--- a/mercurial/chgserver.py	Tue Feb 21 18:22:07 2017 +0100
+++ b/mercurial/chgserver.py	Sun Feb 19 01:00:10 2017 +0900
@@ -179,17 +179,16 @@
             else:
                 self._csystem = csystem
 
-        def system(self, cmd, environ=None, cwd=None, onerr=None,
-                   errprefix=None):
+        def _runsystem(self, cmd, environ, cwd, onerr, errprefix, out):
             # fallback to the original system method if the output needs to be
             # captured (to self._buffers), or the output stream is not stdout
             # (e.g. stderr, cStringIO), because the chg client is not aware of
             # these situations and will behave differently (write to stdout).
-            if (any(s[1] for s in self._bufferstates)
+            if (out is not self.fout
                 or not util.safehasattr(self.fout, 'fileno')
                 or self.fout.fileno() != util.stdout.fileno()):
-                return super(chgui, self).system(cmd, environ, cwd, onerr,
-                                                 errprefix)
+                return util.system(cmd, environ=environ, cwd=cwd, onerr=onerr,
+                                   errprefix=errprefix, out=out)
             self.flush()
             rc = self._csystem(cmd, util.shellenviron(environ), cwd)
             if rc and onerr:

--- a/mercurial/ui.py	Tue Feb 21 18:22:07 2017 +0100
+++ b/mercurial/ui.py	Sun Feb 19 01:00:10 2017 +0900
@@ -1288,8 +1288,14 @@
         if any(s[1] for s in self._bufferstates):
             out = self
         with self.timeblockedsection(blockedtag):
-            return util.system(cmd, environ=environ, cwd=cwd, onerr=onerr,
-                               errprefix=errprefix, out=out)
+            return self._runsystem(cmd, environ=environ, cwd=cwd, onerr=onerr,
+                                   errprefix=errprefix, out=out)
+
+    def _runsystem(self, cmd, environ, cwd, onerr, errprefix, out):
+        """actually execute the given shell command (can be overridden by
+        extensions like chg)"""
+        return util.system(cmd, environ=environ, cwd=cwd, onerr=onerr,
+                           errprefix=errprefix, out=out)
 
     def traceback(self, exc=None, force=False):
         '''print exception traceback if traceback printing enabled or forced.

--- a/tests/test-chg.t	Tue Feb 21 18:22:07 2017 +0100
+++ b/tests/test-chg.t	Sun Feb 19 01:00:10 2017 +0900
@@ -32,6 +32,46 @@
 
   $ cd ..
 
+editor
+------
+
+  $ cat >> pushbuffer.py <<EOF
+  > def reposetup(ui, repo):
+  >     repo.ui.pushbuffer(subproc=True)
+  > EOF
+
+  $ chg init editor
+  $ cd editor
+
+by default, system() should be redirected to the client:
+
+  $ touch foo
+  $ CHGDEBUG= HGEDITOR=cat chg ci -Am channeled --edit 2>&1 \
+  > | egrep "HG:|run 'cat"
+  chg: debug: run 'cat "*"' at '$TESTTMP/editor' (glob)
+  HG: Enter commit message.  Lines beginning with 'HG:' are removed.
+  HG: Leave message empty to abort commit.
+  HG: --
+  HG: user: test
+  HG: branch 'default'
+  HG: added foo
+
+but no redirection should be made if output is captured:
+
+  $ touch bar
+  $ CHGDEBUG= HGEDITOR=cat chg ci -Am bufferred --edit \
+  > --config extensions.pushbuffer="$TESTTMP/pushbuffer.py" 2>&1 \
+  > | egrep "HG:|run 'cat"
+  [1]
+
+check that commit commands succeeded:
+
+  $ hg log -T '{rev}:{desc}\n'
+  1:bufferred
+  0:channeled
+
+  $ cd ..
+
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