bookmark: add a 'check:bookmarks' bundle2 part

This part checks that bookmarks are still at the node they are expected to be.
This allows a pushing client to detect push race where the repository was
updated between the time it discovered the server state and the time it managed
to finish its push.

Such checking already exists when pushing bookmark through pushkey. This new
part can be inserted at the beginning of the bundle, triggering abort earlier.
In addition, we would like to move away from pushey to push bookmark. A step
useful to solve issue5165.

mercurial/bundle2.py

@@ -156,6 +156,7 b' import sys'
 
 from .i18n import _
 from . import (
+    bookmarks,
     changegroup,
     error,
     node as nodemod,
@@ -1787,6 +1788,34 b' def handlereplychangegroup(op, inpart):'
     replyto = int(inpart.params['in-reply-to'])
     op.records.add('changegroup', {'return': ret}, replyto)
 
+@parthandler('check:bookmarks')
+def handlecheckbookmarks(op, inpart):
+    """check location of bookmarks
+
+    This part is to be used to detect push race regarding bookmark, it
+    contains binary encoded (bookmark, node) tuple. If the local state does
+    not marks the one in the part, a PushRaced exception is raised
+    """
+    bookdata = bookmarks.binarydecode(inpart)
+
+    msgstandard = ('repository changed while pushing - please try again '
+                   '(bookmark "%s" move from %s to %s)')
+    msgmissing = ('repository changed while pushing - please try again '
+                  '(bookmark "%s" is missing, expected %s)')
+    msgexist = ('repository changed while pushing - please try again '
+                '(bookmark "%s" set on %s, expected missing)')
+    for book, node in bookdata:
+        currentnode = op.repo._bookmarks.get(book)
+        if currentnode != node:
+            if node is None:
+                finalmsg = msgexist % (book, nodemod.short(currentnode))
+            elif currentnode is None:
+                finalmsg = msgmissing % (book, nodemod.short(node))
+            else:
+                finalmsg = msgstandard % (book, nodemod.short(node),
+                                          nodemod.short(currentnode))
+            raise error.PushRaced(finalmsg)
+
 @parthandler('check:heads')
 def handlecheckheads(op, inpart):
     """check that head of the repo did not change