tests/test-gendoc-ja.t

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

#require docutils gettext

  $ $TESTDIR/check-gendoc ja
  checking for parse errors
  \xe8\xa4\x87\xe8\xa3\xbd\xef\xbc\x8f\xe6\x94\xb9\xe5\x90\x8d\xe3\x81\xab\xe9\x96\xa2\xe3\x81\x99\xe3\x82\x8b\xe5\xb1\xa5\xe6\xad\xb4\xe4\xbf\x9d\xe5\xad\x98\xe3\x81\xae\xe3\x81\x9f\xe3\x82\x81\xe3\x81\xab git \xe5\xb7\xae\xe5\x88\x86\xe5\xbd\xa2\xe5\xbc\x8f\xe3\x82\x92\xe4\xbd\xbf\xe7\x94\xa8(-g/--git \xe6\x8c\x87\xe5\xae\x9a\xe3\x82\x84 \xe8\xa8\xad\xe5\xae\x9a\xe3\x83\x95\xe3\x82\xa1\xe3\x82\xa4\xe3\x83\xab\xe3\x81\xa7\xe3\x81\xae [diff] git=1 \xe8\xa8\x98\xe8\xbf\xb0)\xe3\x81\x99\xe3\x82\x8b\xe3\x81\xae\xe3\x81\xa7\xe3\x81\x82\xe3\x82\x8c\xe3\x81\xb0\xe3\x80\x81 add/remove/copy/rename \xe3\x81\xa8\xe3\x81\x84\xe3\x81\xa3\xe3\x81\x9f hg \xe3\x81\xae\xe3\x82\xb3\xe3\x83\x9e\xe3\x83\xb3\xe3\x83\x89\xe3\x81\xab\xe3\x82\x88\xe3\x82\x8b\xe5\xb1\xa5\xe6\xad\xb4\xe8\xa8\x98\xe9\x8c\xb2\xe3\x82\x82\xe3\x80\x81 \xe9\x80\x9a\xe5\xb8\xb8\xe3\x81\xa8\xe5\xa4\x89\xe3\x82\x8f\xe3\x82\x8a\xe3\x81\xaa\xe3\x81\x8f\xe6\xa9\x9f\xe8\x83\xbd\xe3\x81\x97\xe3\x81\xbe\xe3\x81\x99\xe3\x80\x82 git \xe5\xb7\xae\xe5\x88\x86\xe5\xbd\xa2\xe5\xbc\x8f\xe3\x81\xae\xe8\xa9\xb3\xe7\xb4\xb0\xe3\x81\xab\xe9\x96\xa2\xe3\x81\x97\xe3\x81\xa6\xe3\x81\xaf\xe3\x80\x81 'help diffs' \xe3\x82\x92\xe5\x8f\x82\xe7\x85\xa7\xe3\x81\x97\xe3\x81\xa6\xe3\x81\x8f\xe3\x81\xa0\xe3\x81\x95\xe3\x81\x84\xe3\x80\x82 (esc)
  warning: please use " instead of ' for hg ... "..."