scripts/check_license_headers.sh: delete, rewrite as a Go test

Updates tailscale/corp#29650

Change-Id: Iad4e4ccd9d68ebb1d1a12f335cc5295d0bd05b60
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

license_test.go

@@ -0,0 +1,117 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package tailscaleroot
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"tailscale.com/util/set"
+)
+
+func normalizeLineEndings(b []byte) []byte {
+	return bytes.ReplaceAll(b, []byte("\r\n"), []byte("\n"))
+}
+
+// TestLicenseHeaders checks that all Go files in the tree
+// directory tree have a correct-looking Tailscale license header.
+func TestLicenseHeaders(t *testing.T) {
+	want := normalizeLineEndings([]byte(strings.TrimLeft(`
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+`, "\n")))
+
+	exceptions := set.Of(
+		// Subprocess test harness code
+		"util/winutil/testdata/testrestartableprocesses/main.go",
+		"util/winutil/subprocess_windows_test.go",
+
+		// WireGuard copyright
+		"cmd/tailscale/cli/authenticode_windows.go",
+		"wgengine/router/ifconfig_windows.go",
+
+		// noiseexplorer.com copyright
+		"control/controlbase/noiseexplorer_test.go",
+
+		// Generated eBPF management code
+		"derp/xdp/bpf_bpfeb.go",
+		"derp/xdp/bpf_bpfel.go",
+
+		// Generated kube deepcopy funcs file starts with a Go build tag + an empty line
+		"k8s-operator/apis/v1alpha1/zz_generated.deepcopy.go",
+	)
+
+	err := filepath.Walk(".", func(path string, fi os.FileInfo, err error) error {
+		if err != nil {
+			return fmt.Errorf("path %s: %v", path, err)
+		}
+		if exceptions.Contains(filepath.ToSlash(path)) {
+			return nil
+		}
+		base := filepath.Base(path)
+		switch base {
+		case ".git", "node_modules", "tempfork":
+			return filepath.SkipDir
+		}
+		switch base {
+		case "zsyscall_windows.go":
+			// Generated code.
+			return nil
+		}
+
+		if strings.HasSuffix(base, ".config.ts") {
+			return nil
+		}
+		if strings.HasSuffix(base, "_string.go") {
+			// Generated file from go:generate stringer
+			return nil
+		}
+
+		ext := filepath.Ext(base)
+		switch ext {
+		default:
+			return nil
+		case ".go", ".ts", ".tsx":
+		}
+
+		buf := make([]byte, 512)
+		f, err := os.Open(path)
+		if err != nil {
+			return err
+		}
+		defer f.Close()
+		if n, err := io.ReadAtLeast(f, buf, 512); err != nil && err != io.ErrUnexpectedEOF {
+			return err
+		} else {
+			buf = buf[:n]
+		}
+
+		buf = normalizeLineEndings(buf)
+
+		bufNoTrunc := buf
+		if i := bytes.Index(buf, []byte("\npackage ")); i != -1 {
+			buf = buf[:i]
+		}
+
+		if bytes.Contains(buf, want) {
+			return nil
+		}
+
+		if bytes.Contains(bufNoTrunc, []byte("BSD-3-Clause\npackage ")) {
+			t.Errorf("file %s has license header as a package doc; add a blank line before the package line", path)
+			return nil
+		}
+
+		t.Errorf("file %s is missing Tailscale copyright header:\n\n%s", path, want)
+		return nil
+	})
+	if err != nil {
+		t.Fatalf("Walk: %v", err)
+	}
+}