DAV: Fix behavior when PUT doesn't return ETag (#476)

Thanks to @evert for pointing this out in the #sabredav IRC channel.

tests/test_sync.py

@@ -404,8 +404,11 @@ class SyncMachine(RuleBasedStateMachine):
     def _get_items(storage):
         return sorted(item.raw for etag, item in storage.items.values())
 
-    @rule(target=Storage, read_only=st.booleans(), flaky_etags=st.booleans())
-    def newstorage(self, read_only, flaky_etags):
+    @rule(target=Storage,
+          read_only=st.booleans(),
+          flaky_etags=st.booleans(),
+          null_etag_on_upload=st.booleans())
+    def newstorage(self, read_only, flaky_etags, null_etag_on_upload):
         s = MemoryStorage()
         s.read_only = read_only
         if flaky_etags:
@@ -416,6 +419,12 @@ class SyncMachine(RuleBasedStateMachine):
                 return item, etag
             s.get = get
 
+        if null_etag_on_upload:
+            _old_upload = s.upload
+            _old_update = s.update
+            s.upload = lambda item: (_old_upload(item)[0], None)
+            s.update = lambda h, i, e: _old_update(h, i, e) and None
+
         return s
 
     @rule(target=Status)

vdirsyncer/storage/dav.py

@@ -465,16 +465,24 @@ class DavStorage(Storage):
             data=item.raw.encode('utf-8'),
             headers=headers
         )
+
+        # The server may not return an etag under certain conditions:
+        #
+        #   An origin server MUST NOT send a validator header field (Section
+        #   7.2), such as an ETag or Last-Modified field, in a successful
+        #   response to PUT unless the request's representation data was saved
+        #   without any transformation applied to the body (i.e., the
+        #   resource's new representation data is identical to the
+        #   representation data received in the PUT request) and the validator
+        #   field value reflects the new representation.
+        #
+        # -- https://tools.ietf.org/html/rfc7231#section-4.3.4
+        #
+        # In such cases we return None as etag. The next synchronization will
+        # then detect an etag change (None != some string) and will download
+        # the new item.
         etag = response.headers.get('etag', None)
         href = self._normalize_href(response.url)
-        if not etag:
-            # The server violated the RFC and didn't send an etag. This is
-            # technically a race-condition, but too many popular servers do it.
-            #
-            # ownCloud: https://github.com/owncloud/contacts/issues/920
-            dav_logger.debug('Server did not send etag, fetching {!r}'
-                             .format(href))
-            item2, etag = self.get(href)
         return href, etag
 
     def update(self, href, item, etag):