feat(build.sh) - Add mandatory license check

We could decide to make that optional, too, if this is coming into the way.

hack/build-flags.sh

@@ -1,3 +1,17 @@
+# Copyright 2019 The Knative Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
 function build_flags() {
   local base="${1}"
   local now="$(date -u '+%Y-%m-%d %H:%M:%S')"

hack/build.sh

@@ -61,6 +61,9 @@ run() {
     # Format source code
     go_fmt
 
+    # Check license
+    check_license
+
     # Generate docs
     generate_docs
   fi
@@ -98,6 +101,33 @@ go_test() {
   rm $test_output
 }
 
+check_license() {
+  echo "⚖️  License"
+  local required_keywords=("Authors" "Apache License" "LICENSE-2.0")
+  local extensions_to_check=("sh" "go" "yaml" "yml" "json")
+
+  local check_output=$(mktemp /tmp/kn-client-licence-check.XXXXXX)
+  for ext in "${extensions_to_check[@]}"; do
+    find . -name "*.$ext" -a \! -path "./vendor/*" -a \! -path "./.*" -print0 |
+      while IFS= read -r -d '' path; do
+        for rword in "${required_keywords[@]}"; do
+          if ! grep -q "$rword" "$path"; then
+            echo "   $path" >> $check_output
+          fi
+        done
+      done
+  done
+  if [ -s $check_output ]; then
+    echo "🔥 No license header found in:"
+    sort $check_output | uniq | cat
+    echo "🔥 Please fix and retry."
+    rm $check_output
+    exit 1
+  fi
+  rm $check_output
+}
+
+
 update_deps() {
   echo "🕸️  Update"
   go mod vendor