--- a/mercurial/wireprotoserver.py	Wed Feb 07 20:17:05 2018 -0800
+++ b/mercurial/wireprotoserver.py	Wed Feb 07 20:17:47 2018 -0800
@@ -8,6 +8,7 @@
 
 import abc
 import cgi
+import contextlib
 import struct
 import sys
 
@@ -74,6 +75,20 @@
         """
 
     @abc.abstractmethod
+    def mayberedirectstdio(self):
+        """Context manager to possibly redirect stdio.
+
+        The context manager yields a file-object like object that receives
+        stdout and stderr output when the context manager is active. Or it
+        yields ``None`` if no I/O redirection occurs.
+
+        The intent of this context manager is to capture stdio output
+        so it may be sent in the response. Some transports support streaming
+        stdio to the client in real time. For these transports, stdio output
+        won't be captured.
+        """
+
+    @abc.abstractmethod
     def redirect(self):
         """may setup interception for stdout and stderr
 
@@ -151,6 +166,21 @@
         for s in util.filechunkiter(self._req, limit=length):
             fp.write(s)
 
+    @contextlib.contextmanager
+    def mayberedirectstdio(self):
+        oldout = self._ui.fout
+        olderr = self._ui.ferr
+
+        out = util.stringio()
+
+        try:
+            self._ui.fout = out
+            self._ui.ferr = out
+            yield out
+        finally:
+            self._ui.fout = oldout
+            self._ui.ferr = olderr
+
     def redirect(self):
         self._oldio = self._ui.fout, self._ui.ferr
         self._ui.ferr = self._ui.fout = stringio()
@@ -393,6 +423,10 @@
             fpout.write(self._fin.read(count))
             count = int(self._fin.readline())
 
+    @contextlib.contextmanager
+    def mayberedirectstdio(self):
+        yield None
+
     def redirect(self):
         pass