--- a/mercurial/bundle2.py	Fri Jun 30 03:44:08 2017 +0200
+++ b/mercurial/bundle2.py	Fri Jun 30 03:44:09 2017 +0200
@@ -1343,7 +1343,7 @@
         caps['obsmarkers'] = supportedformat
     if allowpushback:
         caps['pushback'] = ()
-    cpmode = repo.ui.config('server', 'concurrent-push-mode', 'strict')
+    cpmode = repo.ui.config('server', 'concurrent-push-mode')
     if cpmode == 'check-related':
         caps['checkheads'] = ('related',)
     return caps

--- a/mercurial/configitems.py	Fri Jun 30 03:44:08 2017 +0200
+++ b/mercurial/configitems.py	Fri Jun 30 03:44:09 2017 +0200
@@ -122,6 +122,9 @@
 coreconfigitem('server', 'compressionengines',
     default=list,
 )
+coreconfigitem('server', 'concurrent-push-mode',
+    default='strict',
+)
 coreconfigitem('ui', 'clonebundleprefers',
     default=list,
 )