services/keep-web/s3.go

   1 // Copyright (C) The Arvados Authors. All rights reserved.
   2 //
   3 // SPDX-License-Identifier: AGPL-3.0
   4
   5 package main
   6
   7 import (
   8         "crypto/hmac"
   9         "crypto/sha256"
  10         "encoding/xml"
  11         "errors"
  12         "fmt"
  13         "io"
  14         "net/http"
  15         "net/url"
  16         "os"
  17         "path/filepath"
  18         "sort"
  19         "strconv"
  20         "strings"
  21         "time"
  22
  23         "git.arvados.org/arvados.git/sdk/go/arvados"
  24         "git.arvados.org/arvados.git/sdk/go/ctxlog"
  25         "github.com/AdRoll/goamz/s3"
  26 )
  27
  28 const (
  29         s3MaxKeys       = 1000
  30         s3SignAlgorithm = "AWS4-HMAC-SHA256"
  31         s3MaxClockSkew  = 5 * time.Minute
  32 )
  33
  34 func hmacstring(msg string, key []byte) []byte {
  35         h := hmac.New(sha256.New, key)
  36         io.WriteString(h, msg)
  37         return h.Sum(nil)
  38 }
  39
  40 // Signing key for given secret key and request attrs.
  41 func s3signatureKey(key, datestamp, regionName, serviceName string) []byte {
  42         return hmacstring("aws4_request",
  43                 hmacstring(serviceName,
  44                         hmacstring(regionName,
  45                                 hmacstring(datestamp, []byte("AWS4"+key)))))
  46 }
  47
  48 // Canonical query string for S3 V4 signature: sorted keys, spaces
  49 // escaped as %20 instead of +, keyvalues joined with &.
  50 func s3querystring(u *url.URL) string {
  51         keys := make([]string, 0, len(u.Query()))
  52         values := make(map[string]string, len(u.Query()))
  53         for k, vs := range u.Query() {
  54                 k = strings.Replace(url.QueryEscape(k), "+", "%20", -1)
  55                 keys = append(keys, k)
  56                 for _, v := range vs {
  57                         v = strings.Replace(url.QueryEscape(v), "+", "%20", -1)
  58                         if values[k] != "" {
  59                                 values[k] += "&"
  60                         }
  61                         values[k] += k + "=" + v
  62                 }
  63         }
  64         sort.Strings(keys)
  65         for i, k := range keys {
  66                 keys[i] = values[k]
  67         }
  68         return strings.Join(keys, "&")
  69 }
  70
  71 func s3signature(alg, secretKey, scope, signedHeaders string, r *http.Request) (string, error) {
  72         timefmt, timestr := "20060102T150405Z", r.Header.Get("X-Amz-Date")
  73         if timestr == "" {
  74                 timefmt, timestr = time.RFC1123, r.Header.Get("Date")
  75         }
  76         t, err := time.Parse(timefmt, timestr)
  77         if err != nil {
  78                 return "", fmt.Errorf("invalid timestamp %q: %s", timestr, err)
  79         }
  80         if skew := time.Now().Sub(t); skew < -s3MaxClockSkew || skew > s3MaxClockSkew {
  81                 return "", errors.New("exceeded max clock skew")
  82         }
  83
  84         var canonicalHeaders string
  85         for _, h := range strings.Split(signedHeaders, ";") {
  86                 if h == "host" {
  87                         canonicalHeaders += h + ":" + r.URL.Host + "\n"
  88                 } else {
  89                         canonicalHeaders += h + ":" + r.Header.Get(h) + "\n"
  90                 }
  91         }
  92
  93         crhash := sha256.New()
  94         fmt.Fprintf(crhash, "%s\n%s\n%s\n%s\n%s\n%s", r.Method, r.URL.EscapedPath(), s3querystring(r.URL), canonicalHeaders, signedHeaders, r.Header.Get("X-Amz-Content-Sha256"))
  95         crdigest := fmt.Sprintf("%x", crhash.Sum(nil))
  96
  97         payload := fmt.Sprintf("%s\n%s\n%s\n%s", alg, r.Header.Get("X-Amz-Date"), scope, crdigest)
  98
  99         // scope is {datestamp}/{region}/{service}/aws4_request
 100         drs := strings.Split(scope, "/")
 101         if len(drs) != 4 {
 102                 return "", fmt.Errorf("invalid scope %q", scope)
 103         }
 104
 105         key := s3signatureKey(secretKey, drs[0], drs[1], drs[2])
 106         h := hmac.New(sha256.New, key)
 107         h.Write([]byte(payload))
 108         return fmt.Sprintf("%x", h.Sum(nil)), nil
 109 }
 110
 111 // checks3signature verifies the given S3 V4 signature and returns the
 112 // Arvados token that corresponds to the given accessKey. An error is
 113 // returned if accessKey is not a valid token UUID or the signature
 114 // does not match.
 115 func (h *handler) checks3signature(r *http.Request) (string, error) {
 116         var key, scope, signedHeaders, signature string
 117         authstring := strings.TrimPrefix(r.Header.Get("Authorization"), s3SignAlgorithm+" ")
 118         for _, cmpt := range strings.Split(authstring, ",") {
 119                 cmpt = strings.TrimSpace(cmpt)
 120                 split := strings.SplitN(cmpt, "=", 2)
 121                 switch {
 122                 case len(split) != 2:
 123                         // (?) ignore
 124                 case split[0] == "Credential":
 125                         keyandscope := strings.SplitN(split[1], "/", 2)
 126                         if len(keyandscope) == 2 {
 127                                 key, scope = keyandscope[0], keyandscope[1]
 128                         }
 129                 case split[0] == "SignedHeaders":
 130                         signedHeaders = split[1]
 131                 case split[0] == "Signature":
 132                         signature = split[1]
 133                 }
 134         }
 135
 136         client := (&arvados.Client{
 137                 APIHost:  h.Config.cluster.Services.Controller.ExternalURL.Host,
 138                 Insecure: h.Config.cluster.TLS.Insecure,
 139         }).WithRequestID(r.Header.Get("X-Request-Id"))
 140         var aca arvados.APIClientAuthorization
 141         ctx := arvados.ContextWithAuthorization(r.Context(), "Bearer "+h.Config.cluster.SystemRootToken)
 142         err := client.RequestAndDecodeContext(ctx, &aca, "GET", "arvados/v1/api_client_authorizations/"+key, nil, nil)
 143         if err != nil {
 144                 ctxlog.FromContext(ctx).WithError(err).WithField("UUID", key).Info("token lookup failed")
 145                 return "", errors.New("invalid access key")
 146         }
 147         expect, err := s3signature(s3SignAlgorithm, aca.APIToken, scope, signedHeaders, r)
 148         if err != nil {
 149                 return "", err
 150         } else if expect != signature {
 151                 return "", errors.New("signature does not match")
 152         }
 153         return aca.TokenV2(), nil
 154 }
 155
 156 // serveS3 handles r and returns true if r is a request from an S3
 157 // client, otherwise it returns false.
 158 func (h *handler) serveS3(w http.ResponseWriter, r *http.Request) bool {
 159         var token string
 160         if auth := r.Header.Get("Authorization"); strings.HasPrefix(auth, "AWS ") {
 161                 split := strings.SplitN(auth[4:], ":", 2)
 162                 if len(split) < 2 {
 163                         http.Error(w, "malformed Authorization header", http.StatusUnauthorized)
 164                         return true
 165                 }
 166                 token = split[0]
 167         } else if strings.HasPrefix(auth, s3SignAlgorithm+" ") {
 168                 t, err := h.checks3signature(r)
 169                 if err != nil {
 170                         http.Error(w, "signature verification failed: "+err.Error(), http.StatusForbidden)
 171                         return true
 172                 }
 173                 token = t
 174         } else {
 175                 return false
 176         }
 177
 178         _, kc, client, release, err := h.getClients(r.Header.Get("X-Request-Id"), token)
 179         if err != nil {
 180                 http.Error(w, "Pool failed: "+h.clientPool.Err().Error(), http.StatusInternalServerError)
 181                 return true
 182         }
 183         defer release()
 184
 185         fs := client.SiteFileSystem(kc)
 186         fs.ForwardSlashNameSubstitution(h.Config.cluster.Collections.ForwardSlashNameSubstitution)
 187
 188         objectNameGiven := strings.Count(strings.TrimSuffix(r.URL.Path, "/"), "/") > 1
 189
 190         switch {
 191         case r.Method == http.MethodGet && !objectNameGiven:
 192                 // Path is "/{uuid}" or "/{uuid}/", has no object name
 193                 if _, ok := r.URL.Query()["versioning"]; ok {
 194                         // GetBucketVersioning
 195                         w.Header().Set("Content-Type", "application/xml")
 196                         io.WriteString(w, xml.Header)
 197                         fmt.Fprintln(w, `<VersioningConfiguration xmlns="http://s3.amazonaws.com/doc/2006-03-01/"/>`)
 198                 } else {
 199                         // ListObjects
 200                         h.s3list(w, r, fs)
 201                 }
 202                 return true
 203         case r.Method == http.MethodGet || r.Method == http.MethodHead:
 204                 fspath := "/by_id" + r.URL.Path
 205                 fi, err := fs.Stat(fspath)
 206                 if r.Method == "HEAD" && !objectNameGiven {
 207                         // HeadBucket
 208                         if err == nil && fi.IsDir() {
 209                                 w.WriteHeader(http.StatusOK)
 210                         } else if os.IsNotExist(err) {
 211                                 w.WriteHeader(http.StatusNotFound)
 212                         } else {
 213                                 http.Error(w, err.Error(), http.StatusBadGateway)
 214                         }
 215                         return true
 216                 }
 217                 if err == nil && fi.IsDir() && objectNameGiven && strings.HasSuffix(fspath, "/") && h.Config.cluster.Collections.S3FolderObjects {
 218                         w.Header().Set("Content-Type", "application/x-directory")
 219                         w.WriteHeader(http.StatusOK)
 220                         return true
 221                 }
 222                 if os.IsNotExist(err) ||
 223                         (err != nil && err.Error() == "not a directory") ||
 224                         (fi != nil && fi.IsDir()) {
 225                         http.Error(w, "not found", http.StatusNotFound)
 226                         return true
 227                 }
 228                 // shallow copy r, and change URL path
 229                 r := *r
 230                 r.URL.Path = fspath
 231                 http.FileServer(fs).ServeHTTP(w, &r)
 232                 return true
 233         case r.Method == http.MethodPut:
 234                 if !objectNameGiven {
 235                         http.Error(w, "missing object name in PUT request", http.StatusBadRequest)
 236                         return true
 237                 }
 238                 fspath := "by_id" + r.URL.Path
 239                 var objectIsDir bool
 240                 if strings.HasSuffix(fspath, "/") {
 241                         if !h.Config.cluster.Collections.S3FolderObjects {
 242                                 http.Error(w, "invalid object name: trailing slash", http.StatusBadRequest)
 243                                 return true
 244                         }
 245                         n, err := r.Body.Read(make([]byte, 1))
 246                         if err != nil && err != io.EOF {
 247                                 http.Error(w, fmt.Sprintf("error reading request body: %s", err), http.StatusInternalServerError)
 248                                 return true
 249                         } else if n > 0 {
 250                                 http.Error(w, "cannot create object with trailing '/' char unless content is empty", http.StatusBadRequest)
 251                                 return true
 252                         } else if strings.SplitN(r.Header.Get("Content-Type"), ";", 2)[0] != "application/x-directory" {
 253                                 http.Error(w, "cannot create object with trailing '/' char unless Content-Type is 'application/x-directory'", http.StatusBadRequest)
 254                                 return true
 255                         }
 256                         // Given PUT "foo/bar/", we'll use "foo/bar/."
 257                         // in the "ensure parents exist" block below,
 258                         // and then we'll be done.
 259                         fspath += "."
 260                         objectIsDir = true
 261                 }
 262                 fi, err := fs.Stat(fspath)
 263                 if err != nil && err.Error() == "not a directory" {
 264                         // requested foo/bar, but foo is a file
 265                         http.Error(w, "object name conflicts with existing object", http.StatusBadRequest)
 266                         return true
 267                 }
 268                 if strings.HasSuffix(r.URL.Path, "/") && err == nil && !fi.IsDir() {
 269                         // requested foo/bar/, but foo/bar is a file
 270                         http.Error(w, "object name conflicts with existing object", http.StatusBadRequest)
 271                         return true
 272                 }
 273                 // create missing parent/intermediate directories, if any
 274                 for i, c := range fspath {
 275                         if i > 0 && c == '/' {
 276                                 dir := fspath[:i]
 277                                 if strings.HasSuffix(dir, "/") {
 278                                         err = errors.New("invalid object name (consecutive '/' chars)")
 279                                         http.Error(w, err.Error(), http.StatusBadRequest)
 280                                         return true
 281                                 }
 282                                 err = fs.Mkdir(dir, 0755)
 283                                 if err == arvados.ErrInvalidArgument {
 284                                         // Cannot create a directory
 285                                         // here.
 286                                         err = fmt.Errorf("mkdir %q failed: %w", dir, err)
 287                                         http.Error(w, err.Error(), http.StatusBadRequest)
 288                                         return true
 289                                 } else if err != nil && !os.IsExist(err) {
 290                                         err = fmt.Errorf("mkdir %q failed: %w", dir, err)
 291                                         http.Error(w, err.Error(), http.StatusInternalServerError)
 292                                         return true
 293                                 }
 294                         }
 295                 }
 296                 if !objectIsDir {
 297                         f, err := fs.OpenFile(fspath, os.O_WRONLY|os.O_TRUNC|os.O_CREATE, 0644)
 298                         if os.IsNotExist(err) {
 299                                 f, err = fs.OpenFile(fspath, os.O_WRONLY|os.O_TRUNC|os.O_CREATE, 0644)
 300                         }
 301                         if err != nil {
 302                                 err = fmt.Errorf("open %q failed: %w", r.URL.Path, err)
 303                                 http.Error(w, err.Error(), http.StatusBadRequest)
 304                                 return true
 305                         }
 306                         defer f.Close()
 307                         _, err = io.Copy(f, r.Body)
 308                         if err != nil {
 309                                 err = fmt.Errorf("write to %q failed: %w", r.URL.Path, err)
 310                                 http.Error(w, err.Error(), http.StatusBadGateway)
 311                                 return true
 312                         }
 313                         err = f.Close()
 314                         if err != nil {
 315                                 err = fmt.Errorf("write to %q failed: close: %w", r.URL.Path, err)
 316                                 http.Error(w, err.Error(), http.StatusBadGateway)
 317                                 return true
 318                         }
 319                 }
 320                 err = fs.Sync()
 321                 if err != nil {
 322                         err = fmt.Errorf("sync failed: %w", err)
 323                         http.Error(w, err.Error(), http.StatusInternalServerError)
 324                         return true
 325                 }
 326                 w.WriteHeader(http.StatusOK)
 327                 return true
 328         case r.Method == http.MethodDelete:
 329                 if !objectNameGiven || r.URL.Path == "/" {
 330                         http.Error(w, "missing object name in DELETE request", http.StatusBadRequest)
 331                         return true
 332                 }
 333                 fspath := "by_id" + r.URL.Path
 334                 if strings.HasSuffix(fspath, "/") {
 335                         fspath = strings.TrimSuffix(fspath, "/")
 336                         fi, err := fs.Stat(fspath)
 337                         if os.IsNotExist(err) {
 338                                 w.WriteHeader(http.StatusNoContent)
 339                                 return true
 340                         } else if err != nil {
 341                                 http.Error(w, err.Error(), http.StatusInternalServerError)
 342                                 return true
 343                         } else if !fi.IsDir() {
 344                                 // if "foo" exists and is a file, then
 345                                 // "foo/" doesn't exist, so we say
 346                                 // delete was successful.
 347                                 w.WriteHeader(http.StatusNoContent)
 348                                 return true
 349                         }
 350                 } else if fi, err := fs.Stat(fspath); err == nil && fi.IsDir() {
 351                         // if "foo" is a dir, it is visible via S3
 352                         // only as "foo/", not "foo" -- so we leave
 353                         // the dir alone and return 204 to indicate
 354                         // that "foo" does not exist.
 355                         w.WriteHeader(http.StatusNoContent)
 356                         return true
 357                 }
 358                 err = fs.Remove(fspath)
 359                 if os.IsNotExist(err) {
 360                         w.WriteHeader(http.StatusNoContent)
 361                         return true
 362                 }
 363                 if err != nil {
 364                         err = fmt.Errorf("rm failed: %w", err)
 365                         http.Error(w, err.Error(), http.StatusBadRequest)
 366                         return true
 367                 }
 368                 err = fs.Sync()
 369                 if err != nil {
 370                         err = fmt.Errorf("sync failed: %w", err)
 371                         http.Error(w, err.Error(), http.StatusInternalServerError)
 372                         return true
 373                 }
 374                 w.WriteHeader(http.StatusNoContent)
 375                 return true
 376         default:
 377                 http.Error(w, "method not allowed", http.StatusMethodNotAllowed)
 378                 return true
 379         }
 380 }
 381
 382 // Call fn on the given path (directory) and its contents, in
 383 // lexicographic order.
 384 //
 385 // If isRoot==true and path is not a directory, return nil.
 386 //
 387 // If fn returns filepath.SkipDir when called on a directory, don't
 388 // descend into that directory.
 389 func walkFS(fs arvados.CustomFileSystem, path string, isRoot bool, fn func(path string, fi os.FileInfo) error) error {
 390         if isRoot {
 391                 fi, err := fs.Stat(path)
 392                 if os.IsNotExist(err) || (err == nil && !fi.IsDir()) {
 393                         return nil
 394                 } else if err != nil {
 395                         return err
 396                 }
 397                 err = fn(path, fi)
 398                 if err == filepath.SkipDir {
 399                         return nil
 400                 } else if err != nil {
 401                         return err
 402                 }
 403         }
 404         f, err := fs.Open(path)
 405         if os.IsNotExist(err) && isRoot {
 406                 return nil
 407         } else if err != nil {
 408                 return fmt.Errorf("open %q: %w", path, err)
 409         }
 410         defer f.Close()
 411         if path == "/" {
 412                 path = ""
 413         }
 414         fis, err := f.Readdir(-1)
 415         if err != nil {
 416                 return err
 417         }
 418         sort.Slice(fis, func(i, j int) bool { return fis[i].Name() < fis[j].Name() })
 419         for _, fi := range fis {
 420                 err = fn(path+"/"+fi.Name(), fi)
 421                 if err == filepath.SkipDir {
 422                         continue
 423                 } else if err != nil {
 424                         return err
 425                 }
 426                 if fi.IsDir() {
 427                         err = walkFS(fs, path+"/"+fi.Name(), false, fn)
 428                         if err != nil {
 429                                 return err
 430                         }
 431                 }
 432         }
 433         return nil
 434 }
 435
 436 var errDone = errors.New("done")
 437
 438 func (h *handler) s3list(w http.ResponseWriter, r *http.Request, fs arvados.CustomFileSystem) {
 439         var params struct {
 440                 bucket    string
 441                 delimiter string
 442                 marker    string
 443                 maxKeys   int
 444                 prefix    string
 445         }
 446         params.bucket = strings.SplitN(r.URL.Path[1:], "/", 2)[0]
 447         params.delimiter = r.FormValue("delimiter")
 448         params.marker = r.FormValue("marker")
 449         if mk, _ := strconv.ParseInt(r.FormValue("max-keys"), 10, 64); mk > 0 && mk < s3MaxKeys {
 450                 params.maxKeys = int(mk)
 451         } else {
 452                 params.maxKeys = s3MaxKeys
 453         }
 454         params.prefix = r.FormValue("prefix")
 455
 456         bucketdir := "by_id/" + params.bucket
 457         // walkpath is the directory (relative to bucketdir) we need
 458         // to walk: the innermost directory that is guaranteed to
 459         // contain all paths that have the requested prefix. Examples:
 460         // prefix "foo/bar"  => walkpath "foo"
 461         // prefix "foo/bar/" => walkpath "foo/bar"
 462         // prefix "foo"      => walkpath ""
 463         // prefix ""         => walkpath ""
 464         walkpath := params.prefix
 465         if cut := strings.LastIndex(walkpath, "/"); cut >= 0 {
 466                 walkpath = walkpath[:cut]
 467         } else {
 468                 walkpath = ""
 469         }
 470
 471         type commonPrefix struct {
 472                 Prefix string
 473         }
 474         type listResp struct {
 475                 XMLName string `xml:"http://s3.amazonaws.com/doc/2006-03-01/ ListBucketResult"`
 476                 s3.ListResp
 477                 // s3.ListResp marshals an empty tag when
 478                 // CommonPrefixes is nil, which confuses some clients.
 479                 // Fix by using this nested struct instead.
 480                 CommonPrefixes []commonPrefix
 481                 // Similarly, we need omitempty here, because an empty
 482                 // tag confuses some clients (e.g.,
 483                 // github.com/aws/aws-sdk-net never terminates its
 484                 // paging loop).
 485                 NextMarker string `xml:"NextMarker,omitempty"`
 486         }
 487         resp := listResp{
 488                 ListResp: s3.ListResp{
 489                         Name:      strings.SplitN(r.URL.Path[1:], "/", 2)[0],
 490                         Prefix:    params.prefix,
 491                         Delimiter: params.delimiter,
 492                         Marker:    params.marker,
 493                         MaxKeys:   params.maxKeys,
 494                 },
 495         }
 496         commonPrefixes := map[string]bool{}
 497         err := walkFS(fs, strings.TrimSuffix(bucketdir+"/"+walkpath, "/"), true, func(path string, fi os.FileInfo) error {
 498                 if path == bucketdir {
 499                         return nil
 500                 }
 501                 path = path[len(bucketdir)+1:]
 502                 filesize := fi.Size()
 503                 if fi.IsDir() {
 504                         path += "/"
 505                         filesize = 0
 506                 }
 507                 if len(path) <= len(params.prefix) {
 508                         if path > params.prefix[:len(path)] {
 509                                 // with prefix "foobar", walking "fooz" means we're done
 510                                 return errDone
 511                         }
 512                         if path < params.prefix[:len(path)] {
 513                                 // with prefix "foobar", walking "foobag" is pointless
 514                                 return filepath.SkipDir
 515                         }
 516                         if fi.IsDir() && !strings.HasPrefix(params.prefix+"/", path) {
 517                                 // with prefix "foo/bar", walking "fo"
 518                                 // is pointless (but walking "foo" or
 519                                 // "foo/bar" is necessary)
 520                                 return filepath.SkipDir
 521                         }
 522                         if len(path) < len(params.prefix) {
 523                                 // can't skip anything, and this entry
 524                                 // isn't in the results, so just
 525                                 // continue descent
 526                                 return nil
 527                         }
 528                 } else {
 529                         if path[:len(params.prefix)] > params.prefix {
 530                                 // with prefix "foobar", nothing we
 531                                 // see after "foozzz" is relevant
 532                                 return errDone
 533                         }
 534                 }
 535                 if path < params.marker || path < params.prefix {
 536                         return nil
 537                 }
 538                 if fi.IsDir() && !h.Config.cluster.Collections.S3FolderObjects {
 539                         // Note we don't add anything to
 540                         // commonPrefixes here even if delimiter is
 541                         // "/". We descend into the directory, and
 542                         // return a commonPrefix only if we end up
 543                         // finding a regular file inside it.
 544                         return nil
 545                 }
 546                 if params.delimiter != "" {
 547                         idx := strings.Index(path[len(params.prefix):], params.delimiter)
 548                         if idx >= 0 {
 549                                 // with prefix "foobar" and delimiter
 550                                 // "z", when we hit "foobar/baz", we
 551                                 // add "/baz" to commonPrefixes and
 552                                 // stop descending.
 553                                 commonPrefixes[path[:len(params.prefix)+idx+1]] = true
 554                                 return filepath.SkipDir
 555                         }
 556                 }
 557                 if len(resp.Contents)+len(commonPrefixes) >= params.maxKeys {
 558                         resp.IsTruncated = true
 559                         if params.delimiter != "" {
 560                                 resp.NextMarker = path
 561                         }
 562                         return errDone
 563                 }
 564                 resp.Contents = append(resp.Contents, s3.Key{
 565                         Key:          path,
 566                         LastModified: fi.ModTime().UTC().Format("2006-01-02T15:04:05.999") + "Z",
 567                         Size:         filesize,
 568                 })
 569                 return nil
 570         })
 571         if err != nil && err != errDone {
 572                 http.Error(w, err.Error(), http.StatusInternalServerError)
 573                 return
 574         }
 575         if params.delimiter != "" {
 576                 resp.CommonPrefixes = make([]commonPrefix, 0, len(commonPrefixes))
 577                 for prefix := range commonPrefixes {
 578                         resp.CommonPrefixes = append(resp.CommonPrefixes, commonPrefix{prefix})
 579                 }
 580                 sort.Slice(resp.CommonPrefixes, func(i, j int) bool { return resp.CommonPrefixes[i].Prefix < resp.CommonPrefixes[j].Prefix })
 581         }
 582         w.Header().Set("Content-Type", "application/xml")
 583         io.WriteString(w, xml.Header)
 584         if err := xml.NewEncoder(w).Encode(resp); err != nil {
 585                 ctxlog.FromContext(r.Context()).WithError(err).Error("error writing xml response")
 586         }
 587 }