hook: move stdio redirection to context manager

The old code was checking stdio redirection in a loop.
This didn't make sense. The pattern is better expressed
as a context manager IMO, so this commit refactors it
to be one.

Differential Revision: https://phab.mercurial-scm.org/D8338

mercurial/hook.py

@@ -7,6 +7,7
 
 from __future__ import absolute_import
 
+import contextlib
 import os
 import sys
 
@@ -259,14 +260,13 def hook(ui, repo, htype, throw=False, *
     return r
 
 
-def runhooks(ui, repo, htype, hooks, throw=False, **args):
-    args = pycompat.byteskwargs(args)
-    res = {}
[e-mail]
+def redirect_stdio():
+    """Redirects stdout to stderr, if possible."""
+
     oldstdout = -1
-
     try:
-        for hname, cmd in hooks:
-            if oldstdout == -1 and _redirect:
+        if _redirect:
                 try:
                     stdoutno = procutil.stdout.fileno()
                     stderrno = procutil.stderr.fileno()
@@ -279,6 +279,26 def runhooks(ui, repo, htype, hooks, thr
                     # files seem to be bogus, give up on redirecting (WSGI, etc)
                     pass
 
+        yield
+
+    finally:
+        # The stderr is fully buffered on Windows when connected to a pipe.
+        # A forcible flush is required to make small stderr data in the
+        # remote side available to the client immediately.
+        procutil.stderr.flush()
+
+        if _redirect and oldstdout >= 0:
+            procutil.stdout.flush()  # write hook output to stderr fd
+            os.dup2(oldstdout, stdoutno)
+            os.close(oldstdout)
+
+
+def runhooks(ui, repo, htype, hooks, throw=False, **args):
+    args = pycompat.byteskwargs(args)
+    res = {}
+
+    with redirect_stdio():
+        for hname, cmd in hooks:
             if cmd is _fromuntrusted:
                 if throw:
                     raise error.HookAbort(
@@ -312,15 +332,5 def runhooks(ui, repo, htype, hooks, thr
                 raised = False
 
             res[hname] = r, raised
-    finally:
-        # The stderr is fully buffered on Windows when connected to a pipe.
-        # A forcible flush is required to make small stderr data in the
-        # remote side available to the client immediately.
-        procutil.stderr.flush()
-
-        if _redirect and oldstdout >= 0:
-            procutil.stdout.flush()  # write hook output to stderr fd
-            os.dup2(oldstdout, stdoutno)
-            os.close(oldstdout)
 
     return res