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Continuous Integration Cycles

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Laurence Tratt edited this page Oct 31, 2019 · 6 revisions

When working on Yorick, it's possible to create "CI cycles".

Dependency Architecture of ykrustc.

There are two main repos for Yorick:

  • ykrustc: the compiler.
  • yk: the other stuff, including ykpack.

ykpack is the library that deals with SIR serialisation and de-serialisation. The compiler uses it to encode SIR into an ELF section, and the JIT runtime uses it to decode the section.

This leads to a problem: if you change the format of the SIR (change the serialised types in any way that would change the binary representation once serialised), then CI cannot succeed. This is because the yk repo needs to be built with a ykrustc which uses the new ykpack, but yk itself contains the new ykpack.

How do we Break the Cycle?

We break the cycle using a 3-PR model as follows:

  1. Raise the first two PRs

The change author raises two PRs: one for yk and one for ykrustc.

  1. yk review

The reviewer conducts a review of the changes to the yk repo first. We get the branch squashed and ready, but we don't merge it just yet. If we were to try it'd fail. Remember: we need the compiler change in first.

  1. Review ykrustc

Next the reviewer switches to the ykrustc pull request and conducts a review there. We get this branch squashed and ready to merge, but stop short of invoking bors.

  1. Cycle breaker commit

Next the reviewer should ask the PR author to push a "cycle breaker" commit to ykrustc. This commit overrides the compiler's dependency on ykpack, making it instead use the commit hash for the head of the branch we reviewed in stage 2.

Here's an example cycle breaker commit

  1. Merge ykrustc

Once the cycle breaker commit is added, the reviewer can invoke bors to have the compiler change merged.

  1. Merge the ykpack change

Assuming the previous step worked, the CI server has now cached a version of ykrustc which uses the new ykpack. Now we can continue to invoke bors on the branch we reviewed in step 2 and CI will use the newly cached compiler to build it, before merging the branch.

  1. Undo the cycle breaker commit

Now that the ykpack changes are on the master branch of yk, we can revert the cycle breaker commit we introduced to ykrustc in step 4. The change author raises a new PR for this.

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