enforce license on Go files

.circleci/config.yml

@@ -6,3 +6,4 @@ jobs:
     steps:
       - checkout
       - run: go test -mod=readonly ./...
+      - run: find -name '*.go' -not -name '*.pb.go' | xargs go run utils/license.go --

utils/license.go

@@ -0,0 +1,62 @@
+// Copyright 2018 Google LLC
+//
+// 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
+//
+//     https://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.
+
+// +build ignore
+
+package main
+
+import (
+	"bytes"
+	"flag"
+	"io"
+	"log"
+	"os"
+)
+
+var sentinels = [][]byte{
+	[]byte("Copyright"),
+	[]byte("Google LLC"),
+	[]byte("Apache License"),
+	[]byte("limitations under the License."),
+}
+
+func main() {
+	flag.Parse()
+
+	var buf [600]byte
+	exitCode := 0
+
+	for _, fname := range flag.Args() {
+		f, err := os.Open(fname)
+		if err != nil {
+			log.Fatal(err)
+		}
+
+		n, err := io.ReadFull(f, buf[:])
+		if err != nil && err != io.EOF && err != io.ErrUnexpectedEOF {
+			log.Fatal(err)
+		}
+
+		f.Close()
+
+		for _, s := range sentinels {
+			if bytes.Index(buf[:n], s) < 0 {
+				log.Printf("file doesn't have license header, can't find %q: %s", s, fname)
+				exitCode = 1
+			}
+		}
+	}
+
+	os.Exit(exitCode)
+}