google/externalaccount: moves externalaccount package out of internal and exports it

go/programmable-auth-design for context. Adds support for user defined
 supplier methods to return subject tokens and AWS security credentials.

Change-Id: I7bc41f8c5202ae933fce516632f5049bbeb3d378
GitHub-Last-Rev: ac519b242f8315df572f1b205b0670f139bfc6c3
GitHub-Pull-Request: golang/oauth2#690
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/550835
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Leo Siracusa <leosiracusa@google.com>
Reviewed-by: Chris Smith <chrisdsmith@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cody Oss <codyoss@google.com>

google/externalaccount/filecredsource.go

@@ -0,0 +1,61 @@
+// Copyright 2020 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 externalaccount
+
+import (
+	"bytes"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"io/ioutil"
+	"os"
+)
+
+type fileCredentialSource struct {
+	File   string
+	Format Format
+}
+
+func (cs fileCredentialSource) credentialSourceType() string {
+	return "file"
+}
+
+func (cs fileCredentialSource) subjectToken() (string, error) {
+	tokenFile, err := os.Open(cs.File)
+	if err != nil {
+		return "", fmt.Errorf("oauth2/google/externalaccount: failed to open credential file %q", cs.File)
+	}
+	defer tokenFile.Close()
+	tokenBytes, err := ioutil.ReadAll(io.LimitReader(tokenFile, 1<<20))
+	if err != nil {
+		return "", fmt.Errorf("oauth2/google/externalaccount: failed to read credential file: %v", err)
+	}
+	tokenBytes = bytes.TrimSpace(tokenBytes)
+	switch cs.Format.Type {
+	case "json":
+		jsonData := make(map[string]interface{})
+		err = json.Unmarshal(tokenBytes, &jsonData)
+		if err != nil {
+			return "", fmt.Errorf("oauth2/google/externalaccount: failed to unmarshal subject token file: %v", err)
+		}
+		val, ok := jsonData[cs.Format.SubjectTokenFieldName]
+		if !ok {
+			return "", errors.New("oauth2/google/externalaccount: provided subject_token_field_name not found in credentials")
+		}
+		token, ok := val.(string)
+		if !ok {
+			return "", errors.New("oauth2/google/externalaccount: improperly formatted subject token")
+		}
+		return token, nil
+	case "text":
+		return string(tokenBytes), nil
+	case "":
+		return string(tokenBytes), nil
+	default:
+		return "", errors.New("oauth2/google/externalaccount: invalid credential_source file format type")
+	}
+
+}