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                 if strings.HasPrefix(token, "v2_") {
 195                         // User provided a full Arvados token with "/"
 196                         // munged to "_" (see V4 signature validation)
 197                         // but client software used S3 V2 signature.
 198                         token = strings.Replace(token, "_", "/", -1)
 199                 }
 200         } else if strings.HasPrefix(auth, s3SignAlgorithm+" ") {
 201                 t, err := h.checks3signature(r)
 202                 if err != nil {
 203                         http.Error(w, "signature verification failed: "+err.Error(), http.StatusForbidden)
 204                         return true
 205                 }
 206                 token = t
 207         } else {
 208                 return false
 209         }
 210
 211         _, kc, client, release, err := h.getClients(r.Header.Get("X-Request-Id"), token)
 212         if err != nil {
 213                 http.Error(w, "Pool failed: "+h.clientPool.Err().Error(), http.StatusInternalServerError)
 214                 return true
 215         }
 216         defer release()
 217
 218         fs := client.SiteFileSystem(kc)
 219         fs.ForwardSlashNameSubstitution(h.Config.cluster.Collections.ForwardSlashNameSubstitution)
 220
 221         objectNameGiven := strings.Count(strings.TrimSuffix(r.URL.Path, "/"), "/") > 1
 222
 223         switch {
 224         case r.Method == http.MethodGet && !objectNameGiven:
 225                 // Path is "/{uuid}" or "/{uuid}/", has no object name
 226                 if _, ok := r.URL.Query()["versioning"]; ok {
 227                         // GetBucketVersioning
 228                         w.Header().Set("Content-Type", "application/xml")
 229                         io.WriteString(w, xml.Header)
 230                         fmt.Fprintln(w, `<VersioningConfiguration xmlns="http://s3.amazonaws.com/doc/2006-03-01/"/>`)
 231                 } else {
 232                         // ListObjects
 233                         h.s3list(w, r, fs)
 234                 }
 235                 return true
 236         case r.Method == http.MethodGet || r.Method == http.MethodHead:
 237                 fspath := "/by_id" + r.URL.Path
 238                 fi, err := fs.Stat(fspath)
 239                 if r.Method == "HEAD" && !objectNameGiven {
 240                         // HeadBucket
 241                         if err == nil && fi.IsDir() {
 242                                 w.WriteHeader(http.StatusOK)
 243                         } else if os.IsNotExist(err) {
 244                                 w.WriteHeader(http.StatusNotFound)
 245                         } else {
 246                                 http.Error(w, err.Error(), http.StatusBadGateway)
 247                         }
 248                         return true
 249                 }
 250                 if err == nil && fi.IsDir() && objectNameGiven && strings.HasSuffix(fspath, "/") && h.Config.cluster.Collections.S3FolderObjects {
 251                         w.Header().Set("Content-Type", "application/x-directory")
 252                         w.WriteHeader(http.StatusOK)
 253                         return true
 254                 }
 255                 if os.IsNotExist(err) ||
 256                         (err != nil && err.Error() == "not a directory") ||
 257                         (fi != nil && fi.IsDir()) {
 258                         http.Error(w, "not found", http.StatusNotFound)
 259                         return true
 260                 }
 261                 // shallow copy r, and change URL path
 262                 r := *r
 263                 r.URL.Path = fspath
 264                 http.FileServer(fs).ServeHTTP(w, &r)
 265                 return true
 266         case r.Method == http.MethodPut:
 267                 if !objectNameGiven {
 268                         http.Error(w, "missing object name in PUT request", http.StatusBadRequest)
 269                         return true
 270                 }
 271                 fspath := "by_id" + r.URL.Path
 272                 var objectIsDir bool
 273                 if strings.HasSuffix(fspath, "/") {
 274                         if !h.Config.cluster.Collections.S3FolderObjects {
 275                                 http.Error(w, "invalid object name: trailing slash", http.StatusBadRequest)
 276                                 return true
 277                         }
 278                         n, err := r.Body.Read(make([]byte, 1))
 279                         if err != nil && err != io.EOF {
 280                                 http.Error(w, fmt.Sprintf("error reading request body: %s", err), http.StatusInternalServerError)
 281                                 return true
 282                         } else if n > 0 {
 283                                 http.Error(w, "cannot create object with trailing '/' char unless content is empty", http.StatusBadRequest)
 284                                 return true
 285                         } else if strings.SplitN(r.Header.Get("Content-Type"), ";", 2)[0] != "application/x-directory" {
 286                                 http.Error(w, "cannot create object with trailing '/' char unless Content-Type is 'application/x-directory'", http.StatusBadRequest)
 287                                 return true
 288                         }
 289                         // Given PUT "foo/bar/", we'll use "foo/bar/."
 290                         // in the "ensure parents exist" block below,
 291                         // and then we'll be done.
 292                         fspath += "."
 293                         objectIsDir = true
 294                 }
 295                 fi, err := fs.Stat(fspath)
 296                 if err != nil && err.Error() == "not a directory" {
 297                         // requested foo/bar, but foo is a file
 298                         http.Error(w, "object name conflicts with existing object", http.StatusBadRequest)
 299                         return true
 300                 }
 301                 if strings.HasSuffix(r.URL.Path, "/") && err == nil && !fi.IsDir() {
 302                         // requested foo/bar/, but foo/bar is a file
 303                         http.Error(w, "object name conflicts with existing object", http.StatusBadRequest)
 304                         return true
 305                 }
 306                 // create missing parent/intermediate directories, if any
 307                 for i, c := range fspath {
 308                         if i > 0 && c == '/' {
 309                                 dir := fspath[:i]
 310                                 if strings.HasSuffix(dir, "/") {
 311                                         err = errors.New("invalid object name (consecutive '/' chars)")
 312                                         http.Error(w, err.Error(), http.StatusBadRequest)
 313                                         return true
 314                                 }
 315                                 err = fs.Mkdir(dir, 0755)
 316                                 if err == arvados.ErrInvalidArgument {
 317                                         // Cannot create a directory
 318                                         // here.
 319                                         err = fmt.Errorf("mkdir %q failed: %w", dir, err)
 320                                         http.Error(w, err.Error(), http.StatusBadRequest)
 321                                         return true
 322                                 } else if err != nil && !os.IsExist(err) {
 323                                         err = fmt.Errorf("mkdir %q failed: %w", dir, err)
 324                                         http.Error(w, err.Error(), http.StatusInternalServerError)
 325                                         return true
 326                                 }
 327                         }
 328                 }
 329                 if !objectIsDir {
 330                         f, err := fs.OpenFile(fspath, os.O_WRONLY|os.O_TRUNC|os.O_CREATE, 0644)
 331                         if os.IsNotExist(err) {
 332                                 f, err = fs.OpenFile(fspath, os.O_WRONLY|os.O_TRUNC|os.O_CREATE, 0644)
 333                         }
 334                         if err != nil {
 335                                 err = fmt.Errorf("open %q failed: %w", r.URL.Path, err)
 336                                 http.Error(w, err.Error(), http.StatusBadRequest)
 337                                 return true
 338                         }
 339                         defer f.Close()
 340                         _, err = io.Copy(f, r.Body)
 341                         if err != nil {
 342                                 err = fmt.Errorf("write to %q failed: %w", r.URL.Path, err)
 343                                 http.Error(w, err.Error(), http.StatusBadGateway)
 344                                 return true
 345                         }
 346                         err = f.Close()
 347                         if err != nil {
 348                                 err = fmt.Errorf("write to %q failed: close: %w", r.URL.Path, err)
 349                                 http.Error(w, err.Error(), http.StatusBadGateway)
 350                                 return true
 351                         }
 352                 }
 353                 err = fs.Sync()
 354                 if err != nil {
 355                         err = fmt.Errorf("sync failed: %w", err)
 356                         http.Error(w, err.Error(), http.StatusInternalServerError)
 357                         return true
 358                 }
 359                 w.WriteHeader(http.StatusOK)
 360                 return true
 361         case r.Method == http.MethodDelete:
 362                 if !objectNameGiven || r.URL.Path == "/" {
 363                         http.Error(w, "missing object name in DELETE request", http.StatusBadRequest)
 364                         return true
 365                 }
 366                 fspath := "by_id" + r.URL.Path
 367                 if strings.HasSuffix(fspath, "/") {
 368                         fspath = strings.TrimSuffix(fspath, "/")
 369                         fi, err := fs.Stat(fspath)
 370                         if os.IsNotExist(err) {
 371                                 w.WriteHeader(http.StatusNoContent)
 372                                 return true
 373                         } else if err != nil {
 374                                 http.Error(w, err.Error(), http.StatusInternalServerError)
 375                                 return true
 376                         } else if !fi.IsDir() {
 377                                 // if "foo" exists and is a file, then
 378                                 // "foo/" doesn't exist, so we say
 379                                 // delete was successful.
 380                                 w.WriteHeader(http.StatusNoContent)
 381                                 return true
 382                         }
 383                 } else if fi, err := fs.Stat(fspath); err == nil && fi.IsDir() {
 384                         // if "foo" is a dir, it is visible via S3
 385                         // only as "foo/", not "foo" -- so we leave
 386                         // the dir alone and return 204 to indicate
 387                         // that "foo" does not exist.
 388                         w.WriteHeader(http.StatusNoContent)
 389                         return true
 390                 }
 391                 err = fs.Remove(fspath)
 392                 if os.IsNotExist(err) {
 393                         w.WriteHeader(http.StatusNoContent)
 394                         return true
 395                 }
 396                 if err != nil {
 397                         err = fmt.Errorf("rm failed: %w", err)
 398                         http.Error(w, err.Error(), http.StatusBadRequest)
 399                         return true
 400                 }
 401                 err = fs.Sync()
 402                 if err != nil {
 403                         err = fmt.Errorf("sync failed: %w", err)
 404                         http.Error(w, err.Error(), http.StatusInternalServerError)
 405                         return true
 406                 }
 407                 w.WriteHeader(http.StatusNoContent)
 408                 return true
 409         default:
 410                 http.Error(w, "method not allowed", http.StatusMethodNotAllowed)
 411                 return true
 412         }
 413 }
 414
 415 // Call fn on the given path (directory) and its contents, in
 416 // lexicographic order.
 417 //
 418 // If isRoot==true and path is not a directory, return nil.
 419 //
 420 // If fn returns filepath.SkipDir when called on a directory, don't
 421 // descend into that directory.
 422 func walkFS(fs arvados.CustomFileSystem, path string, isRoot bool, fn func(path string, fi os.FileInfo) error) error {
 423         if isRoot {
 424                 fi, err := fs.Stat(path)
 425                 if os.IsNotExist(err) || (err == nil && !fi.IsDir()) {
 426                         return nil
 427                 } else if err != nil {
 428                         return err
 429                 }
 430                 err = fn(path, fi)
 431                 if err == filepath.SkipDir {
 432                         return nil
 433                 } else if err != nil {
 434                         return err
 435                 }
 436         }
 437         f, err := fs.Open(path)
 438         if os.IsNotExist(err) && isRoot {
 439                 return nil
 440         } else if err != nil {
 441                 return fmt.Errorf("open %q: %w", path, err)
 442         }
 443         defer f.Close()
 444         if path == "/" {
 445                 path = ""
 446         }
 447         fis, err := f.Readdir(-1)
 448         if err != nil {
 449                 return err
 450         }
 451         sort.Slice(fis, func(i, j int) bool { return fis[i].Name() < fis[j].Name() })
 452         for _, fi := range fis {
 453                 err = fn(path+"/"+fi.Name(), fi)
 454                 if err == filepath.SkipDir {
 455                         continue
 456                 } else if err != nil {
 457                         return err
 458                 }
 459                 if fi.IsDir() {
 460                         err = walkFS(fs, path+"/"+fi.Name(), false, fn)
 461                         if err != nil {
 462                                 return err
 463                         }
 464                 }
 465         }
 466         return nil
 467 }
 468
 469 var errDone = errors.New("done")
 470
 471 func (h *handler) s3list(w http.ResponseWriter, r *http.Request, fs arvados.CustomFileSystem) {
 472         var params struct {
 473                 bucket    string
 474                 delimiter string
 475                 marker    string
 476                 maxKeys   int
 477                 prefix    string
 478         }
 479         params.bucket = strings.SplitN(r.URL.Path[1:], "/", 2)[0]
 480         params.delimiter = r.FormValue("delimiter")
 481         params.marker = r.FormValue("marker")
 482         if mk, _ := strconv.ParseInt(r.FormValue("max-keys"), 10, 64); mk > 0 && mk < s3MaxKeys {
 483                 params.maxKeys = int(mk)
 484         } else {
 485                 params.maxKeys = s3MaxKeys
 486         }
 487         params.prefix = r.FormValue("prefix")
 488
 489         bucketdir := "by_id/" + params.bucket
 490         // walkpath is the directory (relative to bucketdir) we need
 491         // to walk: the innermost directory that is guaranteed to
 492         // contain all paths that have the requested prefix. Examples:
 493         // prefix "foo/bar"  => walkpath "foo"
 494         // prefix "foo/bar/" => walkpath "foo/bar"
 495         // prefix "foo"      => walkpath ""
 496         // prefix ""         => walkpath ""
 497         walkpath := params.prefix
 498         if cut := strings.LastIndex(walkpath, "/"); cut >= 0 {
 499                 walkpath = walkpath[:cut]
 500         } else {
 501                 walkpath = ""
 502         }
 503
 504         type commonPrefix struct {
 505                 Prefix string
 506         }
 507         type listResp struct {
 508                 XMLName string `xml:"http://s3.amazonaws.com/doc/2006-03-01/ ListBucketResult"`
 509                 s3.ListResp
 510                 // s3.ListResp marshals an empty tag when
 511                 // CommonPrefixes is nil, which confuses some clients.
 512                 // Fix by using this nested struct instead.
 513                 CommonPrefixes []commonPrefix
 514                 // Similarly, we need omitempty here, because an empty
 515                 // tag confuses some clients (e.g.,
 516                 // github.com/aws/aws-sdk-net never terminates its
 517                 // paging loop).
 518                 NextMarker string `xml:"NextMarker,omitempty"`
 519                 // ListObjectsV2 has a KeyCount response field.
 520                 KeyCount int
 521         }
 522         resp := listResp{
 523                 ListResp: s3.ListResp{
 524                         Name:      strings.SplitN(r.URL.Path[1:], "/", 2)[0],
 525                         Prefix:    params.prefix,
 526                         Delimiter: params.delimiter,
 527                         Marker:    params.marker,
 528                         MaxKeys:   params.maxKeys,
 529                 },
 530         }
 531         commonPrefixes := map[string]bool{}
 532         err := walkFS(fs, strings.TrimSuffix(bucketdir+"/"+walkpath, "/"), true, func(path string, fi os.FileInfo) error {
 533                 if path == bucketdir {
 534                         return nil
 535                 }
 536                 path = path[len(bucketdir)+1:]
 537                 filesize := fi.Size()
 538                 if fi.IsDir() {
 539                         path += "/"
 540                         filesize = 0
 541                 }
 542                 if len(path) <= len(params.prefix) {
 543                         if path > params.prefix[:len(path)] {
 544                                 // with prefix "foobar", walking "fooz" means we're done
 545                                 return errDone
 546                         }
 547                         if path < params.prefix[:len(path)] {
 548                                 // with prefix "foobar", walking "foobag" is pointless
 549                                 return filepath.SkipDir
 550                         }
 551                         if fi.IsDir() && !strings.HasPrefix(params.prefix+"/", path) {
 552                                 // with prefix "foo/bar", walking "fo"
 553                                 // is pointless (but walking "foo" or
 554                                 // "foo/bar" is necessary)
 555                                 return filepath.SkipDir
 556                         }
 557                         if len(path) < len(params.prefix) {
 558                                 // can't skip anything, and this entry
 559                                 // isn't in the results, so just
 560                                 // continue descent
 561                                 return nil
 562                         }
 563                 } else {
 564                         if path[:len(params.prefix)] > params.prefix {
 565                                 // with prefix "foobar", nothing we
 566                                 // see after "foozzz" is relevant
 567                                 return errDone
 568                         }
 569                 }
 570                 if path < params.marker || path < params.prefix {
 571                         return nil
 572                 }
 573                 if fi.IsDir() && !h.Config.cluster.Collections.S3FolderObjects {
 574                         // Note we don't add anything to
 575                         // commonPrefixes here even if delimiter is
 576                         // "/". We descend into the directory, and
 577                         // return a commonPrefix only if we end up
 578                         // finding a regular file inside it.
 579                         return nil
 580                 }
 581                 if params.delimiter != "" {
 582                         idx := strings.Index(path[len(params.prefix):], params.delimiter)
 583                         if idx >= 0 {
 584                                 // with prefix "foobar" and delimiter
 585                                 // "z", when we hit "foobar/baz", we
 586                                 // add "/baz" to commonPrefixes and
 587                                 // stop descending.
 588                                 commonPrefixes[path[:len(params.prefix)+idx+1]] = true
 589                                 return filepath.SkipDir
 590                         }
 591                 }
 592                 if len(resp.Contents)+len(commonPrefixes) >= params.maxKeys {
 593                         resp.IsTruncated = true
 594                         if params.delimiter != "" {
 595                                 resp.NextMarker = path
 596                         }
 597                         return errDone
 598                 }
 599                 resp.Contents = append(resp.Contents, s3.Key{
 600                         Key:          path,
 601                         LastModified: fi.ModTime().UTC().Format("2006-01-02T15:04:05.999") + "Z",
 602                         Size:         filesize,
 603                 })
 604                 return nil
 605         })
 606         if err != nil && err != errDone {
 607                 http.Error(w, err.Error(), http.StatusInternalServerError)
 608                 return
 609         }
 610         if params.delimiter != "" {
 611                 resp.CommonPrefixes = make([]commonPrefix, 0, len(commonPrefixes))
 612                 for prefix := range commonPrefixes {
 613                         resp.CommonPrefixes = append(resp.CommonPrefixes, commonPrefix{prefix})
 614                 }
 615                 sort.Slice(resp.CommonPrefixes, func(i, j int) bool { return resp.CommonPrefixes[i].Prefix < resp.CommonPrefixes[j].Prefix })
 616         }
 617         resp.KeyCount = len(resp.Contents)
 618         w.Header().Set("Content-Type", "application/xml")
 619         io.WriteString(w, xml.Header)
 620         if err := xml.NewEncoder(w).Encode(resp); err != nil {
 621                 ctxlog.FromContext(r.Context()).WithError(err).Error("error writing xml response")
 622         }
 623 }