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         "hash"
  14         "io"
  15         "net/http"
  16         "net/url"
  17         "os"
  18         "path/filepath"
  19         "sort"
  20         "strconv"
  21         "strings"
  22         "time"
  23
  24         "git.arvados.org/arvados.git/sdk/go/arvados"
  25         "git.arvados.org/arvados.git/sdk/go/ctxlog"
  26         "github.com/AdRoll/goamz/s3"
  27 )
  28
  29 const (
  30         s3MaxKeys       = 1000
  31         s3SignAlgorithm = "AWS4-HMAC-SHA256"
  32         s3MaxClockSkew  = 5 * time.Minute
  33 )
  34
  35 func hmacstring(msg string, key []byte) []byte {
  36         h := hmac.New(sha256.New, key)
  37         io.WriteString(h, msg)
  38         return h.Sum(nil)
  39 }
  40
  41 func hashdigest(h hash.Hash, payload string) string {
  42         io.WriteString(h, payload)
  43         return fmt.Sprintf("%x", h.Sum(nil))
  44 }
  45
  46 // Signing key for given secret key and request attrs.
  47 func s3signatureKey(key, datestamp, regionName, serviceName string) []byte {
  48         return hmacstring("aws4_request",
  49                 hmacstring(serviceName,
  50                         hmacstring(regionName,
  51                                 hmacstring(datestamp, []byte("AWS4"+key)))))
  52 }
  53
  54 // Canonical query string for S3 V4 signature: sorted keys, spaces
  55 // escaped as %20 instead of +, keyvalues joined with &.
  56 func s3querystring(u *url.URL) string {
  57         keys := make([]string, 0, len(u.Query()))
  58         values := make(map[string]string, len(u.Query()))
  59         for k, vs := range u.Query() {
  60                 k = strings.Replace(url.QueryEscape(k), "+", "%20", -1)
  61                 keys = append(keys, k)
  62                 for _, v := range vs {
  63                         v = strings.Replace(url.QueryEscape(v), "+", "%20", -1)
  64                         if values[k] != "" {
  65                                 values[k] += "&"
  66                         }
  67                         values[k] += k + "=" + v
  68                 }
  69         }
  70         sort.Strings(keys)
  71         for i, k := range keys {
  72                 keys[i] = values[k]
  73         }
  74         return strings.Join(keys, "&")
  75 }
  76
  77 func s3stringToSign(alg, scope, signedHeaders string, r *http.Request) (string, error) {
  78         timefmt, timestr := "20060102T150405Z", r.Header.Get("X-Amz-Date")
  79         if timestr == "" {
  80                 timefmt, timestr = time.RFC1123, r.Header.Get("Date")
  81         }
  82         t, err := time.Parse(timefmt, timestr)
  83         if err != nil {
  84                 return "", fmt.Errorf("invalid timestamp %q: %s", timestr, err)
  85         }
  86         if skew := time.Now().Sub(t); skew < -s3MaxClockSkew || skew > s3MaxClockSkew {
  87                 return "", errors.New("exceeded max clock skew")
  88         }
  89
  90         var canonicalHeaders string
  91         for _, h := range strings.Split(signedHeaders, ";") {
  92                 if h == "host" {
  93                         canonicalHeaders += h + ":" + r.Host + "\n"
  94                 } else {
  95                         canonicalHeaders += h + ":" + r.Header.Get(h) + "\n"
  96                 }
  97         }
  98
  99         canonicalRequest := fmt.Sprintf("%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"))
 100         ctxlog.FromContext(r.Context()).Debugf("s3stringToSign: canonicalRequest %s", canonicalRequest)
 101         return fmt.Sprintf("%s\n%s\n%s\n%s", alg, r.Header.Get("X-Amz-Date"), scope, hashdigest(sha256.New(), canonicalRequest)), nil
 102 }
 103
 104 func s3signature(secretKey, scope, signedHeaders, stringToSign string) (string, error) {
 105         // scope is {datestamp}/{region}/{service}/aws4_request
 106         drs := strings.Split(scope, "/")
 107         if len(drs) != 4 {
 108                 return "", fmt.Errorf("invalid scope %q", scope)
 109         }
 110         key := s3signatureKey(secretKey, drs[0], drs[1], drs[2])
 111         return hashdigest(hmac.New(sha256.New, key), stringToSign), nil
 112 }
 113
 114 // checks3signature verifies the given S3 V4 signature and returns the
 115 // Arvados token that corresponds to the given accessKey. An error is
 116 // returned if accessKey is not a valid token UUID or the signature
 117 // does not match.
 118 func (h *handler) checks3signature(r *http.Request) (string, error) {
 119         var key, scope, signedHeaders, signature string
 120         authstring := strings.TrimPrefix(r.Header.Get("Authorization"), s3SignAlgorithm+" ")
 121         for _, cmpt := range strings.Split(authstring, ",") {
 122                 cmpt = strings.TrimSpace(cmpt)
 123                 split := strings.SplitN(cmpt, "=", 2)
 124                 switch {
 125                 case len(split) != 2:
 126                         // (?) ignore
 127                 case split[0] == "Credential":
 128                         keyandscope := strings.SplitN(split[1], "/", 2)
 129                         if len(keyandscope) == 2 {
 130                                 key, scope = keyandscope[0], keyandscope[1]
 131                         }
 132                 case split[0] == "SignedHeaders":
 133                         signedHeaders = split[1]
 134                 case split[0] == "Signature":
 135                         signature = split[1]
 136                 }
 137         }
 138
 139         client := (&arvados.Client{
 140                 APIHost:  h.Config.cluster.Services.Controller.ExternalURL.Host,
 141                 Insecure: h.Config.cluster.TLS.Insecure,
 142         }).WithRequestID(r.Header.Get("X-Request-Id"))
 143         var aca arvados.APIClientAuthorization
 144         var secret string
 145         var err error
 146         if len(key) == 27 && key[5:12] == "-gj3su-" {
 147                 // Access key is the UUID of an Arvados token, secret
 148                 // key is the secret part.
 149                 ctx := arvados.ContextWithAuthorization(r.Context(), "Bearer "+h.Config.cluster.SystemRootToken)
 150                 err = client.RequestAndDecodeContext(ctx, &aca, "GET", "arvados/v1/api_client_authorizations/"+key, nil, nil)
 151                 secret = aca.APIToken
 152         } else {
 153                 // Access key and secret key are both an entire
 154                 // Arvados token or OIDC access token.
 155                 mungedKey := key
 156                 if strings.HasPrefix(key, "v2_") {
 157                         // Entire Arvados token, with "/" replaced by
 158                         // "_" to avoid colliding with the
 159                         // Authorization header format.
 160                         mungedKey = strings.Replace(key, "_", "/", -1)
 161                 }
 162                 ctx := arvados.ContextWithAuthorization(r.Context(), "Bearer "+mungedKey)
 163                 err = client.RequestAndDecodeContext(ctx, &aca, "GET", "arvados/v1/api_client_authorizations/current", nil, nil)
 164                 secret = key
 165         }
 166         if err != nil {
 167                 ctxlog.FromContext(r.Context()).WithError(err).WithField("UUID", key).Info("token lookup failed")
 168                 return "", errors.New("invalid access key")
 169         }
 170         stringToSign, err := s3stringToSign(s3SignAlgorithm, scope, signedHeaders, r)
 171         if err != nil {
 172                 return "", err
 173         }
 174         expect, err := s3signature(secret, scope, signedHeaders, stringToSign)
 175         if err != nil {
 176                 return "", err
 177         } else if expect != signature {
 178                 return "", fmt.Errorf("signature does not match (scope %q signedHeaders %q stringToSign %q)", scope, signedHeaders, stringToSign)
 179         }
 180         return aca.TokenV2(), nil
 181 }
 182
 183 // serveS3 handles r and returns true if r is a request from an S3
 184 // client, otherwise it returns false.
 185 func (h *handler) serveS3(w http.ResponseWriter, r *http.Request) bool {
 186         var token string
 187         if auth := r.Header.Get("Authorization"); strings.HasPrefix(auth, "AWS ") {
 188                 split := strings.SplitN(auth[4:], ":", 2)
 189                 if len(split) < 2 {
 190                         http.Error(w, "malformed Authorization header", http.StatusUnauthorized)
 191                         return true
 192                 }
 193                 token = split[0]
 194         } else if strings.HasPrefix(auth, s3SignAlgorithm+" ") {
 195                 t, err := h.checks3signature(r)
 196                 if err != nil {
 197                         http.Error(w, "signature verification failed: "+err.Error(), http.StatusForbidden)
 198                         return true
 199                 }
 200                 token = t
 201         } else {
 202                 return false
 203         }
 204
 205         _, kc, client, release, err := h.getClients(r.Header.Get("X-Request-Id"), token)
 206         if err != nil {
 207                 http.Error(w, "Pool failed: "+h.clientPool.Err().Error(), http.StatusInternalServerError)
 208                 return true
 209         }
 210         defer release()
 211
 212         fs := client.SiteFileSystem(kc)
 213         fs.ForwardSlashNameSubstitution(h.Config.cluster.Collections.ForwardSlashNameSubstitution)
 214
 215         objectNameGiven := strings.Count(strings.TrimSuffix(r.URL.Path, "/"), "/") > 1
 216
 217         switch {
 218         case r.Method == http.MethodGet && !objectNameGiven:
 219                 // Path is "/{uuid}" or "/{uuid}/", has no object name
 220                 if _, ok := r.URL.Query()["versioning"]; ok {
 221                         // GetBucketVersioning
 222                         w.Header().Set("Content-Type", "application/xml")
 223                         io.WriteString(w, xml.Header)
 224                         fmt.Fprintln(w, `<VersioningConfiguration xmlns="http://s3.amazonaws.com/doc/2006-03-01/"/>`)
 225                 } else {
 226                         // ListObjects
 227                         h.s3list(w, r, fs)
 228                 }
 229                 return true
 230         case r.Method == http.MethodGet || r.Method == http.MethodHead:
 231                 fspath := "/by_id" + r.URL.Path
 232                 fi, err := fs.Stat(fspath)
 233                 if r.Method == "HEAD" && !objectNameGiven {
 234                         // HeadBucket
 235                         if err == nil && fi.IsDir() {
 236                                 w.WriteHeader(http.StatusOK)
 237                         } else if os.IsNotExist(err) {
 238                                 w.WriteHeader(http.StatusNotFound)
 239                         } else {
 240                                 http.Error(w, err.Error(), http.StatusBadGateway)
 241                         }
 242                         return true
 243                 }
 244                 if err == nil && fi.IsDir() && objectNameGiven && strings.HasSuffix(fspath, "/") && h.Config.cluster.Collections.S3FolderObjects {
 245                         w.Header().Set("Content-Type", "application/x-directory")
 246                         w.WriteHeader(http.StatusOK)
 247                         return true
 248                 }
 249                 if os.IsNotExist(err) ||
 250                         (err != nil && err.Error() == "not a directory") ||
 251                         (fi != nil && fi.IsDir()) {
 252                         http.Error(w, "not found", http.StatusNotFound)
 253                         return true
 254                 }
 255                 // shallow copy r, and change URL path
 256                 r := *r
 257                 r.URL.Path = fspath
 258                 http.FileServer(fs).ServeHTTP(w, &r)
 259                 return true
 260         case r.Method == http.MethodPut:
 261                 if !objectNameGiven {
 262                         http.Error(w, "missing object name in PUT request", http.StatusBadRequest)
 263                         return true
 264                 }
 265                 fspath := "by_id" + r.URL.Path
 266                 var objectIsDir bool
 267                 if strings.HasSuffix(fspath, "/") {
 268                         if !h.Config.cluster.Collections.S3FolderObjects {
 269                                 http.Error(w, "invalid object name: trailing slash", http.StatusBadRequest)
 270                                 return true
 271                         }
 272                         n, err := r.Body.Read(make([]byte, 1))
 273                         if err != nil && err != io.EOF {
 274                                 http.Error(w, fmt.Sprintf("error reading request body: %s", err), http.StatusInternalServerError)
 275                                 return true
 276                         } else if n > 0 {
 277                                 http.Error(w, "cannot create object with trailing '/' char unless content is empty", http.StatusBadRequest)
 278                                 return true
 279                         } else if strings.SplitN(r.Header.Get("Content-Type"), ";", 2)[0] != "application/x-directory" {
 280                                 http.Error(w, "cannot create object with trailing '/' char unless Content-Type is 'application/x-directory'", http.StatusBadRequest)
 281                                 return true
 282                         }
 283                         // Given PUT "foo/bar/", we'll use "foo/bar/."
 284                         // in the "ensure parents exist" block below,
 285                         // and then we'll be done.
 286                         fspath += "."
 287                         objectIsDir = true
 288                 }
 289                 fi, err := fs.Stat(fspath)
 290                 if err != nil && err.Error() == "not a directory" {
 291                         // requested foo/bar, but foo is a file
 292                         http.Error(w, "object name conflicts with existing object", http.StatusBadRequest)
 293                         return true
 294                 }
 295                 if strings.HasSuffix(r.URL.Path, "/") && err == nil && !fi.IsDir() {
 296                         // requested foo/bar/, but foo/bar is a file
 297                         http.Error(w, "object name conflicts with existing object", http.StatusBadRequest)
 298                         return true
 299                 }
 300                 // create missing parent/intermediate directories, if any
 301                 for i, c := range fspath {
 302                         if i > 0 && c == '/' {
 303                                 dir := fspath[:i]
 304                                 if strings.HasSuffix(dir, "/") {
 305                                         err = errors.New("invalid object name (consecutive '/' chars)")
 306                                         http.Error(w, err.Error(), http.StatusBadRequest)
 307                                         return true
 308                                 }
 309                                 err = fs.Mkdir(dir, 0755)
 310                                 if err == arvados.ErrInvalidArgument {
 311                                         // Cannot create a directory
 312                                         // here.
 313                                         err = fmt.Errorf("mkdir %q failed: %w", dir, err)
 314                                         http.Error(w, err.Error(), http.StatusBadRequest)
 315                                         return true
 316                                 } else if err != nil && !os.IsExist(err) {
 317                                         err = fmt.Errorf("mkdir %q failed: %w", dir, err)
 318                                         http.Error(w, err.Error(), http.StatusInternalServerError)
 319                                         return true
 320                                 }
 321                         }
 322                 }
 323                 if !objectIsDir {
 324                         f, err := fs.OpenFile(fspath, os.O_WRONLY|os.O_TRUNC|os.O_CREATE, 0644)
 325                         if os.IsNotExist(err) {
 326                                 f, err = fs.OpenFile(fspath, os.O_WRONLY|os.O_TRUNC|os.O_CREATE, 0644)
 327                         }
 328                         if err != nil {
 329                                 err = fmt.Errorf("open %q failed: %w", r.URL.Path, err)
 330                                 http.Error(w, err.Error(), http.StatusBadRequest)
 331                                 return true
 332                         }
 333                         defer f.Close()
 334                         _, err = io.Copy(f, r.Body)
 335                         if err != nil {
 336                                 err = fmt.Errorf("write to %q failed: %w", r.URL.Path, err)
 337                                 http.Error(w, err.Error(), http.StatusBadGateway)
 338                                 return true
 339                         }
 340                         err = f.Close()
 341                         if err != nil {
 342                                 err = fmt.Errorf("write to %q failed: close: %w", r.URL.Path, err)
 343                                 http.Error(w, err.Error(), http.StatusBadGateway)
 344                                 return true
 345                         }
 346                 }
 347                 err = fs.Sync()
 348                 if err != nil {
 349                         err = fmt.Errorf("sync failed: %w", err)
 350                         http.Error(w, err.Error(), http.StatusInternalServerError)
 351                         return true
 352                 }
 353                 w.WriteHeader(http.StatusOK)
 354                 return true
 355         case r.Method == http.MethodDelete:
 356                 if !objectNameGiven || r.URL.Path == "/" {
 357                         http.Error(w, "missing object name in DELETE request", http.StatusBadRequest)
 358                         return true
 359                 }
 360                 fspath := "by_id" + r.URL.Path
 361                 if strings.HasSuffix(fspath, "/") {
 362                         fspath = strings.TrimSuffix(fspath, "/")
 363                         fi, err := fs.Stat(fspath)
 364                         if os.IsNotExist(err) {
 365                                 w.WriteHeader(http.StatusNoContent)
 366                                 return true
 367                         } else if err != nil {
 368                                 http.Error(w, err.Error(), http.StatusInternalServerError)
 369                                 return true
 370                         } else if !fi.IsDir() {
 371                                 // if "foo" exists and is a file, then
 372                                 // "foo/" doesn't exist, so we say
 373                                 // delete was successful.
 374                                 w.WriteHeader(http.StatusNoContent)
 375                                 return true
 376                         }
 377                 } else if fi, err := fs.Stat(fspath); err == nil && fi.IsDir() {
 378                         // if "foo" is a dir, it is visible via S3
 379                         // only as "foo/", not "foo" -- so we leave
 380                         // the dir alone and return 204 to indicate
 381                         // that "foo" does not exist.
 382                         w.WriteHeader(http.StatusNoContent)
 383                         return true
 384                 }
 385                 err = fs.Remove(fspath)
 386                 if os.IsNotExist(err) {
 387                         w.WriteHeader(http.StatusNoContent)
 388                         return true
 389                 }
 390                 if err != nil {
 391                         err = fmt.Errorf("rm failed: %w", err)
 392                         http.Error(w, err.Error(), http.StatusBadRequest)
 393                         return true
 394                 }
 395                 err = fs.Sync()
 396                 if err != nil {
 397                         err = fmt.Errorf("sync failed: %w", err)
 398                         http.Error(w, err.Error(), http.StatusInternalServerError)
 399                         return true
 400                 }
 401                 w.WriteHeader(http.StatusNoContent)
 402                 return true
 403         default:
 404                 http.Error(w, "method not allowed", http.StatusMethodNotAllowed)
 405                 return true
 406         }
 407 }
 408
 409 // Call fn on the given path (directory) and its contents, in
 410 // lexicographic order.
 411 //
 412 // If isRoot==true and path is not a directory, return nil.
 413 //
 414 // If fn returns filepath.SkipDir when called on a directory, don't
 415 // descend into that directory.
 416 func walkFS(fs arvados.CustomFileSystem, path string, isRoot bool, fn func(path string, fi os.FileInfo) error) error {
 417         if isRoot {
 418                 fi, err := fs.Stat(path)
 419                 if os.IsNotExist(err) || (err == nil && !fi.IsDir()) {
 420                         return nil
 421                 } else if err != nil {
 422                         return err
 423                 }
 424                 err = fn(path, fi)
 425                 if err == filepath.SkipDir {
 426                         return nil
 427                 } else if err != nil {
 428                         return err
 429                 }
 430         }
 431         f, err := fs.Open(path)
 432         if os.IsNotExist(err) && isRoot {
 433                 return nil
 434         } else if err != nil {
 435                 return fmt.Errorf("open %q: %w", path, err)
 436         }
 437         defer f.Close()
 438         if path == "/" {
 439                 path = ""
 440         }
 441         fis, err := f.Readdir(-1)
 442         if err != nil {
 443                 return err
 444         }
 445         sort.Slice(fis, func(i, j int) bool { return fis[i].Name() < fis[j].Name() })
 446         for _, fi := range fis {
 447                 err = fn(path+"/"+fi.Name(), fi)
 448                 if err == filepath.SkipDir {
 449                         continue
 450                 } else if err != nil {
 451                         return err
 452                 }
 453                 if fi.IsDir() {
 454                         err = walkFS(fs, path+"/"+fi.Name(), false, fn)
 455                         if err != nil {
 456                                 return err
 457                         }
 458                 }
 459         }
 460         return nil
 461 }
 462
 463 var errDone = errors.New("done")
 464
 465 func (h *handler) s3list(w http.ResponseWriter, r *http.Request, fs arvados.CustomFileSystem) {
 466         var params struct {
 467                 bucket    string
 468                 delimiter string
 469                 marker    string
 470                 maxKeys   int
 471                 prefix    string
 472         }
 473         params.bucket = strings.SplitN(r.URL.Path[1:], "/", 2)[0]
 474         params.delimiter = r.FormValue("delimiter")
 475         params.marker = r.FormValue("marker")
 476         if mk, _ := strconv.ParseInt(r.FormValue("max-keys"), 10, 64); mk > 0 && mk < s3MaxKeys {
 477                 params.maxKeys = int(mk)
 478         } else {
 479                 params.maxKeys = s3MaxKeys
 480         }
 481         params.prefix = r.FormValue("prefix")
 482
 483         bucketdir := "by_id/" + params.bucket
 484         // walkpath is the directory (relative to bucketdir) we need
 485         // to walk: the innermost directory that is guaranteed to
 486         // contain all paths that have the requested prefix. Examples:
 487         // prefix "foo/bar"  => walkpath "foo"
 488         // prefix "foo/bar/" => walkpath "foo/bar"
 489         // prefix "foo"      => walkpath ""
 490         // prefix ""         => walkpath ""
 491         walkpath := params.prefix
 492         if cut := strings.LastIndex(walkpath, "/"); cut >= 0 {
 493                 walkpath = walkpath[:cut]
 494         } else {
 495                 walkpath = ""
 496         }
 497
 498         type commonPrefix struct {
 499                 Prefix string
 500         }
 501         type listResp struct {
 502                 XMLName string `xml:"http://s3.amazonaws.com/doc/2006-03-01/ ListBucketResult"`
 503                 s3.ListResp
 504                 // s3.ListResp marshals an empty tag when
 505                 // CommonPrefixes is nil, which confuses some clients.
 506                 // Fix by using this nested struct instead.
 507                 CommonPrefixes []commonPrefix
 508                 // Similarly, we need omitempty here, because an empty
 509                 // tag confuses some clients (e.g.,
 510                 // github.com/aws/aws-sdk-net never terminates its
 511                 // paging loop).
 512                 NextMarker string `xml:"NextMarker,omitempty"`
 513                 // ListObjectsV2 has a KeyCount response field.
 514                 KeyCount int
 515         }
 516         resp := listResp{
 517                 ListResp: s3.ListResp{
 518                         Name:      strings.SplitN(r.URL.Path[1:], "/", 2)[0],
 519                         Prefix:    params.prefix,
 520                         Delimiter: params.delimiter,
 521                         Marker:    params.marker,
 522                         MaxKeys:   params.maxKeys,
 523                 },
 524         }
 525         commonPrefixes := map[string]bool{}
 526         err := walkFS(fs, strings.TrimSuffix(bucketdir+"/"+walkpath, "/"), true, func(path string, fi os.FileInfo) error {
 527                 if path == bucketdir {
 528                         return nil
 529                 }
 530                 path = path[len(bucketdir)+1:]
 531                 filesize := fi.Size()
 532                 if fi.IsDir() {
 533                         path += "/"
 534                         filesize = 0
 535                 }
 536                 if len(path) <= len(params.prefix) {
 537                         if path > params.prefix[:len(path)] {
 538                                 // with prefix "foobar", walking "fooz" means we're done
 539                                 return errDone
 540                         }
 541                         if path < params.prefix[:len(path)] {
 542                                 // with prefix "foobar", walking "foobag" is pointless
 543                                 return filepath.SkipDir
 544                         }
 545                         if fi.IsDir() && !strings.HasPrefix(params.prefix+"/", path) {
 546                                 // with prefix "foo/bar", walking "fo"
 547                                 // is pointless (but walking "foo" or
 548                                 // "foo/bar" is necessary)
 549                                 return filepath.SkipDir
 550                         }
 551                         if len(path) < len(params.prefix) {
 552                                 // can't skip anything, and this entry
 553                                 // isn't in the results, so just
 554                                 // continue descent
 555                                 return nil
 556                         }
 557                 } else {
 558                         if path[:len(params.prefix)] > params.prefix {
 559                                 // with prefix "foobar", nothing we
 560                                 // see after "foozzz" is relevant
 561                                 return errDone
 562                         }
 563                 }
 564                 if path < params.marker || path < params.prefix {
 565                         return nil
 566                 }
 567                 if fi.IsDir() && !h.Config.cluster.Collections.S3FolderObjects {
 568                         // Note we don't add anything to
 569                         // commonPrefixes here even if delimiter is
 570                         // "/". We descend into the directory, and
 571                         // return a commonPrefix only if we end up
 572                         // finding a regular file inside it.
 573                         return nil
 574                 }
 575                 if params.delimiter != "" {
 576                         idx := strings.Index(path[len(params.prefix):], params.delimiter)
 577                         if idx >= 0 {
 578                                 // with prefix "foobar" and delimiter
 579                                 // "z", when we hit "foobar/baz", we
 580                                 // add "/baz" to commonPrefixes and
 581                                 // stop descending.
 582                                 commonPrefixes[path[:len(params.prefix)+idx+1]] = true
 583                                 return filepath.SkipDir
 584                         }
 585                 }
 586                 if len(resp.Contents)+len(commonPrefixes) >= params.maxKeys {
 587                         resp.IsTruncated = true
 588                         if params.delimiter != "" {
 589                                 resp.NextMarker = path
 590                         }
 591                         return errDone
 592                 }
 593                 resp.Contents = append(resp.Contents, s3.Key{
 594                         Key:          path,
 595                         LastModified: fi.ModTime().UTC().Format("2006-01-02T15:04:05.999") + "Z",
 596                         Size:         filesize,
 597                 })
 598                 return nil
 599         })
 600         if err != nil && err != errDone {
 601                 http.Error(w, err.Error(), http.StatusInternalServerError)
 602                 return
 603         }
 604         if params.delimiter != "" {
 605                 resp.CommonPrefixes = make([]commonPrefix, 0, len(commonPrefixes))
 606                 for prefix := range commonPrefixes {
 607                         resp.CommonPrefixes = append(resp.CommonPrefixes, commonPrefix{prefix})
 608                 }
 609                 sort.Slice(resp.CommonPrefixes, func(i, j int) bool { return resp.CommonPrefixes[i].Prefix < resp.CommonPrefixes[j].Prefix })
 610         }
 611         resp.KeyCount = len(resp.Contents)
 612         w.Header().Set("Content-Type", "application/xml")
 613         io.WriteString(w, xml.Header)
 614         if err := xml.NewEncoder(w).Encode(resp); err != nil {
 615                 ctxlog.FromContext(r.Context()).WithError(err).Error("error writing xml response")
 616         }
 617 }