wireprotoserver: add context manager mechanism for redirecting stdio

wireprotoserver: add context manager mechanism for redirecting stdio

Today, proto.redirect() sets up redirecting stdio and proto.restore()
undoes that. The API is a bit wonky because restore() is only
implemented on the HTTP protocol. Furthermore, not all calls to
redirect() are obviously paired with calls to restore(). For
example, the call to restore() for "unbundle" requests is handled
by the response handler for the HTTP protocol.

This commit introduces a new method on the protocol handler interface
to maybe capture stdio. It emits a file object or None depending on
whether stdio capture is used by the transport.

To prove it works, the "pushkey" wire protocol command has been
updated to use the new API.

I'm not convinced this is the best mechanism to capture stdio. I may
need to come up with something better once the new wire protocol
emerges into existence. But it is strictly better than before because
it removes variance in the wire protocol handler interface. It
therefore gets us closer to a unified interface between the SSH and
HTTP transports.

Differential Revision: https://phab.mercurial-scm.org/D2081

--- a/mercurial/wireproto.py	Wed Feb 07 20:17:05 2018 -0800
+++ b/mercurial/wireproto.py	Wed Feb 07 20:17:47 2018 -0800
@@ -978,20 +978,12 @@
     else:
         new = encoding.tolocal(new) # normal path
 
-    if util.safehasattr(proto, 'restore'):
-
-        proto.redirect()
-
+    with proto.mayberedirectstdio() as output:
         r = repo.pushkey(encoding.tolocal(namespace), encoding.tolocal(key),
                          encoding.tolocal(old), new) or False
 
-        output = proto.restore()
-
-        return '%s\n%s' % (int(r), output)
-
-    r = repo.pushkey(encoding.tolocal(namespace), encoding.tolocal(key),
-                     encoding.tolocal(old), new)
-    return '%s\n' % int(r)
+    output = output.getvalue() if output else ''
+    return '%s\n%s' % (int(r), output)
 
 @wireprotocommand('stream_out')
 def stream(repo, proto):

--- 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