CQ

This document describes how the Chromium Commit Queue (CQ) is structured and managed. This is specific for the Chromium CQ. Questions about other CQs should be directed to infra-dev@chromium.org.

Purpose

The Chromium CQ exists to test developer changes before they land into chromium/src. It runs all the test suites which a given CL affects, and ensures that they all pass.

Options

The Chromium CQ supports a variety of options that can change what it checks.

These options are supported via git footers. They must appear in the last paragraph of your commit message to be used. See git help footers or git_footers.py for more information.

• Commit: false

  You can mark a CL with this if you are working on experimental code and do not want to risk accidentally submitting it via the CQ. The CQ will immediately stop processing the change if it contains this option.

• Cq-Include-Trybots: <trybots>

  This flag allows you to specify some additional bots to run for this CL, in addition to the default bots. The format for the list of trybots is “bucket:trybot1,trybot2;bucket2:trybot3”.

• No-Presubmit: true

  If you want to skip the presubmit check, you can add this line, and the commit queue won‘t run the presubmit for your change. This should only be used when there’s a bug in the PRESUBMIT scripts. Please check that there‘s a bug filed against the bad script, and if there isn’t, file one.

• No-Tree-Checks: true

  Add this line if you want to skip the tree status checks. This means the CQ will commit a CL even if the tree is closed. Obviously this is strongly discouraged, since the tree is usually closed for a reason. However, in rare cases this is acceptable, primarily to fix build breakages (i.e., your CL will help in reopening the tree).

• No-Try: true

  This should only be used for reverts to green the tree, since it skips try bots and might therefore break the tree. You shouldn't use this otherwise.

• Tbr: <username>

  See policy of when it's acceptable to use TBR (“To be reviewed”). If a change has a TBR line with a valid reviewer, the CQ will skip checks for LGTMs.

FAQ

What exactly does the CQ run?

CQ runs the jobs specified in commit-queue.cfg. See cq-builders.md for an auto generated file with links to information about the builders on the CQ.

Some of these jobs are experimental. This means they are executed on a percentage of CQ builds, and the outcome of the build doesn't affect if the CL can land or not. See the schema linked at the top of the file for more information on what the fields in the config do.

The CQ has the following structure:

• Compile all test suites that might be affected by the CL.
• Runs all test suites that might be affected by the CL.
  • Many test suites are divided into shards. Each shard is run as a separate swarming task.
  • These steps are labeled ‘(with patch)’
• Retry each shard that has a test failure. The retry has the exact same configuration as the original run. No recompile is necessary.
  • If the retry succeeds, then the failure is ignored.
  • These steps are labeled ‘(retry shards with patch)’
  • It's important to retry with the exact same configuration. Attempting to retry the failing test in isolation often produces different behavior.
• Recompile each failing test suite without the CL. Rerun each failing test suite in isolation.
  • If the retry fails, then the fail is ignored, as it's assumed that the test is broken/flaky on tip of tree.
  • These steps are labeled ‘(without patch)’
• Fail the build if there are tests which failed in both ‘(with patch)’ and ‘(retry shards with patch)’ but passed in ‘(without patch)’.

Why did my CL fail the CQ?

Please follow these general guidelines:

1. Check to see if your patch caused the build failures, and fix if possible.
2. If compilation or individual tests are failing on one or more CQ bots and you suspect that your CL is not responsible, please contact your friendly neighborhood sheriff by filing a sheriff bug. If the code in question has appropriate OWNERS, consider contacting or CCing them.
3. If other parts of CQ bot execution (e.g. bot_update) are failing, or you have reason to believe the CQ itself is broken, or you can‘t really tell what’s wrong, please file a trooper bug.

In both cases, when filing bugs, please include links to the build and/or CL (including relevant patchset information) in question.

How do I add a new builder to the CQ?

There are several requirements for a builder to be added to the Commit Queue.

• All the code for this configuration must be in Chromium's public repository or brought in through src/DEPS.
• Setting up the build should be straightforward for a Chromium developer familiar with existing configurations.
• Tests should use existing test harnesses i.e. gtest.
• It should be possible for any committer to replicate any testing run; i.e. tests and their data must be in the public repository.
• Median cycle time needs to be under 40 minutes for trybots. 90th percentile should be around an hour (preferrably shorter).
• Configurations need to catch enough failures to be worth adding to the CQ. Running builds on every CL requires a significant amount of compute resources. If a configuration only fails once every couple of weeks on the waterfalls, then it's probably not worth adding it to the commit queue.

Please email dpranke@chromium.org, who will approve new build configurations.

How do I ensure a trybot runs on all changes to a specific directory?

Several builders are included in the CQ only for changes that affect specific directories. These used to be configured via Cq-Include-Trybots footers injected at CL upload time. They are now configured via location_regexp fields in commit-queue.cfg, e.g.

  builders {
    name: "chromium/try/my-specific-trybot"
    location_regexp: ".+/{+]/path/to/my/specific/directory/.+"
  }

Flakiness

The CQ can sometimes be flaky. Flakiness is when a test on the CQ fails, but should have passed (commonly known as a false negative). There are a few common causes of flaky tests on the CQ:

• Machine issues; weird system processes running, running out of disk space, etc...
• Test issues; individual tests not being independent and relying on the order of tests being run, not mocking out network traffic or other real world interactions.

The CQ mitigates flakiness by retrying failed tests. The core tradeoff in retry policy is that adding retries increases the probability that a flaky test will land on tip of tree sublinearly, but mitigates the impact of the flaky test on unrelated CLs exponentially.

For example, imagine a CL that adds a test that fails with 50% probability. Even with no retries, the test will land with 50% probability. Subsequently, 50% of all unrelated CQ attempts would flakily fail. This effect is cumulative across different flaky tests. Since the CQ has roughly ~20,000 unique flaky tests, without retries, pretty much no CL would ever pass the CQ.

Help!

Have other questions? Run into any issues with the CQ? Email infra-dev@chromium.org, or file a trooper bug.