Adding github action to check for changelog file (#11894)

* Adding github action to check for changelog file

This might have to be slightly different on ENT, where changelog files
should be prefixed with an underscore.

* Fixing comment

* Adding release branches, enhancement more specific

Also tidying my rusty bash

* More descriptive error messages

I also un-nested some if statements.

* Reference this PR too

.github/workflows/changelog-checker.yml

@@ -0,0 +1,49 @@
+# This workflow checks that there is either a 'pr/no-changelog' label applied to a PR
+# or there is a changelog/<pr number>.txt file associated with a PR for a changelog entry
+
+name: Check Changelog
+
+on:
+  pull_request:
+    types: [opened, synchronize, labeled, unlabeled]
+    # Runs on PRs to master
+    branches:
+      - master
+
+jobs:
+  # checks that a changelog entry is present for a PR
+  changelog-check:
+    # If there  a `pr/no-changelog` label we ignore this check
+    if: "!contains(github.event.pull_request.labels.*.name, 'pr/no-changelog')"
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v2
+        with:
+          ref: ${{ github.event.pull_request.head.sha }}
+          fetch-depth: 0 # by default the checkout action doesn't checkout all branches
+      - name: Check for changelog entry in diff
+        run: |
+          # check if there is a diff in the changelog directory
+          changelog_files=$(git --no-pager diff --name-only HEAD "$(git merge-base HEAD "origin/${{ github.event.pull_request.base.ref }}")" -- changelog/${{ github.event.pull_request.number }}.txt)
+
+          # If we do not find a file matching the PR # in changelog/, we fail the check
+          if [ -z "$changelog_files" ]; then
+            echo "Did not find a changelog entry named ${{ github.event.pull_request.number }}.txt"
+            echo "If your changelog file is correct, skip this check with the 'pr/no-changelog' label"
+            echo "Reference - https://github.com/hashicorp/vault/pull/10363 and https://github.com/hashicorp/vault/pull/11894"
+            exit 1
+          elif grep -q ':enhancement$' $changelog_files; then
+            # "Enhancement is not a valid type of changelog entry, but it's a common mistake.
+            echo "Found invalid type (enhancement) in changelog - did you mean improvement?"
+            exit 1
+          elif grep -q ':bugs$' $changelog_files; then
+            echo "Found invalid type (bugs) in changelog - did you mean bug?"
+            exit 1
+          elif ! grep -q '```release-note:' $changelog_files; then
+            # People often make changelog files like ```changelog:, which is incorrect.
+            echo "Changelog file did not contain 'release-note' heading - check formatting."
+            exit 1
+          else
+            echo "Found changelog entry in PR!"
+          fi