bookmarks: prevent pushes of divergent bookmarks (foo@remote)

Before this change, such bookmarks are write-only: a client can push
them but not pull/read them. And because these bookmark can't be read,
even pushes are limited (for instance trying to delete such a bookmark
fails with a vanilla client because the client thinks the bookmark is
neither on the local nor the remote).

This change makes the server refuses such bookmarks, and for earlier
errors, makes the client refuse to send them.

I think the change of behavior is acceptable because I think this is a
bug in push/pull, and I don't think we change the behavior of `hg
unbundle`, because it doesn't seem that `hg bundle` ever store
bookmarks (and even if it did, it would seem weird anyway to try to
send divergent bookmarks).

Differential Revision: https://phab.mercurial-scm.org/D8117

mercurial/bookmarks.py

@@ -487,6 +487,8 b' def listbookmarks(repo):'
 
 
 def pushbookmark(repo, key, old, new):
+    if isdivergent(key):
+        return False
     if bookmarksinstore(repo):
         wlock = util.nullcontextmanager()
     else:

mercurial/bundle2.py

@@ -2368,6 +2368,11 b' def handlebookmark(op, inpart):'
                     b'prepushkey', throw=True, **pycompat.strkwargs(hookargs)
                 )
 
+        for book, node in changes:
+            if bookmarks.isdivergent(book):
+                msg = _(b'cannot accept divergent bookmark %s!') % book
+                raise error.Abort(msg)
+
         bookstore.applychanges(op.repo, op.gettransaction(), changes)
 
         if pushkeycompat:

mercurial/exchange.py

@@ -856,6 +856,10 b' def _processcompared(pushop, pushed, exp'
     for b, scid, dcid in addsrc:
         if b in explicit:
             explicit.remove(b)
+            if bookmod.isdivergent(b):
+                pushop.ui.warn(_(b'cannot push divergent bookmark %s!\n') % b)
+                pushop.bkresult = 2
+            else:
             pushop.outbookmarks.append((b, b'', scid))
     # search for overwritten bookmark
     for b, scid, dcid in list(advdst) + list(diverge) + list(differ):

relnotes/next

@@ -24,6 +24,7 b''
  * Use `hg copy --forget --at-rev REV` to unmark already committed
    copies.
 
+ * prevent pushes of divergent bookmarks (foo@remote)
 
 == Bug Fixes  ==
 

tests/test-bookmarks-pushpull.t

@@ -328,6 +328,17 b' delete a remote bookmark'
 
 #endif
 
+Divergent bookmark cannot be exported
+
+  $ hg book W@default
+  $ hg push -B W@default ../a
+  pushing to ../a
+  searching for changes
+  cannot push divergent bookmark W@default!
+  no changes found
+  [2]
+  $ hg book -d W@default
+
 export the active bookmark
 
   $ hg bookmark V