procutil: introduce context-manager interface for protect/restorestdio

The code looks slightly cleaner since it was pretty easy to pass arguments
in wrong order.

mercurial/commandserver.py

@@ -318,13 +318,12 b' class pipeservice(object):'
         ui = self.ui
         # redirect stdio to null device so that broken extensions or in-process
         # hooks will never cause corruption of channel protocol.
-        fin, fout = procutil.protectstdio(ui.fin, ui.fout)
-        try:
-            sv = server(ui, self.repo, fin, fout)
-            return sv.serve()
-        finally:
-            sv.cleanup()
-            procutil.restorestdio(ui.fin, ui.fout, fin, fout)
+        with procutil.protectedstdio(ui.fin, ui.fout) as (fin, fout):
+            try:
+                sv = server(ui, self.repo, fin, fout)
+                return sv.serve()
+            finally:
+                sv.cleanup()
 
 def _initworkerprocess():
     # use a different process group from the master process, in order to:

mercurial/utils/procutil.py

@@ -9,6 +9,7 b''
 
 from __future__ import absolute_import
 
+import contextlib
 import imp
 import io
 import os
@@ -240,6 +241,15 b' def restorestdio(uin, uout, fin, fout):'
             os.dup2(f.fileno(), uif.fileno())
             f.close()
 
[e-mail]
+def protectedstdio(uin, uout):
+    """Run code block with protected standard streams"""
+    fin, fout = protectstdio(uin, uout)
+    try:
+        yield fin, fout
+    finally:
+        restorestdio(uin, uout, fin, fout)
+
 def shellenviron(environ=None):
     """return environ with optional override, useful for shelling out"""
     def py2shell(val):