Deprecate use of ec['parallel'] and fix updating the template value

[Flamefire]

ec['parallel'] currently doubles as an EC option and as the storage for the calculated parallelism set by the EasyBlock.
This makes it hard to reason about especially as maxparallel has pretty much the same effect. Also changes to ec['parallel'] done by e.g. easyblocks (or the set_parallel method) are not reflected by the template %(parallel)s

Solution: Introduce a property which on write updates the template and some magic to mirror the effect of the now deprecated ec['parallel']

Additional change I'd like to do: Treat parallel = False (legacy) as maxparallel =1 so cfg.parallel is always a number

Requires easybuilders/easybuild-easyconfigs#19374 to avoid deprecation warnings

See #3811 (comment) for the motivation

[boegel]

Makes sense in general, but haven't reviewed this in detail yet (I'm wondering whether we can avoid the parallelLegacy part).

@Flamefire Have you looked into the impact that this will have on both easyblocks (where self.cfg['parallel'] is used) and easyconfigs (where any that use parallel = X will basically be broken because of this)?

I have a feeling that the way this is done currently is too "hard" of a change, we should do it in a way that just results in a warning when self.cfg['parallel'] or parallel = X are being used, we shouldn't break anything...

[Flamefire]

I have a feeling that the way this is done currently is too "hard" of a change, we should do it in a way that just results in a warning when self.cfg['parallel'] or parallel = X are being used, we shouldn't break anything...

This PR is exactly that: NOTHING is broken by that. I took exceptionally great care to make sure of that which requires the parallelLegacy part. Without this it is impossible to NOT break stuff, due to the possibility of custom EasyBlocks.
To answer your questions:

Have you looked into the impact that this will have on both easyblocks (where self.cfg['parallel'] is used)

This will continue to work as before as that value will be stored in parallelLegacy instead (transparently) and a deprecation warning will be shown which tells you to use self.cfg.parallel instead.

easyconfigs (where any that use parallel = X will basically be broken because of this)?

They will also work exactly as before with a deprecation warning that maxparallel should be used instead but otherwise there is no change as the transition var parallelLegacy is used: https://github.com/easybuilders/easybuild-framework/pull/3842/files#diff-00260ae7a519d5825760f53b067b29fb84a3e0d2649e6a27ace99abaca96d7d1R1819

[boegel]

Not a big fan of this, since this opens the door to letting easyblocks use self.cfg['paralleLegacy']...

Can we avoid parking this in the self._config dictionary, and use self._parallel_legacy instead?

If not, we should rename 'paralleLegacy' to something more obscure that better signals that it's an internal thing (like '_parallel_legacy')

[Flamefire]

Can we avoid parking this in the self._config dictionary, and use self._parallel_legacy instead?

Not really as it needs to be in the EC instance not the EB instance. Played around with it a bit and this was the best solution to keep the change as transparent and backwards compatible as possible.
But yes we can rename it although I chose paralleLegacy to be already obscure enough to not be used.

[Flamefire]

I did the requested change after the latest rebase using self._config['_parallelLegacy']

[Flamefire]

@boegel What was the issue here? Anything remaining?

[Flamefire]

@boegel *ping

[boegel]

@Flamefire This can be closed in favor of #4580, right?

[Flamefire]

If there won't be another 4.x then yes.