-    Render (in the manner of the Nevow appserver) a Nevow ``Page`` or a
-    Twisted ``Resource`` against a request with the given query arguments .
-
-    :param resource: The page or resource to render.
-
-    :param query_args: The query arguments to put into the request being
-        rendered.  A mapping from ``bytes`` to ``list`` of ``bytes``.
-
-    :return Deferred: A Deferred that fires with the rendered response body as
-        ``bytes``.
-    """
-    ctx = WebContext(tag=resource)
-    req = FakeRequest(args=query_args)
-    ctx.remember(DefaultExceptionHandler(), ICanHandleException)
-    ctx.remember(req, IRequest)
-    ctx.remember(None, IData)
-
-    def maybe_concat(res):
-        if isinstance(res, bytes):
-            return req.v + res
-        return req.v
-
-    resource = INevowResource(resource)
-    d = maybeDeferred(resource.renderHTTP, ctx)
-    d.addErrback(processingFailed, req, ctx)
-    d.addCallback(maybe_concat)
-    return d
+if PY2:
+    # We can only use Nevow on Python 2 and Tahoe-LAFS still *does* use Nevow
+    # so prefer the Nevow-based renderer if we can get it.
+    from .common_nevow import (
+        render,
+    )
+else:
+    # However, Tahoe-LAFS *will* use Twisted Web before too much longer so go
+    # ahead and let some tests run against the Twisted Web-based renderer on
+    # Python 3.  Later this will become the only codepath.
+    from .common_tweb import (
+        render,
+    )