Pills: Conditional Pipeline decorators

Pipeline decorators are a really peculiar feature of Azure DevOps, because they allow you to specify a series of tasks that are run for EVERY pipeline in your organization, so they are rarely needed, but nevertheless they are a nice tool to know because there are situation when they are useful. Moreover, in latest Sprint 194 update they are expanded to support new functionalities, like running before or after specific tasks.

The major criticism against a pipeline decorator is it runs in every pipeline so it is a really impacting technique to use, but sometimes we forget that we have a rich syntax in pipeline to specify conditional execution. This allows you to change the decorator to run only if you have a specific variable defined, or you can exclude if you have a specific variable defined. These two scenarios allows you to implement an op-in or opt-out procedure.

Having a decorator to run in EVERY pipeline without the ability to even opt-out is usually a too risky/impacting decision.

The solution is really simple, just put a condition in the decorator definition, condition can be positive if you want to use opt-in or negative if you want to use opt-out. Let’s see an example with the decorator that runs after each build to remove all non source controlled files.

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steps:
- ${{ if not(eq(variables['skip-clean-decorator'], 'true'))}}:
  - task: CmdLine@2
    displayName: "do a git clean (injected from decorator)"
    inputs:
      workingDirectory: $(Build.SourcesDirectory)
      failOnStderr: false
      script: |
        git checkout -- .
        git clean -xdf        
    continueOnError: true

Pay attention to the indentation, task section must be indented after condition or it will not run.

As you can see the very first line of the decorator is a condition. In this case the condition is if the variable skip-clean-decorator is not equal to true. This means that if you define the variable skip-clean-decorator with value true, the decorator will not run. In this example I’ve used an opt-out solution, my decorator runs in every build, excepts those one that explicitly disable it.

If you change the condition removing the not you will use the opt-in model, the decorator does not run unless you explicitly enable it defining a specific variable.

It is a good practice to at least use opt-out mechanism for your decorators so you can always disable them if you need to.

If you didn’t know that a decorator can run conditionally you can now use it in a more effective way. Decorators are a really powerful weapon in your arsenal.

Gian Maria.