Add CQ markdown doc

Adds a general CQ documentation page. Will add an auto generated
markdown doc based on cq.cfg once this lands.

Bug: 902507
Change-Id: I153efab72885faa2d70051f4372bfbcc2164f6d4
Reviewed-on: https://chromium-review.googlesource.com/c/1321579
Commit-Queue: Stephen Martinis <martiniss@chromium.org>
Reviewed-by: Sergey Berezin <sergeyberezin@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606270}

docs/infra/cq.md

+# 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.
+
+[TOC]
+
+## Purpose
+
+The Chromium CQ exists to test developer changes before they land into
+[chromium/src](https://chromium.googlesource.com/chromium/src/). It runs all the
+test suites which a given CL affects, and ensures that they all pass.
+
+## Options
+
+* `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".
+
+* `NOPRESUBMIT=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](https://crbug.com/new).
+
+* `NOTREECHECKS=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).
+
+* `NOTRY=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](https://chromium.googlesource.com/chromium/src/+/master/docs/code_reviews.md#TBR-To-Be-Reviewed)
+  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 CQ run?
+
+CQ runs the jobs specified in [cq.cfg](../../infra/config/branch/cq.cfg).
+
+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.
+
+### 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.
+1. 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](https://bugs.chromium.org/p/chromium/issues/entry?template=Defect%20report%20from%20developer&labels=Sheriff-Chromium&summary=%5BBrief%20description%20of%20problem%5D&comment=What%27s%20wrong?).
+   If the code in question has appropriate OWNERS, consider contacting or CCing
+   them.
+1. 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](g.co/bugatrooper).
+
+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](../../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](../../third_party/googletest).
+* 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.
+
+## 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.
+
+CQ handles flakiness mainly by retrying tests. It retries at a few different
+levels:
+
+1. Per test retries.
+
+   Most test suites have test retries built into them, which
+   retry failed tests a few times.
+1. Per build retries.
+
+   After a test suite fails in a build, the build will retry the test suite
+   again, both without patch, and with the patch applied.
+1. Per CQ run retries.
+
+   If a build fails, CQ will retry the individual trybot which failed.
+
+## Help!
+
+Have other questions? Run into any issues with the CQ? Email
+infra-dev@chromium.org, or file a [trooper bug](g.co/bugatrooper).