--- a/hgext3rd/evolve/obscache.py	Wed May 03 20:56:26 2017 +0200
+++ b/hgext3rd/evolve/obscache.py	Wed May 03 21:47:06 2017 +0200
@@ -13,6 +13,7 @@
 import errno
 
 from mercurial import (
+    error,
     localrepo,
     obsolete,
     phases,
@@ -20,6 +21,8 @@
     util,
 )
 
+from mercurial.i18n import _
+
 from . import (
     exthelper,
 )
@@ -160,6 +163,31 @@
 
     return True, startrev, startidx
 
+
+# XXX copied as is from Mercurial 4.2 and added the "offset" parameters
+@util.nogc
+def _readmarkers(data, offset=None):
+    """Read and enumerate markers from raw data"""
+    off = 0
+    diskversion = struct.unpack('>B', data[off:off + 1])[0]
+    if offset is None:
+        off += 1
+    else:
+        assert 1 <= offset
+        off = offset
+    if diskversion not in obsolete.formats:
+        raise error.Abort(_('parsing obsolete marker: unknown version %r')
+                          % diskversion)
+    return diskversion, obsolete.formats[diskversion][0](data, off)
+
+def markersfrom(obsstore, byteoffset, firstmarker):
+    if '_all' in vars(obsstore):
+        # if the data are in memory, just use that
+        return obsstore._all[firstmarker:]
+    else:
+        obsdata = obsstore.svfs.tryread('obsstore')
+        return _readmarkers(obsdata, byteoffset)[1]
+
 class obscache(object):
     """cache the "does a rev" is the precursors of some obsmarkers data
 
@@ -258,23 +286,10 @@
 
         # process the new obsmarkers
         if startidx is not None:
-            markers = repo.obsstore._all
-            # Note:
-            #
-            #   There are no actually needs to load the full obsstore here,
-            #   since we only read the latest ones.  We do it for simplicity in
-            #   the first implementation. Loading the full obsstore has a
-            #   performance cost and should go away in this case too. We have
-            #   two simples options for that:
-            #
-            #   1) provide and API to start reading markers from a byte offset
-            #      (we have that data in the cache key)
-            #
-            #   2) directly update the cache at a lower level, in the code
-            #      responsible for adding a markers.
-            #
-            #   Option 2 is probably a bit more invasive, but more solid on the long run
-            markers = [markers[i] for i in xrange(startidx, len(repo.obsstore))]
+            if startidx == 0: # all markers
+                markers = repo.obsstore._all
+            else:
+                markers = markersfrom(repo.obsstore, self._cachekey[3], startidx)
             self._updatemarkers(repo, markers)
 
         self._cachekey = getcachekey(repo)