go/build: add check for allowed vendor packages

Add a test that checks that all vendored packages in both src/ and
src/cmd are on an allowlist.

For #65678.

Change-Id: Ie9982f8b69c9c0fc6fef41c9dadeb464ecd7781a
Reviewed-on: https://go-review.googlesource.com/c/go/+/564355
Reviewed-by: Bryan Mills <bcmills@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

src/go/build/vendor_test.go

@@ -0,0 +1,79 @@
+// 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 build
+
+import (
+	"internal/testenv"
+	"runtime"
+	"strings"
+	"testing"
+)
+
+// Prefixes for packages that can be vendored into the go repo.
+// The prefixes are component-wise; for example, "golang.org/x"
+// matches "golang.org/x/build" but not "golang.org/xyz".
+//
+// DO NOT ADD TO THIS LIST TO FIX BUILDS.
+// Vendoring a new package requires prior discussion.
+var allowedPackagePrefixes = []string{
+	"golang.org/x",
+	"github.com/google/pprof",
+	"github.com/ianlancetaylor/demangle",
+	"rsc.io/markdown",
+}
+
+// Verify that the vendor directories contain only packages matching the list above.
+func TestVendorPackages(t *testing.T) {
+	_, thisFile, _, _ := runtime.Caller(0)
+	goBin := testenv.GoToolPath(t)
+	listCmd := testenv.Command(t, goBin, "list", "std", "cmd")
+	out, err := listCmd.Output()
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, fullPkg := range strings.Split(string(out), "\n") {
+		pkg, found := strings.CutPrefix(fullPkg, "vendor/")
+		if !found {
+			_, pkg, found = strings.Cut(fullPkg, "/vendor/")
+			if !found {
+				continue
+			}
+		}
+		if !isAllowed(pkg) {
+			t.Errorf(`
+		Package %q should not be vendored into this repo.
+		After getting approval from the Go team, add it to allowedPackagePrefixes
+		in %s.`,
+				pkg, thisFile)
+		}
+	}
+}
+
+func isAllowed(pkg string) bool {
+	for _, pre := range allowedPackagePrefixes {
+		if pkg == pre || strings.HasPrefix(pkg, pre+"/") {
+			return true
+		}
+	}
+	return false
+}
+
+func TestIsAllowed(t *testing.T) {
+	for _, test := range []struct {
+		in   string
+		want bool
+	}{
+		{"evil.com/bad", false},
+		{"golang.org/x/build", true},
+		{"rsc.io/markdown", true},
+		{"rsc.io/markdowntonabbey", false},
+		{"rsc.io/markdown/sub", true},
+	} {
+		got := isAllowed(test.in)
+		if got != test.want {
+			t.Errorf("%q: got %t, want %t", test.in, got, test.want)
+		}
+	}
+}