Rollup Procedure


The Rust project has a policy that every pull request must be tested after merge before it can be pushed to master. As PR volume increases this can scale poorly, especially given the long (~3.5hr) current CI duration for Rust.

Enter rollups! Changes that are small, not performance sensitive, or not platform dependent are marked with the rollup command to bors (@bors r+ rollup to approve a PR and mark as a rollup, @bors rollup to mark a previously approved PR, @bors rollup- to un-mark as a rollup). ‘Performing a Rollup’ then means collecting these changes into one PR and merging them all at once. The rollup command accepts four values always, maybe, iffy, and never. See the Rollups section of the review policies for guidance on what these different statuses mean.

You can see the list of rollup PRs on Rust’s Homu queue, they are listed at the bottom of the ‘approved’ queue with a priority of ‘rollup’ meaning they will not be merged by themselves until everything in front of them in the queue has been merged.

Making a Rollup

  1. Using the interface on Homu queue, select pull requests and then use “rollup” button to make a rollup pull request. (The text about fairness can be ignored.) Important note: consider for addition PRs marked as rollup=always, rollup=maybe and rollup=iffy, based on the review policies of the Rollups section. Be extra careful when deciding what to include, in particular on rollup=maybe and rollup=iffy PRs. We should try as much as possible to avoid risking and hit regressions (bugs or perf). Also consider that contributors often forget to tag things with rollup=never, when they should have done so, so when PRs are not explicitly tagged with rollup, be extra careful.

  2. Run the following command in the pull request thread:

    @bors r+ rollup=never p=5
  3. If the rollup fails, use the logs rust-log-analyzer provides to bisect the failure to a specific PR and do @bors r-. If the PR is running, you need to do @bors r- retry. Otherwise, your rollup succeeded. If it did, proceed to the next rollup (every now and then let rollup=never and toolstate PRs progress).

  4. Recreate the rollup without the offending PR starting again from 1.. There’s a link in the rollup PR’s body to automatically prefill the rollup UI with the existing PRs (minus any PRs that have been r-d)

Selecting Pull Requests

The queue is sorted by rollup status. In general, a good rollup includes one or two iffy PRs (if available), a bunch of maybe (unmarked) PRs, and a large pile of always PRs. A rollup should never include rollup=never PRs.

The actual absolute size of the rollup can depend based on experience, people new to making rollups might start with including 1 iffy, 4 maybes, and 5 alwayss, but more experienced people might even make a rollup of 1-2 iffys, 8 maybes, and 10 alwayss! Massive rollups are rarely needed, but as your intuition grows you’ll get better at judging risk when including PRs in a rollup.

Don’t hesitate to downgrade the rollup status of a PR! If your intuition tells you that a rollup=always PR has some chances for failures, mark it rollup=maybe or rollup=iffy. A lot of the unmarked maybe PRs are categorized as such because the reviewer may not have considered rollupability, so it’s always worth picking them with a critical eye. Similarly, if a PR causes your rollup to fail, it’s worth considering changing its rollup status

Generally, PRs, that touch CI configuration or the bootstrapping process are probably iffy and should be handled with care. On the other hand, PRs that just edit docs are usually rollup=always.

Avoid having too many PRs with large diffs or submodule changes in the same rollup. Also avoid having PRs you suspect will have large perf impacts, and mark them as rollup=never.

It’s tempting to avoid including iffy PRs at all since ideally you want your rollup to succeed. However, it’s worth remembering that the job of the PR queue is to test PRs, not to land them. As such, a rollup that fails because of an iffy PR is a good thing, since that PR would have to be tested at some point anyway and it would have taken up the same amount of time to test if it never got included in a rollup. One way to look at rollups when it comes to iffy PRs is that a rollup is a way for a bunch of other PRs to piggyback on the CI cycle that the iffy PR needs anyway. If rollups avoid iffy PRs entirely what ends up happening is that these PRs tend to languish in the queue for a long time, which isn’t good.

Similarly, make sure to leave some spare CI cycles so that never PRs also get a chance! If you’re the only person making rollups it’s worth letting them run during times you’re not paying attention to the queue, but these days there are rollup authors in multiple time zones, so it’s often best to just keep an eye on the relative size of the queue and put aside a couple CI cycles for never PRs, especially if they pile up.

Try to be fair with rollups: Rollups are a way for things to jump the queue. For rollup=maybe PRs, try to include the oldest one (at the top of the section) so that newer PRs aren’t jumping the queue over older PRs entirely. You don’t have to include every PR older than PRs included in your rollup, but try to include the oldest. Similar to the perspective around iffy, it’s useful to look at a rollup as a way for other PRs to piggyback on the CI cycle of the oldest PR in queue.

Failed rollups

If the rollup has failed, run the @bors retry command if the failure was spurious (e.g. due to a network problem or a timeout). If it wasn’t spurious, find the offending PR and throw it out by copying a link to the rust-logs-analyzer comment, and writing Failed in <link_to_comment>, @bors r-. Hopefully, the author or reviewer will give feedback to get the PR fixed or confirm that it’s not at fault. The failed rollup PR can be closed.

Once you’ve removed the offending PR, re-create your rollup without it (see 1.). Sometimes however, it is hard to find the offending PR. If so, use your intuition to avoid the PRs that you suspect are the problem and recreate the rollup. Another strategy is to raise the priority of the PRs you suspect, mark them as rollup=never (or iffy) and let bors test them standalone to dismiss or confirm your hypothesis.

If a rollup continues to fail you can run the @bors rollup=never command to never rollup the PR in question.