[PATCH CREW] parsers: add apiversion module constant to C implementation of parsers

[Chris Jerdonek]

On Tue, Dec 3, 2013 at 1:43 PM, Matt Mackall <mpm at selenic.com> wrote:
> On Tue, 2013-12-03 at 03:35 -0800, Chris Jerdonek wrote:
>> # HG changeset patch
>> # User Chris Jerdonek <chris.jerdonek at gmail.com>
>> # Date 1386070245 28800
>> #      Tue Dec 03 03:30:45 2013 -0800
>> # Node ID 2b49d4087af707302eb2acf23b2ec9cea17a4adc
>> # Parent  7eda5bb9ec8ff230d035164efde065d4f5ae92a2
>> parsers: add apiversion module constant to C implementation of parsers
>>
>> This change adds a module constant called "apiversion" to the C implementation
>> of the parsers module.  Going forward, the presence of this constant can be
>> used to distinguish the C implementation of parsers from the pure Python
>> version (e.g. from within unit test modules).  The value is initially set to 1.
>
> This is still tightly coupled. Hint: bisect travels backwards and
> forwards in history. I don't want to run 'make' if I'm not tracking down
> an extension bug.

You wouldn't need to.  The idea was to put certain C-specific tests
behind an if-block.  If the module constant didn't exist or the number
didn't match exactly (which could happen if the C wasn't recompiled),
the tests just wouldn't run.  Unless I'm missing something, that
pattern is forward and backwards-compatible.

>> In addition, this value can be incremented as a way to signal changes in
>> the C API.
>
> Also, this is a known recipe for failure, as already demonstrated in the
> very bug this was created in response to. Human beings simply cannot be
> entrusted to do this correctly, or more importantly, in a timely
> fashion.
>
> In C: add a new interface (do not change old interfaces)
> In Python: check for existence of new interface

The version-checking behavior doesn't require a change in the
interface, which is why I chose something general.  It just changes
what happens if you import the module when using the wrong version.
But I can change to something more meaningful.

--Chris