tests/test-issue4074.t

vfs: use checkambigatclosing in checkambig=True but atomictemp=False case

In Mercurial source tree, opening a file in "a"/"a+" mode like below
doesn't specify atomictemp=True for vfs, and this avoids file stat
ambiguity check by atomictempfile.

  - writing changes out in revlog layer uses "a+" mode
  - truncation in repair.strip() uses "a" mode
  - truncation in transaction._playback() uses "a" mode

If steps below occurs at "the same time in sec", all of mtime, ctime
and size are same between (1) and (3).

  1. append data to revlog-style file (and close transaction)
  2. discard appended data by truncation (strip or rollback)
  3. append same size but different data to revlog-style file again

Therefore, cache validation doesn't work after (3) as expected.

This patch uses checkambigatclosing in checkambig=True but
atomictemp=False case, to check (and get rid of) file stat ambiguity
at closing.

This is a part of ExactCacheValidationPlan.

    https://www.mercurial-scm.org/wiki/ExactCacheValidationPlan

#require no-pure

A script to generate nasty diff worst-case scenarios:

  $ cat > s.py <<EOF
  > import random
  > for x in xrange(100000):
  >     print
  >     if random.randint(0, 100) >= 50:
  >         x += 1
  >     print hex(x)
  > EOF

  $ hg init a
  $ cd a

Check in a big file:

  $ python ../s.py > a
  $ hg ci -qAm0

Modify it:

  $ python ../s.py > a

Time a check-in, should never take more than 10 seconds user time:

  $ hg ci --time -m1
  time: real .* secs .user [0-9][.].* sys .* (re)