Add test that ensures we have no external dependencies

vendor_test.go

@@ -0,0 +1,66 @@
+// Copyright 2018 Twitch Interactive, Inc.  All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License"). You may not
+// use this file except in compliance with the License. A copy of the License is
+// located at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// or in the "license" file accompanying this file. This file 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.
+
+package twirp
+
+import (
+	"go/build"
+	"os"
+	"strings"
+	"testing"
+)
+
+func TestNoExternalDeps(t *testing.T) {
+	// Twirp commits its vendor directory so that 'go get' works for its main
+	// packages, but vendoring dependencies of the 'twirp' package could cause
+	// problems for users.
+	//
+	// The simplest way to make things safe is to have no non-stdlib dependencies
+	// in the twirp package.
+
+	wd, err := os.Getwd()
+	if err != nil {
+		t.Fatalf("unable to get current working directory: %v", err)
+	}
+
+	// Gather all imports of the current package recursively
+	allPkgs := make(map[string]bool)
+
+	var walkImports func(string)
+	walkImports = func(pkgName string) {
+		if allPkgs[pkgName] {
+			// already visited
+			return
+		}
+		allPkgs[pkgName] = true
+
+		pkg, err := build.Default.Import(pkgName, wd, 0)
+		if err != nil {
+			t.Fatalf("unable to import package %s: %s", pkgName, err)
+		}
+		for _, imported := range pkg.Imports {
+			// Standard libary packages don't have a '.' in them.
+			if !strings.Contains(imported, ".") {
+				continue
+			}
+			// This is a non-stdlib package. It's okay if it's a twirp package - as
+			// long as it doesn't have any external deps itself.
+			if strings.HasPrefix(imported, "github.com/twitchtv/twirp") {
+				walkImports(imported)
+			} else {
+				t.Errorf("imported external dependency: %v, imported by %v", imported, pkgName)
+			}
+		}
+	}
+	walkImports("github.com/twitchtv/twirp")
+}