oauth2/google: add config type to use Cloud SDK credentials

Change-Id: Ied7fecc0cb155c33faca7766b81221eacb3aa0c0
Reviewed-on: https://go-review.googlesource.com/1670
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Burcu Dogan <jbd@google.com>

internal/oauth2.go

@@ -6,10 +6,14 @@
 package internal
 
 import (
+	"bufio"
 	"crypto/rsa"
 	"crypto/x509"
 	"encoding/pem"
 	"errors"
+	"fmt"
+	"io"
+	"strings"
 )
 
 // ParseKey converts the binary contents of a private key file
@@ -35,3 +39,31 @@ func ParseKey(key []byte) (*rsa.PrivateKey, error) {
 	}
 	return parsed, nil
 }
+
+func ParseINI(ini io.Reader) (map[string]map[string]string, error) {
+	result := map[string]map[string]string{
+		"": map[string]string{}, // root section
+	}
+	scanner := bufio.NewScanner(ini)
+	currentSection := ""
+	for scanner.Scan() {
+		line := strings.TrimSpace(scanner.Text())
+		if strings.HasPrefix(line, ";") {
+			// comment.
+			continue
+		}
+		if strings.HasPrefix(line, "[") && strings.HasSuffix(line, "]") {
+			currentSection = strings.TrimSpace(line[1 : len(line)-1])
+			result[currentSection] = map[string]string{}
+			continue
+		}
+		parts := strings.SplitN(line, "=", 2)
+		if len(parts) == 2 && parts[0] != "" {
+			result[currentSection][strings.TrimSpace(parts[0])] = strings.TrimSpace(parts[1])
+		}
+	}
+	if err := scanner.Err(); err != nil {
+		return nil, fmt.Errorf("error scanning ini: %v", err)
+	}
+	return result, nil
+}