wireprotoserver: support logging SSH server I/O to a file descriptor

We will soon introduce a debug command and tests for low-level I/O
behavior of the SSH wire protocol.

To facilitate this, we need to instrument the SSH server so it
can log its I/O as events occur.

We teach the SSH server to convert its stdout and stderr file objects
into file object proxies. We configure these proxies to log to a
file descriptor whose file number is specified via a config option.
The idea is to have a future debug command start the SSH server
process with access to an extra file descriptor that can be used
by the server process to log I/O. Monitoring only the write I/O
will be more robust than monitoring both writes and reads from the
client process because read operations are not deterministic. This
will matter for tests that capture raw I/O activity.

Differential Revision: https://phab.mercurial-scm.org/D2463

mercurial/wireprotoserver.py

@@ -588,12 +588,19 b' def _runsshserver(ui, repo, fin, fout, e'
                                          state)
 
 class sshserver(object):
-    def __init__(self, ui, repo):
+    def __init__(self, ui, repo, logfh=None):
         self._ui = ui
         self._repo = repo
         self._fin = ui.fin
         self._fout = ui.fout
 
+        # Log write I/O to stdout and stderr if configured.
+        if logfh:
+            self._fout = util.makeloggingfileobject(
+                logfh, self._fout, 'o', logdata=True)
+            ui.ferr = util.makeloggingfileobject(
+                logfh, ui.ferr, 'e', logdata=True)
+
         hook.redirect(True)
         ui.fout = repo.ui.fout = ui.ferr