contrib/vim/hgtest.vim

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

" Vim syntax file
" Language: Mercurial unified tests
" Author: Steve Losh ([e-mail])
"
" Place this file in ~/.vim/syntax/ and add the following line to your
" ~/.vimrc to enable:
" au BufNewFile,BufRead *.t set filetype=hgtest
"
" If you want folding you'll need the following line as well:
" let hgtest_fold=1
"
" You might also want to set the starting foldlevel for hgtest files:
" autocmd Syntax hgtest setlocal foldlevel=1

if exists("b:current_syntax")
  finish
endif

syn include @Shell syntax/sh.vim

syn match hgtestComment /^[^ ].*$/
syn region hgtestOutput start=/^  [^$>]/ start=/^  $/ end=/\v.(\n\n*[^ ])\@=/me=s end=/^  [$>]/me=e-3 end=/^$/ fold containedin=hgtestBlock
syn match hgtestCommandStart /^  \$ / containedin=hgtestCommand
syn region hgtestCommand start=/^  \$ /hs=s+4,rs=s+4 end=/^  [^>]/me=e-3 end=/^  $/me=e-2 containedin=hgtestBlock contains=@Shell keepend
syn region hgtestBlock start=/^  /ms=e-2 end=/\v.(\n\n*[^ ])\@=/me=s end=/^$/me=e-1 fold keepend

hi link hgtestCommandStart Keyword
hi link hgtestComment Normal
hi link hgtestOutput Comment

if exists("hgtest_fold")
  setlocal foldmethod=syntax
endif

syn sync match hgtestSync grouphere NONE "^$"
syn sync maxlines=200

" It's okay to set tab settings here, because an indent of two spaces is specified
" by the file format.
setlocal tabstop=2 softtabstop=2 shiftwidth=2 expandtab

let b:current_syntax = "hgtest"