When a child process exits due to an exception, a traceback is written, but stderr is not flushed. Thus I see a header like "Process 1:\n", but no traceback.

I don't have a development environment or any experience with Mecurial, so I'm afraid there's no patch, but it's a one-liner.

In   /Lib/multiprocess/process.py :: Process._bootstrap

    except:
        exitcode = 1
        import traceback
        sys.stderr.write('Process %s:\n' % self.name)
        sys.stderr.flush()
        traceback.print_exc()

Append a "sys.stderr.flush()" to the suite.

It surprised me that flushing was even necessary. I would've thought that the standard streams would all be closed just before the process terminated, regardless of exit status. But I observe that unless I explicitly flush stdout and stderr before terminating, the output is lost entirely, even if the exit is not abnormal. This isn't the desired behavior, is it?