Index: twisted/web2/static.py
===================================================================
--- twisted/web2/static.py      (revision 15884)
+++ twisted/web2/static.py      (working copy)
@@ -17,7 +17,7 @@
 
 # Twisted Imports
 from twisted.python import filepath
-from twisted.internet.defer import succeed, maybeDeferred
+from twisted.internet.defer import maybeDeferred
 from zope.interface import implements
 
 class MetaDataMixin(object):
@@ -74,6 +74,32 @@
         return True
 
 class StaticRenderMixin(resource.RenderMixin, MetaDataMixin):
+    def checkPreconditions(self, request):
+        #
+        # http.checkPreconditions() gets called by the server after every
+        # request.  Unfortunately, we need to know to bail out before request
+        # processing, especially for methods that modify server state (eg. PUT).
+        # We also would like to do so even for methods that don't, if those
+        # methods might be expensive to process.
+        #
+        # Unfortunately, http.checkPreconditions() doesn't work for GET and HEAD
+        # unless it is given a response object, so we won't check those here and
+        # leave it for later.
+        #
+        # Also unfortunate, having checked precoditions here for all other
+        # methods, the server is going to check them again after processing,
+        # which is, at best, redundant.
+        #
+        if request.method not in ("GET", "HEAD"):
+            http.checkPreconditions(
+                request,
+                entityExists = self.exists(),
+                etag         = self.etag(),
+                lastModified = self.lastModified(),
+            )
+
+        super(StaticRenderMixin, self).checkPreconditions(request)
+
     def renderHTTP(self, request):
         """
         See L{resource.RenderMixIn.renderHTTP}.
Index: twisted/web2/resource.py
===================================================================
--- twisted/web2/resource.py    (revision 15882)
+++ twisted/web2/resource.py    (working copy)
@@ -25,6 +25,15 @@
             self._allowed_methods = tuple([name[5:] for name in dir(self) if name.startswith('http_')])
         return self._allowed_methods
 
+    def checkPreconditions(self, request):
+        """
+        Checks all preconditions imposed by this resource upon a request made
+        against it.
+        @param request: the request to process.
+        @raise http.HTTPError: if any precondition fails.
+        """
+        pass
+
     def renderHTTP(self, request):
         """
         See L{iweb.IResource.renderHTTP}.
@@ -52,6 +61,11 @@
             response.headers.setHeader("allow", self.allowedMethods())
             return response
 
+        try:
+            self.checkPreconditions(request)
+        except http.HTTPError, e:
+            return e.response
+
         return method(request)
 
     def http_OPTIONS(self, request):