Extract etag computation so it can be customized or disabled.

tornado/web.py

@@ -598,10 +598,8 @@ class RequestHandler(object):
             if (self._status_code == 200 and
                 self.request.method in ("GET", "HEAD") and
                 "Etag" not in self._headers):
-                hasher = hashlib.sha1()
-                for part in self._write_buffer:
-                    hasher.update(part)
-                etag = '"%s"' % hasher.hexdigest()
+                etag = self.compute_etag()
+                if etag is not None:
                     inm = self.request.headers.get("If-None-Match")
                     if inm and inm.find(etag) != -1:
                         self._write_buffer = []
@@ -874,6 +872,17 @@ class RequestHandler(object):
     def reverse_url(self, name, *args):
         return self.application.reverse_url(name, *args)
 
+    def compute_etag(self):
+        """Computes the etag header to be used for this request.
+
+        May be overridden to provide custom etag implementations,
+        or may return None to disable tornado's default etag support.
+        """
+        hasher = hashlib.sha1()
+        for part in self._write_buffer:
+            hasher.update(part)
+        return '"%s"' % hasher.hexdigest()
+
     def _stack_context_handle_exception(self, type, value, traceback):
         try:
             # For historical reasons _handle_request_exception only takes