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