view CONTRIBUTING @ 44066:f5d2720f3bea

revlog-native: introduced ABI version in capsule

Concerns that an inconsistency could arise between the actual contents
of the capsule in revlog.c and the Rust consumer have been raised after
the switch to the array of data and function pointers in f384d68d8ea8.

It has been suggested that the `version` from parsers.c could be use for
this. In this change, we introduce instead a separate ABI version number,
which should have the following advantages:

- no need to change the consuming Rust code for changes that have nothing
  to do with the contents of the capsule
- the version number in parsers.c is not explicitely flagged as ABI. It's
  not obvious to me whether an ABI change that would be invisible to Python
  would warrant an increment

The drawback is that developers now have to consider two version numbers.

We expect the added cost of the check to be negligible because it occurs
at instantiation of `CIndex` only, which in turn is tied to instantiation
of Python objects such as `LazyAncestors` and `MixedIndex`. Frequent calls
to `Cindex::new` should also probably hit the CPU branch predictor.

Differential Revision: https://phab.mercurial-scm.org/D7856

Our full contribution guidelines are in our wiki, please see:

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

If you just want a checklist to follow, you can go straight to

https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist

If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).