internal/copyright: add test that copyright notices exist

We shouldn't spend human code review time checking this.
Let the computer check.

Change-Id: I6de9d733c128d833b958b0e43a52b564e8f82dd3
Reviewed-on: https://go-review.googlesource.com/c/go/+/630417
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Sam Thanawalla <samthanawalla@google.com>

src/cmd/go/internal/auth/gitauth_test.go

@@ -1,3 +1,7 @@
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
 package auth
 
 import (

src/cmd/go/internal/web/intercept/intercept.go

@@ -1,3 +1,7 @@
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
 package intercept
 
 import (

src/crypto/internal/fips140/check/checktest/asm.s

@@ -1,3 +1,7 @@
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
 //go:build !purego && !wasm
 
 #include "textflag.h"

src/crypto/tls/bogo_shim_test.go

@@ -1,3 +1,7 @@
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
 package tls
 
 import (

src/internal/copyright/copyright_test.go

@@ -0,0 +1,69 @@
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package copyright
+
+import (
+	"bytes"
+	"internal/testenv"
+	"io"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+var copyright = []byte("Copyright")
+
+var permitted = [][]byte{
+	[]byte("// Code generated by "),
+	[]byte("// Code generated from "),
+	[]byte("// Created by cgo -cdefs"),
+	[]byte("// DO NOT EDIT\n// generated by:"),
+	[]byte("// Empty assembly file"),
+	[]byte("// Generated using cgo"),
+	[]byte("// Original source:\n//\thttp://www.zorinaq.com/papers/md5-amd64.html"), // public domain crypto/md5
+	[]byte("// created by cgo -cdefs"),
+	[]byte("// go run mkasm.go"),
+	[]byte("// mkerrors"),
+	[]byte("// mksys"),
+	[]byte("// run\n// Code generated by"), // cmd/compile/internal/test/constFold_test.go
+}
+
+func TestCopyright(t *testing.T) {
+	buf := make([]byte, 2048)
+	filepath.WalkDir(filepath.Join(testenv.GOROOT(t), "src"), func(path string, d fs.DirEntry, err error) error {
+		if d.Name() == "testdata" || d.Name() == "vendor" {
+			return filepath.SkipDir
+		}
+		switch filepath.Ext(d.Name()) {
+		default:
+			return nil
+		case ".s", ".go":
+			// check
+		}
+
+		f, err := os.Open(path)
+		if err != nil {
+			t.Error(err)
+			return nil
+		}
+		n, err := f.Read(buf)
+		if err != nil && err != io.EOF {
+			t.Error(err)
+			return nil
+		}
+		b := buf[:n]
+		if bytes.Contains(b, copyright) {
+			return nil
+		}
+		for _, ok := range permitted {
+			if bytes.HasPrefix(b, ok) {
+				return nil
+			}
+		}
+		t.Errorf("%s: missing copyright notice", path)
+		return nil
+	})
+}

src/internal/runtime/sys/dit_arm64.s

@@ -1,3 +1,7 @@
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
 #include "textflag.h"
 
 TEXT ·EnableDIT(SB),$0-1

src/internal/types/errors/generrordocs.go

@@ -1,9 +1,9 @@
-//go:build ignore
-
 // Copyright 2023 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build ignore
+
 // generrordocs creates a Markdown file for each (compiler) error code
 // and its associated documentation.
 // Note: this program must be run in this directory.

src/log/slog/example_discard_test.go

@@ -1,3 +1,7 @@
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
 package slog_test
 
 import (

src/path/filepath/path_windows.go

@@ -1,3 +1,7 @@
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
 package filepath
 
 import (