Azure DevOps, get the triggering branch of the triggering pipeline

Question

I've got two (YAML) pipelines in Azure DevOps, call them B (application build) and D (application deployment). D deploys to Dev, QA, and Test, each in a separate stage. The QA and Test environments are configured in DevOps to require approval before their respective stages in D are executed. The YAML files behind B and D are in the same DevOps project.

The code repository is Azure DevOps Git. B is triggered by completed merges to the main branch. D is triggered by successful completion of B. In case it matters, the means by which I've configured D to be triggered by successful completion of B is via

leading to

So far, this arrangement has worked well.

Now I want B to be triggered by not only feature pushes to main but by hotfix pushes to any branch named release/*. So,

  trigger:
    - main
    - release/*

The difference is that the hotfixes should be deployed only to Test, not to Dev or QA.

Therefore, in D, I want to make execution of the Dev and QA deployment stages conditional on the triggering branch of B having been main. Is there some way in D to access from B the value that in B can be referenced as $(Build.SourceBranch)?

UPDATE: I now learn that the manner I described above for having D triggered by B is itself outmoded, and I should be using something like

resources:
  pipelines:
    - pipeline: sourcePipeline
      source: 'B'
      trigger:
        branches:
          - main
          - release/*

Is that the correct way to set this up? Do I need to specify the branches here or are they relevant? (I saw one example that simply has trigger: true, which I'm guessing means that the second pipeline should always be run after the first completes. Perhaps branches are specified above only when B may be triggered by lots of branches but D should run after B only when B was triggered by a subset of those.)

If this is the preferred approach and I switch to it, does the answer to my question become that I can now access B's triggering branch in D through $(resources.pipeline.sourcePipeline.SourceBranch)?

Answer 1

In the absence of a response I continued with my project and engaged in some painful experimentation. In the end, my guesses turned out to be correct:

• Replace the Classic method of triggering D off of B with the YAML method, like this:

  resources:
    pipelines:
      - pipeline: sourcePipeline
        source: 'B'
        trigger: true

And this requires replacing Build variables like Build.SourceBranch with pipeline variables: resources.pipeline.sourcePipeline.SourceBranch is correct.

Finally (bonus answer to a question I hadn't asked here), it turns out you can't use condition: on stages that are template invocations. So instead I had to create a canRun parameter for the template being invoked, setting it to the same expression I would have used in a condition:, and then referencing canRun in condition:s in the template.