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                 ctx := arvados.ContextWithAuthorization(r.Context(), "Bearer "+key)
 156                 err = client.RequestAndDecodeContext(ctx, &aca, "GET", "arvados/v1/api_client_authorizations/current", nil, nil)
 157                 secret = key
 158         }
 159         if err != nil {
 160                 ctxlog.FromContext(r.Context()).WithError(err).WithField("UUID", key).Info("token lookup failed")
 161                 return "", errors.New("invalid access key")
 162         }
 163         stringToSign, err := s3stringToSign(s3SignAlgorithm, scope, signedHeaders, r)
 164         if err != nil {
 165                 return "", err
 166         }
 167         expect, err := s3signature(secret, scope, signedHeaders, stringToSign)
 168         if err != nil {
 169                 return "", err
 170         } else if expect != signature {
 171                 return "", fmt.Errorf("signature does not match (scope %q signedHeaders %q stringToSign %q)", scope, signedHeaders, stringToSign)
 172         }
 173         return secret, nil
 174 }
 175
 176 // serveS3 handles r and returns true if r is a request from an S3
 177 // client, otherwise it returns false.
 178 func (h *handler) serveS3(w http.ResponseWriter, r *http.Request) bool {
 179         var token string
 180         if auth := r.Header.Get("Authorization"); strings.HasPrefix(auth, "AWS ") {
 181                 split := strings.SplitN(auth[4:], ":", 2)
 182                 if len(split) < 2 {
 183                         http.Error(w, "malformed Authorization header", http.StatusUnauthorized)
 184                         return true
 185                 }
 186                 token = split[0]
 187         } else if strings.HasPrefix(auth, s3SignAlgorithm+" ") {
 188                 t, err := h.checks3signature(r)
 189                 if err != nil {
 190                         http.Error(w, "signature verification failed: "+err.Error(), http.StatusForbidden)
 191                         return true
 192                 }
 193                 token = t
 194         } else {
 195                 return false
 196         }
 197
 198         _, kc, client, release, err := h.getClients(r.Header.Get("X-Request-Id"), token)
 199         if err != nil {
 200                 http.Error(w, "Pool failed: "+h.clientPool.Err().Error(), http.StatusInternalServerError)
 201                 return true
 202         }
 203         defer release()
 204
 205         fs := client.SiteFileSystem(kc)
 206         fs.ForwardSlashNameSubstitution(h.Config.cluster.Collections.ForwardSlashNameSubstitution)
 207
 208         var objectNameGiven bool
 209         fspath := "/by_id"
 210         if id := parseCollectionIDFromDNSName(r.Host); id != "" {
 211                 fspath += "/" + id
 212                 objectNameGiven = true
 213         } else {
 214                 objectNameGiven = strings.Count(strings.TrimSuffix(r.URL.Path, "/"), "/") > 1
 215         }
 216         fspath += r.URL.Path
 217
 218         switch {
 219         case r.Method == http.MethodGet && !objectNameGiven:
 220                 // Path is "/{uuid}" or "/{uuid}/", has no object name
 221                 if _, ok := r.URL.Query()["versioning"]; ok {
 222                         // GetBucketVersioning
 223                         w.Header().Set("Content-Type", "application/xml")
 224                         io.WriteString(w, xml.Header)
 225                         fmt.Fprintln(w, `<VersioningConfiguration xmlns="http://s3.amazonaws.com/doc/2006-03-01/"/>`)
 226                 } else {
 227                         // ListObjects
 228                         h.s3list(w, r, fs)
 229                 }
 230                 return true
 231         case r.Method == http.MethodGet || r.Method == http.MethodHead:
 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                 var objectIsDir bool
 266                 if strings.HasSuffix(fspath, "/") {
 267                         if !h.Config.cluster.Collections.S3FolderObjects {
 268                                 http.Error(w, "invalid object name: trailing slash", http.StatusBadRequest)
 269                                 return true
 270                         }
 271                         n, err := r.Body.Read(make([]byte, 1))
 272                         if err != nil && err != io.EOF {
 273                                 http.Error(w, fmt.Sprintf("error reading request body: %s", err), http.StatusInternalServerError)
 274                                 return true
 275                         } else if n > 0 {
 276                                 http.Error(w, "cannot create object with trailing '/' char unless content is empty", http.StatusBadRequest)
 277                                 return true
 278                         } else if strings.SplitN(r.Header.Get("Content-Type"), ";", 2)[0] != "application/x-directory" {
 279                                 http.Error(w, "cannot create object with trailing '/' char unless Content-Type is 'application/x-directory'", http.StatusBadRequest)
 280                                 return true
 281                         }
 282                         // Given PUT "foo/bar/", we'll use "foo/bar/."
 283                         // in the "ensure parents exist" block below,
 284                         // and then we'll be done.
 285                         fspath += "."
 286                         objectIsDir = true
 287                 }
 288                 fi, err := fs.Stat(fspath)
 289                 if err != nil && err.Error() == "not a directory" {
 290                         // requested foo/bar, but foo is a file
 291                         http.Error(w, "object name conflicts with existing object", http.StatusBadRequest)
 292                         return true
 293                 }
 294                 if strings.HasSuffix(r.URL.Path, "/") && err == nil && !fi.IsDir() {
 295                         // requested foo/bar/, but foo/bar is a file
 296                         http.Error(w, "object name conflicts with existing object", http.StatusBadRequest)
 297                         return true
 298                 }
 299                 // create missing parent/intermediate directories, if any
 300                 for i, c := range fspath {
 301                         if i > 0 && c == '/' {
 302                                 dir := fspath[:i]
 303                                 if strings.HasSuffix(dir, "/") {
 304                                         err = errors.New("invalid object name (consecutive '/' chars)")
 305                                         http.Error(w, err.Error(), http.StatusBadRequest)
 306                                         return true
 307                                 }
 308                                 err = fs.Mkdir(dir, 0755)
 309                                 if err == arvados.ErrInvalidArgument {
 310                                         // Cannot create a directory
 311                                         // here.
 312                                         err = fmt.Errorf("mkdir %q failed: %w", dir, err)
 313                                         http.Error(w, err.Error(), http.StatusBadRequest)
 314                                         return true
 315                                 } else if err != nil && !os.IsExist(err) {
 316                                         err = fmt.Errorf("mkdir %q failed: %w", dir, err)
 317                                         http.Error(w, err.Error(), http.StatusInternalServerError)
 318                                         return true
 319                                 }
 320                         }
 321                 }
 322                 if !objectIsDir {
 323                         f, err := fs.OpenFile(fspath, os.O_WRONLY|os.O_TRUNC|os.O_CREATE, 0644)
 324                         if os.IsNotExist(err) {
 325                                 f, err = fs.OpenFile(fspath, os.O_WRONLY|os.O_TRUNC|os.O_CREATE, 0644)
 326                         }
 327                         if err != nil {
 328                                 err = fmt.Errorf("open %q failed: %w", r.URL.Path, err)
 329                                 http.Error(w, err.Error(), http.StatusBadRequest)
 330                                 return true
 331                         }
 332                         defer f.Close()
 333                         _, err = io.Copy(f, r.Body)
 334                         if err != nil {
 335                                 err = fmt.Errorf("write to %q failed: %w", r.URL.Path, err)
 336                                 http.Error(w, err.Error(), http.StatusBadGateway)
 337                                 return true
 338                         }
 339                         err = f.Close()
 340                         if err != nil {
 341                                 err = fmt.Errorf("write to %q failed: close: %w", r.URL.Path, err)
 342                                 http.Error(w, err.Error(), http.StatusBadGateway)
 343                                 return true
 344                         }
 345                 }
 346                 err = fs.Sync()
 347                 if err != nil {
 348                         err = fmt.Errorf("sync failed: %w", err)
 349                         http.Error(w, err.Error(), http.StatusInternalServerError)
 350                         return true
 351                 }
 352                 w.WriteHeader(http.StatusOK)
 353                 return true
 354         case r.Method == http.MethodDelete:
 355                 if !objectNameGiven || r.URL.Path == "/" {
 356                         http.Error(w, "missing object name in DELETE request", http.StatusBadRequest)
 357                         return true
 358                 }
 359                 if strings.HasSuffix(fspath, "/") {
 360                         fspath = strings.TrimSuffix(fspath, "/")
 361                         fi, err := fs.Stat(fspath)
 362                         if os.IsNotExist(err) {
 363                                 w.WriteHeader(http.StatusNoContent)
 364                                 return true
 365                         } else if err != nil {
 366                                 http.Error(w, err.Error(), http.StatusInternalServerError)
 367                                 return true
 368                         } else if !fi.IsDir() {
 369                                 // if "foo" exists and is a file, then
 370                                 // "foo/" doesn't exist, so we say
 371                                 // delete was successful.
 372                                 w.WriteHeader(http.StatusNoContent)
 373                                 return true
 374                         }
 375                 } else if fi, err := fs.Stat(fspath); err == nil && fi.IsDir() {
 376                         // if "foo" is a dir, it is visible via S3
 377                         // only as "foo/", not "foo" -- so we leave
 378                         // the dir alone and return 204 to indicate
 379                         // that "foo" does not exist.
 380                         w.WriteHeader(http.StatusNoContent)
 381                         return true
 382                 }
 383                 err = fs.Remove(fspath)
 384                 if os.IsNotExist(err) {
 385                         w.WriteHeader(http.StatusNoContent)
 386                         return true
 387                 }
 388                 if err != nil {
 389                         err = fmt.Errorf("rm failed: %w", err)
 390                         http.Error(w, err.Error(), http.StatusBadRequest)
 391                         return true
 392                 }
 393                 err = fs.Sync()
 394                 if err != nil {
 395                         err = fmt.Errorf("sync failed: %w", err)
 396                         http.Error(w, err.Error(), http.StatusInternalServerError)
 397                         return true
 398                 }
 399                 w.WriteHeader(http.StatusNoContent)
 400                 return true
 401         default:
 402                 http.Error(w, "method not allowed", http.StatusMethodNotAllowed)
 403                 return true
 404         }
 405 }
 406
 407 // Call fn on the given path (directory) and its contents, in
 408 // lexicographic order.
 409 //
 410 // If isRoot==true and path is not a directory, return nil.
 411 //
 412 // If fn returns filepath.SkipDir when called on a directory, don't
 413 // descend into that directory.
 414 func walkFS(fs arvados.CustomFileSystem, path string, isRoot bool, fn func(path string, fi os.FileInfo) error) error {
 415         if isRoot {
 416                 fi, err := fs.Stat(path)
 417                 if os.IsNotExist(err) || (err == nil && !fi.IsDir()) {
 418                         return nil
 419                 } else if err != nil {
 420                         return err
 421                 }
 422                 err = fn(path, fi)
 423                 if err == filepath.SkipDir {
 424                         return nil
 425                 } else if err != nil {
 426                         return err
 427                 }
 428         }
 429         f, err := fs.Open(path)
 430         if os.IsNotExist(err) && isRoot {
 431                 return nil
 432         } else if err != nil {
 433                 return fmt.Errorf("open %q: %w", path, err)
 434         }
 435         defer f.Close()
 436         if path == "/" {
 437                 path = ""
 438         }
 439         fis, err := f.Readdir(-1)
 440         if err != nil {
 441                 return err
 442         }
 443         sort.Slice(fis, func(i, j int) bool { return fis[i].Name() < fis[j].Name() })
 444         for _, fi := range fis {
 445                 err = fn(path+"/"+fi.Name(), fi)
 446                 if err == filepath.SkipDir {
 447                         continue
 448                 } else if err != nil {
 449                         return err
 450                 }
 451                 if fi.IsDir() {
 452                         err = walkFS(fs, path+"/"+fi.Name(), false, fn)
 453                         if err != nil {
 454                                 return err
 455                         }
 456                 }
 457         }
 458         return nil
 459 }
 460
 461 var errDone = errors.New("done")
 462
 463 func (h *handler) s3list(w http.ResponseWriter, r *http.Request, fs arvados.CustomFileSystem) {
 464         var params struct {
 465                 bucket    string
 466                 delimiter string
 467                 marker    string
 468                 maxKeys   int
 469                 prefix    string
 470         }
 471         params.bucket = strings.SplitN(r.URL.Path[1:], "/", 2)[0]
 472         params.delimiter = r.FormValue("delimiter")
 473         params.marker = r.FormValue("marker")
 474         if mk, _ := strconv.ParseInt(r.FormValue("max-keys"), 10, 64); mk > 0 && mk < s3MaxKeys {
 475                 params.maxKeys = int(mk)
 476         } else {
 477                 params.maxKeys = s3MaxKeys
 478         }
 479         params.prefix = r.FormValue("prefix")
 480
 481         bucketdir := "by_id/" + params.bucket
 482         // walkpath is the directory (relative to bucketdir) we need
 483         // to walk: the innermost directory that is guaranteed to
 484         // contain all paths that have the requested prefix. Examples:
 485         // prefix "foo/bar"  => walkpath "foo"
 486         // prefix "foo/bar/" => walkpath "foo/bar"
 487         // prefix "foo"      => walkpath ""
 488         // prefix ""         => walkpath ""
 489         walkpath := params.prefix
 490         if cut := strings.LastIndex(walkpath, "/"); cut >= 0 {
 491                 walkpath = walkpath[:cut]
 492         } else {
 493                 walkpath = ""
 494         }
 495
 496         type commonPrefix struct {
 497                 Prefix string
 498         }
 499         type listResp struct {
 500                 XMLName string `xml:"http://s3.amazonaws.com/doc/2006-03-01/ ListBucketResult"`
 501                 s3.ListResp
 502                 // s3.ListResp marshals an empty tag when
 503                 // CommonPrefixes is nil, which confuses some clients.
 504                 // Fix by using this nested struct instead.
 505                 CommonPrefixes []commonPrefix
 506                 // Similarly, we need omitempty here, because an empty
 507                 // tag confuses some clients (e.g.,
 508                 // github.com/aws/aws-sdk-net never terminates its
 509                 // paging loop).
 510                 NextMarker string `xml:"NextMarker,omitempty"`
 511                 // ListObjectsV2 has a KeyCount response field.
 512                 KeyCount int
 513         }
 514         resp := listResp{
 515                 ListResp: s3.ListResp{
 516                         Name:      strings.SplitN(r.URL.Path[1:], "/", 2)[0],
 517                         Prefix:    params.prefix,
 518                         Delimiter: params.delimiter,
 519                         Marker:    params.marker,
 520                         MaxKeys:   params.maxKeys,
 521                 },
 522         }
 523         commonPrefixes := map[string]bool{}
 524         err := walkFS(fs, strings.TrimSuffix(bucketdir+"/"+walkpath, "/"), true, func(path string, fi os.FileInfo) error {
 525                 if path == bucketdir {
 526                         return nil
 527                 }
 528                 path = path[len(bucketdir)+1:]
 529                 filesize := fi.Size()
 530                 if fi.IsDir() {
 531                         path += "/"
 532                         filesize = 0
 533                 }
 534                 if len(path) <= len(params.prefix) {
 535                         if path > params.prefix[:len(path)] {
 536                                 // with prefix "foobar", walking "fooz" means we're done
 537                                 return errDone
 538                         }
 539                         if path < params.prefix[:len(path)] {
 540                                 // with prefix "foobar", walking "foobag" is pointless
 541                                 return filepath.SkipDir
 542                         }
 543                         if fi.IsDir() && !strings.HasPrefix(params.prefix+"/", path) {
 544                                 // with prefix "foo/bar", walking "fo"
 545                                 // is pointless (but walking "foo" or
 546                                 // "foo/bar" is necessary)
 547                                 return filepath.SkipDir
 548                         }
 549                         if len(path) < len(params.prefix) {
 550                                 // can't skip anything, and this entry
 551                                 // isn't in the results, so just
 552                                 // continue descent
 553                                 return nil
 554                         }
 555                 } else {
 556                         if path[:len(params.prefix)] > params.prefix {
 557                                 // with prefix "foobar", nothing we
 558                                 // see after "foozzz" is relevant
 559                                 return errDone
 560                         }
 561                 }
 562                 if path < params.marker || path < params.prefix {
 563                         return nil
 564                 }
 565                 if fi.IsDir() && !h.Config.cluster.Collections.S3FolderObjects {
 566                         // Note we don't add anything to
 567                         // commonPrefixes here even if delimiter is
 568                         // "/". We descend into the directory, and
 569                         // return a commonPrefix only if we end up
 570                         // finding a regular file inside it.
 571                         return nil
 572                 }
 573                 if params.delimiter != "" {
 574                         idx := strings.Index(path[len(params.prefix):], params.delimiter)
 575                         if idx >= 0 {
 576                                 // with prefix "foobar" and delimiter
 577                                 // "z", when we hit "foobar/baz", we
 578                                 // add "/baz" to commonPrefixes and
 579                                 // stop descending.
 580                                 commonPrefixes[path[:len(params.prefix)+idx+1]] = true
 581                                 return filepath.SkipDir
 582                         }
 583                 }
 584                 if len(resp.Contents)+len(commonPrefixes) >= params.maxKeys {
 585                         resp.IsTruncated = true
 586                         if params.delimiter != "" {
 587                                 resp.NextMarker = path
 588                         }
 589                         return errDone
 590                 }
 591                 resp.Contents = append(resp.Contents, s3.Key{
 592                         Key:          path,
 593                         LastModified: fi.ModTime().UTC().Format("2006-01-02T15:04:05.999") + "Z",
 594                         Size:         filesize,
 595                 })
 596                 return nil
 597         })
 598         if err != nil && err != errDone {
 599                 http.Error(w, err.Error(), http.StatusInternalServerError)
 600                 return
 601         }
 602         if params.delimiter != "" {
 603                 resp.CommonPrefixes = make([]commonPrefix, 0, len(commonPrefixes))
 604                 for prefix := range commonPrefixes {
 605                         resp.CommonPrefixes = append(resp.CommonPrefixes, commonPrefix{prefix})
 606                 }
 607                 sort.Slice(resp.CommonPrefixes, func(i, j int) bool { return resp.CommonPrefixes[i].Prefix < resp.CommonPrefixes[j].Prefix })
 608         }
 609         resp.KeyCount = len(resp.Contents)
 610         w.Header().Set("Content-Type", "application/xml")
 611         io.WriteString(w, xml.Header)
 612         if err := xml.NewEncoder(w).Encode(resp); err != nil {
 613                 ctxlog.FromContext(r.Context()).WithError(err).Error("error writing xml response")
 614         }
 615 }