diff --git a/mercurial/util.py b/mercurial/util.py
--- a/mercurial/util.py
+++ b/mercurial/util.py
@@ -715,11 +715,13 @@ class fileobjectobserver(object):
     def _writedata(self, data):
         if not self.logdata:
             self.fh.write('\n')
+            self.fh.flush()
             return
 
         # Simple case writes all data on a single line.
         if b'\n' not in data:
             self.fh.write(': %s\n' % escapedata(data))
+            self.fh.flush()
             return
 
         # Data with newlines is written to multiple lines.
@@ -727,6 +729,7 @@ class fileobjectobserver(object):
         lines = data.splitlines(True)
         for line in lines:
             self.fh.write('%s>     %s\n' % (self.name, escapedata(line)))
+        self.fh.flush()
 
     def read(self, res, size=-1):
         if not self.reads:
diff --git a/mercurial/wireproto.py b/mercurial/wireproto.py
--- a/mercurial/wireproto.py
+++ b/mercurial/wireproto.py
@@ -1077,6 +1077,7 @@ def unbundle(repo, proto, heads):
                     util.stderr.write("abort: %s\n" % exc)
                     if exc.hint is not None:
                         util.stderr.write("(%s)\n" % exc.hint)
+                    util.stderr.flush()
                     return pushres(0, output.getvalue() if output else '')
                 except error.PushRaced:
                     return pusherr(pycompat.bytestr(exc),