--- a/mercurial/bundle2.py	Tue Sep 19 22:01:31 2017 +0200
+++ b/mercurial/bundle2.py	Tue Sep 19 22:23:41 2017 +0200
@@ -1836,23 +1836,10 @@
                 kwargs[key] = inpart.params[key]
         raise error.PushkeyFailed(partid=str(inpart.id), **kwargs)
 
-def _readphaseheads(inpart):
-    headsbyphase = [[] for i in phases.allphases]
-    entrysize = phases._fphasesentry.size
-    while True:
-        entry = inpart.read(entrysize)
-        if len(entry) < entrysize:
-            if entry:
-                raise error.Abort(_('bad phase-heads bundle part'))
-            break
-        phase, node = phases._fphasesentry.unpack(entry)
-        headsbyphase[phase].append(node)
-    return headsbyphase
-
 @parthandler('phase-heads')
 def handlephases(op, inpart):
     """apply phases from bundle part to repo"""
-    headsbyphase = _readphaseheads(inpart)
+    headsbyphase = phases.binarydecode(inpart)
     phases.updatephases(op.repo.unfiltered(), op.gettransaction(), headsbyphase)
     op.records.add('phase-heads', {})
 

--- a/mercurial/debugcommands.py	Tue Sep 19 22:01:31 2017 +0200
+++ b/mercurial/debugcommands.py	Tue Sep 19 22:23:41 2017 +0200
@@ -310,7 +310,7 @@
 def _debugphaseheads(ui, data, indent=0):
     """display version and markers contained in 'data'"""
     indent_string = ' ' * indent
-    headsbyphase = bundle2._readphaseheads(data)
+    headsbyphase = phases.binarydecode(data)
     for phase in phases.allphases:
         for head in headsbyphase[phase]:
             ui.write(indent_string)

--- a/mercurial/phases.py	Tue Sep 19 22:01:31 2017 +0200
+++ b/mercurial/phases.py	Tue Sep 19 22:23:41 2017 +0200
@@ -169,6 +169,22 @@
             binarydata.append(_fphasesentry.pack(phase, head))
     return ''.join(binarydata)
 
+def binarydecode(stream):
+    """decode a binary stream into a 'phase -> nodes' mapping
+
+    Since phases are integer the mapping is actually a python list."""
+    headsbyphase = [[] for i in allphases]
+    entrysize = _fphasesentry.size
+    while True:
+        entry = stream.read(entrysize)
+        if len(entry) < entrysize:
+            if entry:
+                raise error.Abort(_('bad phase-heads stream'))
+            break
+        phase, node = _fphasesentry.unpack(entry)
+        headsbyphase[phase].append(node)
+    return headsbyphase
+
 def _trackphasechange(data, rev, old, new):
     """add a phase move the <data> dictionnary