diff -r 261d37b94d31 -r 189e06b2d719 mercurial/revlogutils/deltas.py
--- a/mercurial/revlogutils/deltas.py	Wed Jan 30 18:15:38 2019 +0100
+++ b/mercurial/revlogutils/deltas.py	Thu Jan 24 18:22:47 2019 -0500
@@ -755,7 +755,8 @@
     The group order aims at providing fast or small candidates first.
     """
     gdelta = revlog._generaldelta
-    sparse = revlog._sparserevlog
+    # gate sparse behind general-delta because of issue6056
+    sparse = gdelta and revlog._sparserevlog
     curr = len(revlog)
     prev = curr - 1
     deltachain = lambda rev: revlog._deltachain(rev)[0]