What are possible "selectors" for the "on:" snapcraft.yaml directive?

rpjday · March 8, 2022, 10:38pm

Just noticed in snapcraft schema file that the “on:” directive is of the form:

“usage”: “on <selector>[,<selector>…]:”

which normally selects among some architecture, but the above refers to a more general “selector” for that conditional – what can be a selector other than an architecture?

P.S. The above suggests one can have something like:

on: amd64,arm64:

Yes?

jamesh · March 9, 2022, 3:44am

Currently Snapcraft only supports the host architecture selectors you already know about:

github.com

snapcore/snapcraft/blob/a04646f12623c145ceb71ad085781e0015eaa20c/snapcraft/internal/project_loader/grammar/_on.py#L76-L90


def _check(self) -> bool:
    """Check if a statement main body should be processed.
    :return: True if main body should be processed, False if elses should
             be processed.
    :rtype: bool
    """
    # A new ProjectOptions instance defaults to the host architecture
    # whereas self._project_options would yield the target architecture
    host_arch = snapcraft.ProjectOptions().deb_arch
    # The only selector currently supported is the host arch. Since
    # selectors are matched with an AND, not OR, there should only be one
    # selector.
    return (len(self.selectors) == 1) and (host_arch in self.selectors)

The documentation references generic selectors so that the syntax could be reused if we need something other than host architecture selectors.

The comments suggest that if multiple selectors are specified in a single statement, then the block should only be processed if they are all true. So writing a block like on amd64,arm64: would be a bit useless, since those two selectors are mutually exclusive.

rpjday · March 9, 2022, 10:23am

Ah, it is suddenly all clear, thanks. I think I will add that to the appropriate docs and courseware.