Conditionally skip blocks

[talex5]

I'd like to skip certain blocks/sections, where the test is performed at runtime. For example, when running on macos in the opam sandbox we can't run any tests that use the network, and when running in a Docker container we might not be able to use IPv6.

Is there a good way to do this already?

If not, what's the best way to implement this? Perhaps if a block raises Mdx_skip then it could just be ignored?

[Leonidas-from-XIV]

You mean, only correct the block unless it raises Mdx_skip? I wonder if this isn't adding functionality to MDX that would better be solved on another level, e.g. do not run the MDX test via dune in some conditions.

[talex5]

Yes. For example, oUnit has a skip_if function that raises a special exception (https://github.com/gildor478/ounit/blob/master/src/lib/ounit2/advanced/oUnitAssert.ml#L37), and crowbar has a similar guard function (https://github.com/stedolan/crowbar/blob/0cbe3ea7e990a7d233360e6a74b1cb5e712501ad/src/crowbar.ml#L93).

I don't think it can be solved easily with dune. For example, most of the Eio README file should still be tested by opam-repo-ci, just not the parts using the network when run on macos within the opam sandbox.