cmdserver: write channel header and payload by a single write() call

This makes a channeledoutput thread-safe as long as the underlying fwrite() is
thread-safe. Both POSIX and Windows implementations are documented as MT-safe.

MT-safety is necessary to use ui.fout and ui.ferr in hgweb.

mercurial/commandserver.py

@@ -54,8 +54,8 b' class channeledoutput(object):'
     def write(self, data):
         if not data:
             return
-        self.out.write(struct.pack('>cI', self.channel, len(data)))
-        self.out.write(data)
+        # single write() to guarantee the same atomicity as the underlying file
+        self.out.write(struct.pack('>cI', self.channel, len(data)) + data)
         self.out.flush()
 
     def __getattr__(self, attr):