Variants should be allowed to share common stock

[pixelmachine]

This would be useful in several situations I've come across, generally for limited run, bespoke, or non-physical items. A specific example I've encountered is meals or other prepared food products, where there may be differently priced variations but a specific limited total number across all variants.

[pixelmachine]

This has come up again for me in the form of tickets. There are a set number of tickets but depending on the variant the price may change. There are ways around it but it would be nicer if variants could just share stock.

Routes:

1. Stock is optionally defined on the product rather than variant
2. Stock is optionally defined only on the default variant
3. Stock is sync'd across variants

I'll probably do 3 in a module but I'm worried it could potentially lead to some race conditions in high traffic situations.

[pixelmachine]

Here's another example on Craft's StackExchange