Add notify-on-failure action

Adds a composite action that can be used to send Slack notifications
for failed GitHub Action workflows. Intended to be used with the
pathogen-repo-build workflow, but I've made it a separate composite
action because it may be useful for monitoring GitHub Actions that are
not running Nextstrain builds.

README.md

@@ -36,6 +36,7 @@ Invoked by our GitHub Actions workflows, including the reusable workflows below.
 - [Setup debugger](actions/setup-debugger/action.yaml) for interactive debugging of workflow jobs
   ([README](actions/setup-debugger/README.md))
 - [Workflow context](actions/workflow-context/action.yaml)
+- [Notify on failure](actions/notify-on-failure/action.yaml)
 
 See also GitHub's [documentation on creating custom actions](https://docs.github.com/en/actions/creating-actions/about-custom-actions).
 

actions/notify-on-failure/action.yaml

@@ -0,0 +1,31 @@
+name: Notify Slack on failure
+description: >-
+  Sends Slack notification with link to a GitHub Action workflow run that failed.
+
+  Intended to be used for automated pathogen build workflows to ensure that the
+  entire team is notified of a GitHub Action failure and not just the GitHub
+  user who originally launched and/or scheduled the workflow.
+
+  Requires two environment variables to be set in the workflow:
+  - SLACK_TOKEN
+  - SLACK_CHANNELS
+
+  SLACK_TOKEN is the OAuth token for the Slack app that has permissions to send messages.
+  SLACK_CHANNELS is the name or channel id for the intended Slack channel.
+
+runs:
+  using: composite
+  steps:
+  - uses: actions/checkout@v3
+    with:
+      repository: nextstrain/ingest
+      path: nextstrain-ingest
+      sparse-checkout: |
+        notify-on-job-fail
+
+  - name: Notify job failure
+    shell: bash
+    run: ./nextstrain-ingest/notify-on-job-fail "$WORKFLOW_NAME" "$JOB_REPO"
+    env:
+      WORKFLOW_NAME: ${{ github.workflow }}
+      JOB_REPO: ${{ github.repository }}