[CI] Add Design Doc for Premerge Advisor

This patch adds a design document for the premerge advisor that we plan on
implementing.

Author: boomanaiden154

premerge/premerge-advisor.md

@@ -0,0 +1,168 @@
+# Premerge Advisor
+
+## Introduction
+
+While the premerge infrastructure is generally reliable, tests that are broken
+at HEAD, or tests that are flaky can significantly degrade the user experience.
+Failures unrelated to the PR being worked on can lead to warning fatigue which
+can compound this problem when people land failing tests into main that they
+believe are unrelated. The premerge advisor is designed to run after premerge
+testing and signal to the user whether the failures can be safely ignored because
+they are flaky or broken at head, or whether they should be investigated further.
+
+## Background/Motivation
+
+The usefulness of the premerge system is significantly impacted by how much
+people trust it. People trust the system less when the issues reported to them
+are unrelated to the changes that they are currently working on. False positives
+occur regularly whether due to flaky tests, or due to main being broken. Efforts
+to fix these issues at the source and keep main always green and deflake tests
+are laudable, but not a scalable solution to the problem of false positive
+failures. It is also not reasonable to expect PR authors to spend time
+familiarizing themselves with all known flaky tests and dig through postcommit
+testing logs to see if the failures in their premerge run also occur in main.
+These alternatives are further explored in the section on
+[alternatives considered](#alternatives-considered). Having tooling to
+automatically run through the steps that one would otherwise need to perform
+manually would ensure the analysis on every failed premerge run is thorough and
+likely to be correct.
+
+## Design
+
+The premerge advisor will consist of three main parts: jobs uploading failure
+information, a web server and database to store and query failure information,
+and tooling to write out a verdict about the failures to comments on Github.
+When a job runs premerge or postcommit and there are build/test failures, it
+will upload information to the web server containing information on the failure
+like the test/build action that failed and the exact log. The web server will
+then store this information in a format that makes it easy to query for later.
+Every premerge run the encounters build/test failures will then query the web
+server to see if there are any matching build/test failures for the version of
+`main` that the PR has been merged into. If there are, the web server can
+respond with the failure information and signal that the failures can be safely
+ignored for that premerge run. If the failures are novel, then the web server
+can signal that the failures should be investigated more thoroughly. If there
+are failures, the premerge advisor can then write out a comment on the PR
+explaining its findings.
+
+### Processing and Storing Failures
+
+A key part of the premerge advisor is infrastructure to store and process build
+failure information so that it can be queried later. We plan on having jobs
+extract failure logs and upload them to a web server. This web server in
+addition to having an endpoint to accept uploads will have an endpoint that will
+accept test failure information (logs and filenames) and return whether or not
+they are broken at `main`, flaky, or novel test failures due to the PR.
+
+For the premerge jobs running through Github Actions, we plan on using the
+existing `generate_test_report` scripts that are currently used for generating
+summaries on job failures. When the job ends and there is a failure, there would
+be a script that runs, utilizing the `generate_test_report` library to extract
+failure information, and then uploading the information to the web server.
+Information on how the premerge jobs will query the server and display results
+about flaky/already broken tests is in
+[the section below](#informing-the-user). We plan on having both the premerge
+and postcommit jobs upload failure information to the web server. This enables
+the web server to know about failures that are not the result of mid-air
+collisions before postcommit testing has been completed. Postcommit testing can
+take upwards of 30 minutes, during which the premerge advisor would not be able
+to advise that these failures are due to a broken `main`.
+
+We plan on implementing the web server in python with the `flask` library. All
+contributors to the premerge infrastructure are already familiar with Python,
+building web servers in python with `flask` is relatively easy, and we do not
+need high performance or advanced features. For storage, we plan on using SQLite
+as it has support built in to python, does not require any additional complexity
+in terms of infrastructure setup, and is reliable.
+
+Given we have two identical clusters that need to be able to function
+independently of each other, we also need to duplicate the web server for the
+premerge advisor. Queries and failure information uploads will go to the
+cluster-local premerge advisor web server. Periodically (eg once every thirty
+seconds), the web server will query the web server on the other cluster to see
+if there is any new data that has not been propgated back to the other side yet.
+It is easy to figure out what one side is missing as Github workflow runs are
+numbered sequentially and git commits are also explicitly ordered. One side just
+needs to send the latest commit SHA and workflow run it has all previous data
+for, and the other side can reply with the data that it has past that point.
+Explicitly synchronizing everytime without assumptions about the state of the
+other side has benefits over just writing through, like ensuring that a cluster
+that has been down for a significant amount of time is seamlessly able to
+recover. Synchronization does not need to be perfect as test failures that are
+flaky or broken at head will almost certainly show up in both clusters
+relatively quickly, and minor discrepancies for queries between the clusters are
+not a big deal.
+
+### Informing the User
+
+Once a workflow has completed, no actions related to the premerge advisor will
+be performed if there are no failures. If there are failures, they will be
+uploaded to the web server. Afterwards, the premerge workflow then makes a
+request to the server asking if it can explain the failures as either existing
+in `main` or as flaky.
+
+After the response from the server has been recieved, the workflow will then
+construct a comment. It will contain the failure information, and if relevant,
+information/links showing the tests are flaky (and the flakiness rate) or are
+broken in `main`. If all of the test failures are due to flakes or failures in
+`main`, the comment will indicate to the user that they can safely merge their
+PR despite the test failures. We plan to construct the comment in a manner
+similar to the code formatting action. We will create one comment on the first
+workflow failure and then update that comment everytime we get new data. This
+prevents creating much noise. This does mean that the comment might get buried
+in long running reviews, but those are not the common case and people should
+learn to expect to look for the comment earlier in the thread in such cases.
+
+## Alternatives Considered
+
+There are two main alternatives to this work. One would be to do nothing and let
+users figure out these failures on their own, potentially with documentation to
+better inform them of the process. The other alternative would be to work on
+keeping `main` green all the time and deflake tests rather than work on a
+premerge advisor. Both of these alternatives are considered below.
+
+### Deflaking Tests and Keeping Main Green
+
+Instead of putting effort into building the premerge advisor, we could also be
+putting effort into deflaking tests and making process changes to ensure `main`
+is not broken. These fixes have the bonus of being useful for more than just
+premerge, also improving reliability for buildbots and any downstream testing.
+While we probably could achieve this at least temporarily with process changes
+and a concentrated deflaking effort, we do not believe this is feasible or
+scalable.
+
+In order to ensure that main is not broken by new patches, we need to ensure
+that every commit is tested directly on top of `main` before landing. This is
+not feasible given LLVM's current processes where pushing directly to main is a
+critical component of several developer's workflows. We would also need to
+reduce the risk of "mid-air collisions", patches that pass premerge testing, but
+fail on `main` when landed due to the patch in its original state not being
+compatible with the new state of main. This would most likely involve merge
+queues which would introduce new CI load and are also not compatible with LLVM's
+existing practices for the same reason requiring premerge checks to be run
+before landing are not.
+
+Doing an initial effort for deflaking tests is also not scalable from an
+engineering effort perspective. While we might be able to deflake existing
+tests, additional flaky tests will get added in the future, and it is likely not
+feasible to dedicate enough resources to deflake them. Policy improvements
+around reverting patches that introduce flaky tests might make this more
+scalable, but relies on quick detection of flaky tests, which might be difficult
+for tests that experience flaky failures very rarely.
+
+### Not Doing Anything
+
+Alternatively, we could not implement this at all. This system adds quite a bit
+of complexity and adds new failure modes. False positive failures also are not
+that frequent. However, even a relatively small percentage of failures like we
+are observing significantly impacts trust in the premerge system, which
+compounds the problem. People learn not to trust the results, ignore true
+failures caused by their patch, and then land it, causing many downstream
+failures. The frequency of these incidents (typically multiple times per week)
+means that it is pretty likely most LLVM developers will run into this class of
+issue sooner or later.
+
+The complexity is also well confined to the components specific to this new
+infrastructure, and the new failure modes can be mitigated through proper error
+handling at the interface between the existing premerge system and the new
+premerge advisor infrastructure.