Currently snapcraft only supports a flat list of stage packages to be staged regardless of target architecture. This PR resolves LP: [#1637282](https://bugs.launchpad.net/snapcraft/+bug/1637282) by introducing a more complex grammar that allows one to filter stage packages depending on various selectors (target arch for right now), as well as specify optional packages. The grammar is made up of two statements: `on` and `try`. ```yaml - on <selector>[,<selector>...]: - ... - else[ fail]: - ... ``` The body of the `on` clause is taken into account if every (AND, not OR) selector is true for the target build environment. Currently the only selectors supported are target architectures (e.g. amd64). If the `on` clause doesn't match and it's immediately followed by an `else` clause, the `else` clause must be satisfied. An `on` clause without an `else` clause is considered satisfied even if no selector matched. The `else fail` form allows erroring out if an `on` clause was not matched. ```yaml - try: - ... - else: - ... ``` The body of the `try` clause is taken into account only when all packages contained within it are valid. If not, if it's immediately followed by `else` clauses they are tried in order, and one of them must be satisfied. A `try` clause with no `else` clause is considered satisfied even if it contains invalid packages. This PR is larger than I'd like it to be, I'm sorry about that-- I didn't see a logical place to split. Lots of the line count is tests. Note also that I tried to test a number of combinations, but the grammar is recursive, allowing for unlimited combinations. This could use some good poking.