contrib/docker/apache-server/Dockerfile

worker: rewrite error handling so os._exit covers all cases

Previously the worker error handling is like:

    pid = os.fork()   --+
    if pid == 0:        |
        ....            | problematic
        ....          --+
        try:          --+
            ....        | worker error handling
                      --+

If a signal arrives when Python is executing the "problematic" lines, an
external error handling (dispatch.py) will take over the control flow and
it's no longer guaranteed "os._exit" is called (see 86cd09bc13ba for why it
is necessary).

This patch rewrites the error handling so it covers all possible code paths
for a worker even during fork.

Note: "os.getpid() == parentpid" is used to test if the process is parent or
not intentionally, instead of checking "pid", because "pid = os.fork()" may
be not atomic - it's possible that that a signal hits the worker before the
assignment completes [1].  The newly added test replaces "os.fork" to
exercise that extreme case.

[1]: CPython compiles "pid = os.fork()" to 2 byte codes: "CALL_FUNCTION" and
"STORE_FAST", so it's probably not atomic:

    def f():
        pid = os.fork()

    dis.dis(f)
      2           0 LOAD_GLOBAL              0 (os)
                  3 LOAD_ATTR                1 (fork)
                  6 CALL_FUNCTION            0
                  9 STORE_FAST               0 (pid)
                 12 LOAD_CONST               0 (None)
                 15 RETURN_VALUE

FROM debian:wheezy

ENV DEBIAN_FRONTEND noninteractive
ENV WSGI_PROCESSES 4
ENV WSGI_THREADS 1
ENV WSGI_MAX_REQUESTS 100000

EXPOSE 80
VOLUME ["/var/hg/htdocs", "/var/hg/repos"]

RUN apt-get update && apt-get -y install libapache2-mod-wsgi python-dev vim

# Install our own Apache site.
RUN a2dissite 000-default
ADD vhost.conf /etc/apache2/sites-available/hg
RUN a2ensite hg

ADD hgwebconfig /defaulthgwebconfig

ADD entrypoint.sh /entrypoint.sh
ENTRYPOINT ["/entrypoint.sh"]

CMD ["/usr/sbin/apache2", "-DFOREGROUND"]