--- a/mercurial/commands.py	Mon May 05 21:26:40 2014 +0900
+++ b/mercurial/commands.py	Mon May 05 21:26:40 2014 +0900
@@ -5684,15 +5684,18 @@
             date = util.parsedate(date)
 
         if opts.get('edit'):
-            message = ui.edit(message, ui.username())
-            repo.savecommitmessage(message)
+            def editor(repo, ctx, subs):
+                return ui.edit(ctx.description() + "\n", ctx.user())
+        else:
+            editor = False
 
         # don't allow tagging the null rev
         if (not opts.get('remove') and
             scmutil.revsingle(repo, rev_).rev() == nullrev):
             raise util.Abort(_("cannot tag null revision"))
 
-        repo.tag(names, r, message, opts.get('local'), opts.get('user'), date)
+        repo.tag(names, r, message, opts.get('local'), opts.get('user'), date,
+                 editor=editor)
     finally:
         release(lock, wlock)
 

--- a/mercurial/localrepo.py	Mon May 05 21:26:40 2014 +0900
+++ b/mercurial/localrepo.py	Mon May 05 21:26:40 2014 +0900
@@ -479,7 +479,8 @@
         return hook.hook(self.ui, self, name, throw, **args)
 
     @unfilteredmethod
-    def _tag(self, names, node, message, local, user, date, extra={}):
+    def _tag(self, names, node, message, local, user, date, extra={},
+             editor=False):
         if isinstance(names, str):
             names = (names,)
 
@@ -539,14 +540,15 @@
             self[None].add(['.hgtags'])
 
         m = matchmod.exact(self.root, '', ['.hgtags'])
-        tagnode = self.commit(message, user, date, extra=extra, match=m)
+        tagnode = self.commit(message, user, date, extra=extra, match=m,
+                              editor=editor)
 
         for name in names:
             self.hook('tag', node=hex(node), tag=name, local=local)
 
         return tagnode
 
-    def tag(self, names, node, message, local, user, date):
+    def tag(self, names, node, message, local, user, date, editor=False):
         '''tag a revision with one or more symbolic names.
 
         names is a list of strings or, when adding a single tag, names may be a
@@ -574,7 +576,7 @@
                                        '(please commit .hgtags manually)'))
 
         self.tags() # instantiate the cache
-        self._tag(names, node, message, local, user, date)
+        self._tag(names, node, message, local, user, date, editor=editor)
 
     @filteredpropertycache
     def _tagscache(self):

--- a/tests/test-tag.t	Mon May 05 21:26:40 2014 +0900
+++ b/tests/test-tag.t	Mon May 05 21:26:40 2014 +0900
@@ -225,8 +225,11 @@
 
 at first, test saving last-message.txt
 
+(test that editor is not invoked before transaction starting)
+
   $ cat > .hg/hgrc << '__EOF__'
   > [hooks]
+  > # this failure occurs before editor invocation
   > pretag.test-saving-lastmessage = false
   > __EOF__
   $ rm -f .hg/last-message.txt
@@ -234,12 +237,37 @@
   abort: pretag.test-saving-lastmessage hook exited with status 1
   [255]
   $ cat .hg/last-message.txt
+  cat: .hg/last-message.txt: No such file or directory
+  [1]
+
+(test that editor is invoked and commit message is saved into
+"last-message.txt")
+
+  $ cat >> .hg/hgrc << '__EOF__'
+  > [hooks]
+  > pretag.test-saving-lastmessage =
+  > # this failure occurs after editor invocation
+  > pretxncommit.unexpectedabort = false
+  > __EOF__
+
+  $ rm -f .hg/last-message.txt
+  $ HGEDITOR="\"sh\" \"`pwd`/editor.sh\"" hg tag custom-tag -e
+  transaction abort!
+  rollback completed
+  note: commit message saved in .hg/last-message.txt
+  abort: pretxncommit.unexpectedabort hook exited with status 1
+  [255]
+  $ cat .hg/last-message.txt
   custom tag message
   second line
-  $ cat > .hg/hgrc << '__EOF__'
+
+  $ cat >> .hg/hgrc << '__EOF__'
   > [hooks]
-  > pretag.test-saving-lastmessage =
+  > pretxncommit.unexpectedabort =
   > __EOF__
+  $ hg status .hgtags
+  M .hgtags
+  $ hg revert --no-backup -q .hgtags
 
 then, test custom commit message itself