Add license-checker action that fails when any backported file contains BUSL header (#18485)

* Add license-checker action that fails when any backported file contains BUSL header

* Quote echoed variable to retain line breaks

* Add ticket to reference for more details

.github/scripts/license_checker.sh

@@ -0,0 +1,16 @@
+#!/bin/bash
+# Copyright (c) HashiCorp, Inc.
+# SPDX-License-Identifier: BUSL-1.1
+
+
+busl_files=$(grep -r 'SPDX-License-Identifier: BUSL' --exclude=./.github/scripts/license_checker.sh .)
+
+# If we do not find a file in .changelog/, we fail the check
+if [ -n "$busl_files" ]; then
+    echo "Found BUSL occurrences in the PR branch! (See NET-5258 for details)"
+    echo -n "$busl_files"
+    exit 1
+else
+    echo "Did not find any occurrences of BUSL in the PR branch"
+    exit 0
+fi

.github/workflows/license-checker.yml

@@ -0,0 +1,27 @@
+# Copyright (c) HashiCorp, Inc.
+# SPDX-License-Identifier: MPL-2.0
+
+# This workflow checks that the BUSL license is not mentioned anywhere in
+# a PR targeting a release that should maintain the MPL-2.0 license.
+name: License Checker
+
+on:
+  pull_request:
+    types: [opened]
+    branches:
+      - release/1.14.*
+      - release/1.15.*
+      - release/1.16.*
+
+jobs:
+  # checks that the diff does not contain any reference to
+  # the BUSL license and thus retains the MPL-2.0 license
+  license-check:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@c85c95e3d7251135ab7dc9ce3241c5835cc595a9 # v3.5.3
+        with:
+          ref: ${{ github.event.pull_request.head.sha }}
+          fetch-depth: 0 # by default the checkout action doesn't checkout all branches
+      - name: Check for BUSL text in diff
+        run: ./.github/scripts/license_checker.sh