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
$ echo 'raise Exception("bit bucket overflow")' > badext.py
$ abspath=`pwd`/badext.py
$ cat <<EOF >> $HGRCPATH
> [extensions]
> gpg =
> hgext.gpg =
> badext = $abspath
> badext2 =
> EOF
$ hg -q help help
*** failed to import extension badext from $TESTTMP/badext.py: bit bucket overflow
*** failed to import extension badext2: No module named badext2
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