Thursday, April 5, 2018

Flaky test behavior with TFS build agents

On one of my projects, we noticed flaky test behavior where the test task somethings succeeded but sometimes failed to correctly initialize and run the test adapters. As the issue seemed related not to a specific test, but to the setup of the test task itself we had no clue what whas going on.

By comparing the succeeded and failed cases, we noticed that when the test task failed, we had the following warning in our build log:

Agent 'AgentXXX' is using a deprecated version '1.105.7'. You will not be able to use this version of agent with the next major version of Team Foundation Server. Migrate to the latest 2.x version of agent. For more information, see https://go.microsoft.com/fwlink/?linkid=851067.

This brought us to the solution. We had 2 build agents running on our build server. One agent was an older 1.x version  where the other one was using the 2.x version. Every time the build was triggered on the new agent it succeeded, where it failed on the old one.

To fix it, we configured the demands of our Build definition to include a VSTest_15.0 condition. Now the build is always executed on the new agent.

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