tests/test-merge-subrepos.t

util: improve iterfile so it chooses code path wisely

We have performance concerns on "iterfile" as it is 4X slower on normal
files. While modern systems have the nice property that reading a "fast"
(on-disk) file cannot be interrupted and should be made use of.

This patch dumps the related knowledge in comments. And "iterfile" chooses
code paths wisely:

  1. If it's CPython 3, or PyPY, use the fast path.
  2. If fp is a normal file, use the fast path.
  3. If fp is not a normal file and CPython version >= 2.7.4, use the same
     workaround (4x slower) as before.
  4. If fp is not a normal file and CPython version < 2.7.4, use another
     workaround (2x slower but may block longer then necessary) which
     basically re-invents the buffer + readline logic in Python.

This will give us good confidence on both correctness and performance
dealing with EINTR in iterfile(fp) for all known supported Python versions.

  $ hg init

  $ echo a > a
  $ hg ci -qAm 'add a'

  $ hg init subrepo
  $ echo 'subrepo = http://example.net/libfoo' > .hgsub
  $ hg ci -qAm 'added subrepo'

  $ hg up -qC 0
  $ echo ax > a
  $ hg ci -m 'changed a'
  created new head

  $ hg up -qC 1
  $ cd subrepo
  $ echo b > b
  $ hg add b
  $ cd ..

Should fail, since there are added files to subrepo:

  $ hg merge
  abort: uncommitted changes in subrepository 'subrepo'
  [255]