Flaky test package: test/xds

[zasweq]

Alongside #6913 and #6912, I have ran the test/xds suite on master since I added tests to it for my xDS Server fix #6889. I have encountered numerous flakes on g3, particularly those outlined in custom lb tests for distribution #6601. However, I have encountered almost every client and server side xDS test flake with a context timeout for a RPC expected to proceed. Each has different logs/events preceeding it's timeout, but every test seems susceptible to timeout. The flakes are generally rare, but due to the number of tests in the test suite you can successfully trigger by running the full test suite enough times. My initial inkling tells me there's some synchronization needed or something gets stuck in the management server/testing xDS Client flow. This also manifests in rare flakes for my xDS Server fix, where I expect something like an err that represents Accept and Close, and I get a context timeout instead.

[arvindbr8]

another one for TestServerSideXDS_WithValidAndInvalidSecurityConfiguration: https://github.com/grpc/grpc-go/actions/runs/7480796959/job/20361025267?pr=6916

[zasweq]

https://github.com/grpc/grpc-go/actions/runs/7618526481/job/20749870802?pr=6933

[arvindbr8]

https://github.com/grpc/grpc-go/actions/runs/7716359885/job/21032969470?pr=6949

[arvindbr8]

https://github.com/grpc/grpc-go/actions/runs/7791646589/job/21248143511?pr=6965

[zasweq]

https://github.com/grpc/grpc-go/actions/runs/8546803299/job/23417838293?pr=7085

[arjan-bal]

https://github.com/grpc/grpc-go/actions/runs/9790522733/job/27032314481?pr=7390

[arjan-bal]

@zasweq I investigated this and the problem seems to be due to the xDS management server getting stuck while writing to this buffered channel

grpc-go/test/xds/xds_server_certificate_providers_test.go

Line 249 in d27ddb5

case nackCh <- struct{}{}:

In the logs of failing runs for TestServerSideXDS_WithValidAndInvalidSecurityConfiguration, I noticed that the resource snapshot update request is sent to the xds management server before the xds client is able to connect to the xds server. This somehow results in more than 1 Listener requests being sent to the xds server which get stuck waiting to write to the buffered channel.

This seems to be a problem with the test and not the implementation. Adding a 50 millis sleep after starting both the servers did get rid of the flakiness in TestServerSideXDS_WithValidAndInvalidSecurityConfiguration.

[zasweq]

Ah nice thank you for figuring this out!