rewriteutil: add a precheck function to check if revs can be rewritten

The precheck function is intended to be used before we start rewritting
changesets.

Differential Revision: https://phab.mercurial-scm.org/D1503

mercurial/rewriteutil.py

@@ -7,11 +7,39 b''
 
 from __future__ import absolute_import
 
+from .i18n import _
+
 from . import (
+    error,
+    node,
     obsolete,
     revset,
 )
 
+def precheck(repo, revs, action='rewrite'):
+    """check if revs can be rewritten
+    action is used to control the error message.
+
+    Make sure this function is called after taking the lock.
+    """
+    if node.nullrev in revs:
+        msg = _("cannot %s null changeset") % (action)
+        hint = _("no changeset checked out")
+        raise error.Abort(msg, hint=hint)
+
+    publicrevs = repo.revs('%ld and public()', revs)
+    if len(repo[None].parents()) > 1:
+        raise error.Abort(_("cannot %s while merging") % action)
+
+    if publicrevs:
+        msg = _("cannot %s public changesets") % (action)
+        hint = _("see 'hg help phases' for details")
+        raise error.Abort(msg, hint=hint)
+
+    newunstable = disallowednewunstable(repo, revs)
+    if newunstable:
+        raise error.Abort(_("cannot %s changeset with children") % action)
+
 def disallowednewunstable(repo, revs):
     """Checks whether editing the revs will create new unstable changesets and
     are we allowed to create them.