tests/mockblackbox.py

transaction: separate calculating TXNID from creating transaction object

Before this patch, transaction ID (TXNID) is calculated from
`transaction` object itself by `id()`, but this prevents TXNID from
being passed to `pretxnopen` hooks, which should be executed before
starting transaction processing (also any preparations for it, like
writing journal files out).

As a preparation for passing TXNID to `pretxnopen` hooks, this patch
separates calculation of TXNID from creation of `transaction` object.

This patch uses "random" library for reasonable unique ID. "uuid"
library can't be used, because it was introduced since Python 2.5 and
isn't suitable for Mercurial 3.4.x stable line.

`%f` formatting for `random.random()` is used with explicit precision
number 40, because default precision for `%f` is 6. 40 should be long
enough, even if 10**9 transactions are executed in a short time (a
second or less).

On the other hand, `time.time()` is used to ensures uniqueness of
TXNID in a long time, for safety.

BTW, platform not providing `/dev/urandom` or so may cause failure of
`import random` itself with some Python versions (see Python
issue15340 for detail http://bugs.python.org/issue15340).

But this patch uses "random" without any workaround, because:

  - "random" is already used directly in some code paths,
  - such platforms are very rare (e.g. Tru64 and HPUX), and
    http://bugs.python.org/issue15340#msg170000
  - updating Python runtime can avoid this issue

from mercurial import util

def makedate():
    return 0, 0
def getuser():
    return 'bob'

# mock the date and user apis so the output is always the same
def uisetup(ui):
    util.makedate = makedate
    util.getuser = getuser