Fix couldn't load package bug, if multiple GOPATH exists and target code is not in first GOPATH

.travis.yml

@@ -5,5 +5,4 @@ go:
   - 1.8
   - master
 script:
-  - go test ./...
-
+  - GOPATH="$GOPATH:/tmp/jsonenums-test/go1:/tmp/jsonenums-test/go2" go test ./...

parser/parser.go

@@ -23,7 +23,6 @@ import (
 	"go/token"
 	"go/types"
 	"log"
-	"path/filepath"
 	"strings"
 
 	"golang.org/x/tools/go/loader"
@@ -39,20 +38,20 @@ type Package struct {
 
 // ParsePackage parses the package in the given directory and returns it.
 func ParsePackage(directory string) (*Package, error) {
-	relDir, err := filepath.Rel(filepath.Join(build.Default.GOPATH, "src"), directory)
+	p, err := build.ImportDir(directory, build.FindOnly)
 	if err != nil {
-		return nil, fmt.Errorf("provided directory not under GOPATH (%s): %v",
-			build.Default.GOPATH, err)
+		return nil, fmt.Errorf("provided directory (%s) may not under GOPATH (%s): %v",
+			directory, build.Default.GOPATH, err)
 	}
 
 	conf := loader.Config{TypeChecker: types.Config{FakeImportC: true}}
-	conf.Import(relDir)
+	conf.Import(p.ImportPath)
 	program, err := conf.Load()
 	if err != nil {
 		return nil, fmt.Errorf("couldn't load package: %v", err)
 	}
 
-	pkgInfo := program.Package(relDir)
+	pkgInfo := program.Package(p.ImportPath)
 	return &Package{
 		Name:  pkgInfo.Pkg.Name(),
 		files: pkgInfo.Files,

parser/parser_test.go

@@ -0,0 +1,52 @@
+// Copyright 2017 Google 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.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to writing, software distributed
+// under the License is distributed on a "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 parser
+
+import (
+	"go/build"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func must(t *testing.T, err error) {
+	if err != nil {
+		t.Fatal(err)
+	}
+}
+
+var fakeCode = `
+package main
+import "fmt"
+func main() {
+	fmt.Println("Hello world!")
+}
+`
+
+func TestParseFromMultipleGopath(t *testing.T) {
+	gopaths := filepath.SplitList(build.Default.GOPATH)
+	if len(gopaths) < 2 {
+		t.Skipf("No multiple GOPATH (%s) exists, skiping..", build.Default.GOPATH)
+	}
+	gopath := gopaths[len(gopaths)-1]
+	dir := filepath.Join(gopath, "src", "foo")
+	defer must(t, os.RemoveAll(dir))
+	must(t, os.MkdirAll(dir, 0755))
+	must(t, ioutil.WriteFile(filepath.Join(dir, "main.go"), []byte(fakeCode), 0644))
+
+	if _, err := ParsePackage(dir); err != nil {
+		t.Fatalf("Parse package (%v): %v", dir, err)
+	}
+}