Fix concurrency and cancel in progress contexts

When evaluating expressions for cancel-in-progress, certain
parameters may not be available at the time of evaluation.

For instance, the github.job context is not accessible, as it's
specific to the running job and not the concurrency group.

Change-type: patch
Signed-off-by: Kyle Harding <[email protected]>

.github/workflows/yocto-build-deploy.yml

@@ -134,10 +134,32 @@ on:
         type: string
 
 # https://docs.github.com/en/actions/using-jobs/using-concurrency
+# https://docs.github.com/en/actions/writing-workflows/choosing-what-your-workflow-does/control-the-concurrency-of-workflows-and-jobs
+# The following concurrency group cancels in-progress jobs or runs on pull_request events only;
+# if github.head_ref is undefined, the concurrency group will fallback to the run ID,
+# which is guaranteed to be both unique and defined for the run.
+
+# From: https://github.com/orgs/community/discussions/69704#discussioncomment-7803351
+
+# The available contexts for cancel-in-progress expressions are:
+# - github: This context provides access to various GitHub-specific variables,
+#   such as github.event_name, github.ref, and github.workflow.
+# - inputs: This context allows you to access input parameters defined in the workflow.
+#   This is particularly useful for conditional cancellation based on user-specified settings.
+# - vars: This context provides access to workflow-defined variables,
+#   which can be used to store intermediate values or constants.
+# When evaluating expressions for cancel-in-progress, certain parameters may not be available at the time of evaluation.
+# For instance, the github.job context is not accessible, as it's specific to the running job and not the concurrency group.
+
+# Note that we do not use github.ref here, as PRs from forks will have a
+# ref of 'refs/heads/master' and collide with each other. Instead, we use github.head_ref
 concurrency:
-  group: ${{ github.workflow }}-${{ github.event.number || github.ref }}-${{ inputs.machine }}-${{ inputs.deploy-environment}}
-  # cancel jobs in progress for updated PRs, but not merge or tag events
-  cancel-in-progress: ${{ github.event.action == 'synchronize' }}
+  group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}-${{ inputs.machine }}-${{ inputs.deploy-environment }}
+  # Cancel jobs in-progress for open PRs, but not merged or closed PRs, by checking for the merge ref.
+  # Note that for pull_request_target events (PRs from forks), the github.ref value is
+  # usually 'refs/heads/master' so we can't rely on that to determine if it is a merge event or not.
+  # As a result pull_request_target events will never cancel in-progress jobs and will be queued instead.
+  cancel-in-progress: ${{ startsWith(github.ref, 'refs/pull/') }}
 
 env:
   WORKSPACE: ${{ github.workspace }}