Multi-Stage Azure Pipeline with a path trigger - first stage always runs

Question

I have a multi-stage pipeline for my terraform code. This is my first pipeline so I am sure I'm missing something but there appears to be some inconsistent behaviour that is rather frustrating.

The pipeline trigger is defined as follows:

trigger:
  branches:
    include:
      - master
  paths:
    include:
      - terraform/*

My repo has the following folder structure:

IaC
 |
 +-- terraform
 |    |
 |    +-- main.tf
 |   
 |    
 +-- azure-pipeline.yml
 +-- readme.md

My two stages are defined as follows (I cut out the job steps, its just tarraform commands):

stages:
  - stage: plan
    displayName: 'terraform plan'
    condition: eq(variables['Build.Reason'], 'PullRequest')
    ...
  - stage: apply
    displayName: 'terraform apply'
    condition: ne(variables['Build.Reason'], 'PullRequest')
    ...

Whenever I update code in the terraform directory and create a PR, the pipeline works exactly as expected. It does the terraform plan for the reviewer to see and once the PR is approved, the second stage fires and it does a terraform apply.

The issue I am having is whenever I update a file outside the terraform directory and create a PR, the first stage still runs! Oddly, once the PR is approved, the second stage does not.

Are my stage conditions to simple? Does a multi-stage pipeline always have to do something?

Answer 1

Your pipeline runs for pull requests not because of the trigger, but because of a build validation policy you've set for one or more target branches ("whenever I create a PR to this branch, start this pipeline"). It's the build validation, as part of a branch's policies: https://docs.microsoft.com/en-us/azure/devops/repos/git/branch-policies?view=azure-devops#build-validation

You probably didn't specify a path filter in the build validation settings. Here's an example of three different build validations in a single branch policy, all with different path filters:

In short: The path trigger in your YAML has no effect on how a pipeline is triggered for a PR. The trigger you've specified is one that triggers on commits to the master branch, if the commit includes files in the terraform directory.