listV2: Continuation and NextContinuation tokens are encoded with base64 (#8337)

Minio V2 listing uses object names/prefixes as continuation tokens. This
is problematic when object names contain some characters that are forbidden
in XML documents. This PR will use base64 encoded form of continuation
and next continuation tokens to address that corner case.

cmd/api-resources.go

@@ -17,6 +17,7 @@
 package cmd
 
 import (
+	"encoding/base64"
 	"net/url"
 	"strconv"
 )
@@ -86,11 +87,19 @@ func getListObjectsV2Args(values url.Values) (prefix, token, startAfter, delimit
 	}
 
 	prefix = values.Get("prefix")
-	token = values.Get("continuation-token")
 	startAfter = values.Get("start-after")
 	delimiter = values.Get("delimiter")
 	fetchOwner = values.Get("fetch-owner") == "true"
 	encodingType = values.Get("encoding-type")
+
+	if token = values.Get("continuation-token"); token != "" {
+		decodedToken, err := base64.StdEncoding.DecodeString(token)
+		if err != nil {
+			errCode = ErrIncorrectContinuationToken
+			return
+		}
+		token = string(decodedToken)
+	}
 	return
 }
 

cmd/api-resources_test.go

@@ -34,7 +34,7 @@ func TestListObjectsV2Resources(t *testing.T) {
 		{
 			values: url.Values{
 				"prefix":             []string{"photos/"},
-				"continuation-token": []string{"token"},
+				"continuation-token": []string{"dG9rZW4="},
 				"start-after":        []string{"start-after"},
 				"delimiter":          []string{SlashSeparator},
 				"fetch-owner":        []string{"true"},
@@ -53,7 +53,7 @@ func TestListObjectsV2Resources(t *testing.T) {
 		{
 			values: url.Values{
 				"prefix":             []string{"photos/"},
-				"continuation-token": []string{"token"},
+				"continuation-token": []string{"dG9rZW4="},
 				"start-after":        []string{"start-after"},
 				"delimiter":          []string{SlashSeparator},
 				"fetch-owner":        []string{"true"},

cmd/api-response.go

@@ -18,6 +18,7 @@ package cmd
 
 import (
 	"context"
+	"encoding/base64"
 	"encoding/xml"
 	"net/http"
 	"net/url"
@@ -498,8 +499,8 @@ func generateListObjectsV2Response(bucket, prefix, token, nextToken, startAfter,
 	data.Delimiter = s3EncodeName(delimiter, encodingType)
 	data.Prefix = s3EncodeName(prefix, encodingType)
 	data.MaxKeys = maxKeys
-	data.ContinuationToken = s3EncodeName(token, encodingType)
-	data.NextContinuationToken = s3EncodeName(nextToken, encodingType)
+	data.ContinuationToken = base64.StdEncoding.EncodeToString([]byte(token))
+	data.NextContinuationToken = base64.StdEncoding.EncodeToString([]byte(nextToken))
 	data.IsTruncated = isTruncated
 	for _, prefix := range prefixes {
 		var prefixItem = CommonPrefix{}