--- a/mercurial/chgserver.py	Tue Sep 25 08:53:20 2018 -0700
+++ b/mercurial/chgserver.py	Wed Sep 26 08:46:56 2018 -0700
@@ -198,13 +198,15 @@
                 self._csystem = csystem
 
         def _runsystem(self, cmd, environ, cwd, 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).
+            # fallback to the original system method if
+            #  a. the output stream is not stdout (e.g. stderr, cStringIO),
+            #  b. or stdout is redirected by protectstdio(),
+            # because the chg client is not aware of these situations and
+            # will behave differently (i.e. write to stdout).
             if (out is not self.fout
                 or not util.safehasattr(self.fout, 'fileno')
-                or self.fout.fileno() != procutil.stdout.fileno()):
+                or self.fout.fileno() != procutil.stdout.fileno()
+                or self._finoutredirected):
                 return procutil.system(cmd, environ=environ, cwd=cwd, out=out)
             self.flush()
             return self._csystem(cmd, procutil.shellenviron(environ), cwd)

--- a/mercurial/ui.py	Tue Sep 25 08:53:20 2018 -0700
+++ b/mercurial/ui.py	Wed Sep 26 08:46:56 2018 -0700
@@ -234,6 +234,7 @@
             self.fout = src.fout
             self.ferr = src.ferr
             self.fin = src.fin
+            self._finoutredirected = src._finoutredirected
             self.pageractive = src.pageractive
             self._disablepager = src._disablepager
             self._tweaked = src._tweaked
@@ -258,6 +259,7 @@
             self.fout = procutil.stdout
             self.ferr = procutil.stderr
             self.fin = procutil.stdin
+            self._finoutredirected = False
             self.pageractive = False
             self._disablepager = False
             self._tweaked = False

--- a/mercurial/utils/procutil.py	Tue Sep 25 08:53:20 2018 -0700
+++ b/mercurial/utils/procutil.py	Wed Sep 26 08:46:56 2018 -0700
@@ -278,13 +278,13 @@
     """
     uout.flush()
     fin, fout = uin, uout
-    if uin is stdin:
+    if _testfileno(uin, stdin):
         newfd = os.dup(uin.fileno())
         nullfd = os.open(os.devnull, os.O_RDONLY)
         os.dup2(nullfd, uin.fileno())
         os.close(nullfd)
         fin = os.fdopen(newfd, r'rb')
-    if uout is stdout:
+    if _testfileno(uout, stdout):
         newfd = os.dup(uout.fileno())
         os.dup2(stderr.fileno(), uout.fileno())
         fout = os.fdopen(newfd, r'wb')

--- a/mercurial/wireprotoserver.py	Tue Sep 25 08:53:20 2018 -0700
+++ b/mercurial/wireprotoserver.py	Wed Sep 26 08:46:56 2018 -0700
@@ -783,6 +783,8 @@
         self._ui = ui
         self._repo = repo
         self._fin, self._fout = procutil.protectstdio(ui.fin, ui.fout)
+        # TODO: manage the redirection flag internally by ui
+        ui._finoutredirected = (self._fin, self._fout) != (ui.fin, ui.fout)
 
         # Log write I/O to stdout and stderr if configured.
         if logfh:

--- a/tests/test-ssh.t	Tue Sep 25 08:53:20 2018 -0700
+++ b/tests/test-ssh.t	Wed Sep 26 08:46:56 2018 -0700
@@ -313,6 +313,26 @@
   summary:     z
   
 
+#if chg
+
+try again with remote chg, which should succeed as well
+
+  $ hg rollback -R ../remote
+  repository tip rolled back to revision 4 (undo serve)
+
+  $ hg push --config ui.remotecmd=chg
+  pushing to ssh://user@dummy/remote
+  searching for changes
+  remote has heads on branch 'default' that are not known locally: 6c0482d977a3
+  remote: adding changesets
+  remote: adding manifests
+  remote: adding file changes
+  remote: added 1 changesets with 1 changes to 1 files
+  remote: KABOOM
+  remote: KABOOM IN PROCESS
+
+#endif
+
 clone bookmarks
 
   $ hg -R ../remote bookmark test
@@ -554,6 +574,8 @@
   Got arguments 1:user@dummy 2:hg -R remote serve --stdio
   Got arguments 1:user@dummy 2:hg -R remote serve --stdio
   changegroup-in-remote hook: HG_BUNDLE2=1 HG_HOOKNAME=changegroup HG_HOOKTYPE=changegroup HG_NODE=1383141674ec756a6056f6a9097618482fe0f4a6 HG_NODE_LAST=1383141674ec756a6056f6a9097618482fe0f4a6 HG_SOURCE=serve HG_TXNID=TXN:$ID$ HG_URL=remote:ssh:$LOCALIP
+  Got arguments 1:user@dummy 2:chg -R remote serve --stdio (chg !)
+  changegroup-in-remote hook: HG_BUNDLE2=1 HG_HOOKNAME=changegroup HG_HOOKTYPE=changegroup HG_NODE=1383141674ec756a6056f6a9097618482fe0f4a6 HG_NODE_LAST=1383141674ec756a6056f6a9097618482fe0f4a6 HG_SOURCE=serve HG_TXNID=TXN:$ID$ HG_URL=remote:ssh:$LOCALIP (chg !)
   Got arguments 1:user@dummy 2:hg -R remote serve --stdio
   Got arguments 1:user@dummy 2:hg init 'a repo'
   Got arguments 1:user@dummy 2:hg -R 'a repo' serve --stdio