tests/test-addremove.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

  $ hg init rep
  $ cd rep
  $ mkdir dir
  $ touch foo dir/bar
  $ hg -v addremove
  adding dir/bar
  adding foo
  $ hg -v commit -m "add 1"
  committing files:
  dir/bar
  foo
  committing manifest
  committing changelog
  committed changeset 0:6f7f953567a2
  $ cd dir/
  $ touch ../foo_2 bar_2
  $ hg -v addremove
  adding dir/bar_2
  adding foo_2
  $ hg -v commit -m "add 2"
  committing files:
  dir/bar_2
  foo_2
  committing manifest
  committing changelog
  committed changeset 1:e65414bf35c5
  $ cd ..
  $ hg forget foo
  $ hg -v addremove
  adding foo
  $ hg forget foo
#if windows
  $ hg -v addremove nonexistent
  nonexistent: The system cannot find the file specified
  [1]
#else
  $ hg -v addremove nonexistent
  nonexistent: No such file or directory
  [1]
#endif
  $ cd ..

  $ hg init subdir
  $ cd subdir
  $ mkdir dir
  $ cd dir
  $ touch a.py
  $ hg addremove 'glob:*.py'
  adding a.py
  $ hg forget a.py
  $ hg addremove -I 'glob:*.py'
  adding a.py
  $ hg forget a.py
  $ hg addremove
  adding dir/a.py
  $ cd ..

  $ hg init sim
  $ cd sim
  $ echo a > a
  $ echo a >> a
  $ echo a >> a
  $ echo c > c
  $ hg commit -Ama
  adding a
  adding c
  $ mv a b
  $ rm c
  $ echo d > d
  $ hg addremove -n -s 50 # issue 1696
  removing a
  adding b
  removing c
  adding d
  recording removal of a as rename to b (100% similar)
  $ hg addremove -s 50
  removing a
  adding b
  removing c
  adding d
  recording removal of a as rename to b (100% similar)
  $ hg commit -mb
  $ cp b c
  $ hg forget b
  $ hg addremove -s 50
  adding b
  adding c

  $ rm c
#if windows
  $ hg ci -A -m "c" nonexistent
  nonexistent: The system cannot find the file specified
  abort: failed to mark all new/missing files as added/removed
  [255]
#else
  $ hg ci -A -m "c" nonexistent
  nonexistent: No such file or directory
  abort: failed to mark all new/missing files as added/removed
  [255]
#endif
  $ hg st
  ! c
  $ cd ..