Merge branch '16774-keep-web-errors' refs #16774
[arvados.git] / services / keep-web / s3.go
index c6f501e492e6b5f627e9dbae79ec12aec98b1214..7fb90789a55b164bd2465bf8d2454cf2d30e4f52 100644 (file)
 package main
 
 import (
+       "crypto/hmac"
+       "crypto/sha256"
+       "encoding/xml"
+       "errors"
        "fmt"
+       "hash"
        "io"
        "net/http"
+       "net/url"
        "os"
+       "path/filepath"
+       "regexp"
+       "sort"
+       "strconv"
        "strings"
+       "time"
+
+       "git.arvados.org/arvados.git/sdk/go/arvados"
+       "git.arvados.org/arvados.git/sdk/go/ctxlog"
+       "github.com/AdRoll/goamz/s3"
+)
+
+const (
+       s3MaxKeys       = 1000
+       s3SignAlgorithm = "AWS4-HMAC-SHA256"
+       s3MaxClockSkew  = 5 * time.Minute
 )
 
+func hmacstring(msg string, key []byte) []byte {
+       h := hmac.New(sha256.New, key)
+       io.WriteString(h, msg)
+       return h.Sum(nil)
+}
+
+func hashdigest(h hash.Hash, payload string) string {
+       io.WriteString(h, payload)
+       return fmt.Sprintf("%x", h.Sum(nil))
+}
+
+// Signing key for given secret key and request attrs.
+func s3signatureKey(key, datestamp, regionName, serviceName string) []byte {
+       return hmacstring("aws4_request",
+               hmacstring(serviceName,
+                       hmacstring(regionName,
+                               hmacstring(datestamp, []byte("AWS4"+key)))))
+}
+
+// Canonical query string for S3 V4 signature: sorted keys, spaces
+// escaped as %20 instead of +, keyvalues joined with &.
+func s3querystring(u *url.URL) string {
+       keys := make([]string, 0, len(u.Query()))
+       values := make(map[string]string, len(u.Query()))
+       for k, vs := range u.Query() {
+               k = strings.Replace(url.QueryEscape(k), "+", "%20", -1)
+               keys = append(keys, k)
+               for _, v := range vs {
+                       v = strings.Replace(url.QueryEscape(v), "+", "%20", -1)
+                       if values[k] != "" {
+                               values[k] += "&"
+                       }
+                       values[k] += k + "=" + v
+               }
+       }
+       sort.Strings(keys)
+       for i, k := range keys {
+               keys[i] = values[k]
+       }
+       return strings.Join(keys, "&")
+}
+
+func s3stringToSign(alg, scope, signedHeaders string, r *http.Request) (string, error) {
+       timefmt, timestr := "20060102T150405Z", r.Header.Get("X-Amz-Date")
+       if timestr == "" {
+               timefmt, timestr = time.RFC1123, r.Header.Get("Date")
+       }
+       t, err := time.Parse(timefmt, timestr)
+       if err != nil {
+               return "", fmt.Errorf("invalid timestamp %q: %s", timestr, err)
+       }
+       if skew := time.Now().Sub(t); skew < -s3MaxClockSkew || skew > s3MaxClockSkew {
+               return "", errors.New("exceeded max clock skew")
+       }
+
+       var canonicalHeaders string
+       for _, h := range strings.Split(signedHeaders, ";") {
+               if h == "host" {
+                       canonicalHeaders += h + ":" + r.Host + "\n"
+               } else {
+                       canonicalHeaders += h + ":" + r.Header.Get(h) + "\n"
+               }
+       }
+
+       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"))
+       ctxlog.FromContext(r.Context()).Debugf("s3stringToSign: canonicalRequest %s", canonicalRequest)
+       return fmt.Sprintf("%s\n%s\n%s\n%s", alg, r.Header.Get("X-Amz-Date"), scope, hashdigest(sha256.New(), canonicalRequest)), nil
+}
+
+func s3signature(secretKey, scope, signedHeaders, stringToSign string) (string, error) {
+       // scope is {datestamp}/{region}/{service}/aws4_request
+       drs := strings.Split(scope, "/")
+       if len(drs) != 4 {
+               return "", fmt.Errorf("invalid scope %q", scope)
+       }
+       key := s3signatureKey(secretKey, drs[0], drs[1], drs[2])
+       return hashdigest(hmac.New(sha256.New, key), stringToSign), nil
+}
+
+var v2tokenUnderscore = regexp.MustCompile(`^v2_[a-z0-9]{5}-gj3su-[a-z0-9]{15}_`)
+
+func unescapeKey(key string) string {
+       if v2tokenUnderscore.MatchString(key) {
+               // Entire Arvados token, with "/" replaced by "_" to
+               // avoid colliding with the Authorization header
+               // format.
+               return strings.Replace(key, "_", "/", -1)
+       } else if s, err := url.PathUnescape(key); err == nil {
+               return s
+       } else {
+               return key
+       }
+}
+
+// checks3signature verifies the given S3 V4 signature and returns the
+// Arvados token that corresponds to the given accessKey. An error is
+// returned if accessKey is not a valid token UUID or the signature
+// does not match.
+func (h *handler) checks3signature(r *http.Request) (string, error) {
+       var key, scope, signedHeaders, signature string
+       authstring := strings.TrimPrefix(r.Header.Get("Authorization"), s3SignAlgorithm+" ")
+       for _, cmpt := range strings.Split(authstring, ",") {
+               cmpt = strings.TrimSpace(cmpt)
+               split := strings.SplitN(cmpt, "=", 2)
+               switch {
+               case len(split) != 2:
+                       // (?) ignore
+               case split[0] == "Credential":
+                       keyandscope := strings.SplitN(split[1], "/", 2)
+                       if len(keyandscope) == 2 {
+                               key, scope = keyandscope[0], keyandscope[1]
+                       }
+               case split[0] == "SignedHeaders":
+                       signedHeaders = split[1]
+               case split[0] == "Signature":
+                       signature = split[1]
+               }
+       }
+
+       client := (&arvados.Client{
+               APIHost:  h.Config.cluster.Services.Controller.ExternalURL.Host,
+               Insecure: h.Config.cluster.TLS.Insecure,
+       }).WithRequestID(r.Header.Get("X-Request-Id"))
+       var aca arvados.APIClientAuthorization
+       var secret string
+       var err error
+       if len(key) == 27 && key[5:12] == "-gj3su-" {
+               // Access key is the UUID of an Arvados token, secret
+               // key is the secret part.
+               ctx := arvados.ContextWithAuthorization(r.Context(), "Bearer "+h.Config.cluster.SystemRootToken)
+               err = client.RequestAndDecodeContext(ctx, &aca, "GET", "arvados/v1/api_client_authorizations/"+key, nil, nil)
+               secret = aca.APIToken
+       } else {
+               // Access key and secret key are both an entire
+               // Arvados token or OIDC access token.
+               ctx := arvados.ContextWithAuthorization(r.Context(), "Bearer "+unescapeKey(key))
+               err = client.RequestAndDecodeContext(ctx, &aca, "GET", "arvados/v1/api_client_authorizations/current", nil, nil)
+               secret = key
+       }
+       if err != nil {
+               ctxlog.FromContext(r.Context()).WithError(err).WithField("UUID", key).Info("token lookup failed")
+               return "", errors.New("invalid access key")
+       }
+       stringToSign, err := s3stringToSign(s3SignAlgorithm, scope, signedHeaders, r)
+       if err != nil {
+               return "", err
+       }
+       expect, err := s3signature(secret, scope, signedHeaders, stringToSign)
+       if err != nil {
+               return "", err
+       } else if expect != signature {
+               return "", fmt.Errorf("signature does not match (scope %q signedHeaders %q stringToSign %q)", scope, signedHeaders, stringToSign)
+       }
+       return aca.TokenV2(), nil
+}
+
+func s3ErrorResponse(w http.ResponseWriter, s3code string, message string, resource string, code int) {
+       w.Header().Set("Content-Type", "application/xml")
+       w.Header().Set("X-Content-Type-Options", "nosniff")
+       w.WriteHeader(code)
+       var errstruct struct {
+               Code      string
+               Message   string
+               Resource  string
+               RequestId string
+       }
+       errstruct.Code = s3code
+       errstruct.Message = message
+       errstruct.Resource = resource
+       errstruct.RequestId = ""
+       enc := xml.NewEncoder(w)
+       fmt.Fprint(w, xml.Header)
+       enc.EncodeElement(errstruct, xml.StartElement{Name: xml.Name{Local: "Error"}})
+}
+
+var NoSuchKey = "NoSuchKey"
+var NoSuchBucket = "NoSuchBucket"
+var InvalidArgument = "InvalidArgument"
+var InternalError = "InternalError"
+var UnauthorizedAccess = "UnauthorizedAccess"
+var InvalidRequest = "InvalidRequest"
+var SignatureDoesNotMatch = "SignatureDoesNotMatch"
+
 // serveS3 handles r and returns true if r is a request from an S3
 // client, otherwise it returns false.
 func (h *handler) serveS3(w http.ResponseWriter, r *http.Request) bool {
@@ -19,89 +223,437 @@ func (h *handler) serveS3(w http.ResponseWriter, r *http.Request) bool {
        if auth := r.Header.Get("Authorization"); strings.HasPrefix(auth, "AWS ") {
                split := strings.SplitN(auth[4:], ":", 2)
                if len(split) < 2 {
-                       w.WriteHeader(http.StatusUnauthorized)
+                       s3ErrorResponse(w, InvalidRequest, "malformed Authorization header", r.URL.Path, http.StatusUnauthorized)
                        return true
                }
-               token = split[0]
-       } else if strings.HasPrefix(auth, "AWS4-HMAC-SHA256 ") {
-               w.WriteHeader(http.StatusBadRequest)
-               fmt.Println(w, "V4 signature is not supported")
-               return true
+               token = unescapeKey(split[0])
+       } else if strings.HasPrefix(auth, s3SignAlgorithm+" ") {
+               t, err := h.checks3signature(r)
+               if err != nil {
+                       s3ErrorResponse(w, SignatureDoesNotMatch, "signature verification failed: "+err.Error(), r.URL.Path, http.StatusForbidden)
+                       return true
+               }
+               token = t
        } else {
                return false
        }
 
        _, kc, client, release, err := h.getClients(r.Header.Get("X-Request-Id"), token)
        if err != nil {
-               http.Error(w, "Pool failed: "+h.clientPool.Err().Error(), http.StatusInternalServerError)
+               s3ErrorResponse(w, InternalError, "Pool failed: "+h.clientPool.Err().Error(), r.URL.Path, http.StatusInternalServerError)
                return true
        }
        defer release()
 
-       r.URL.Path = "/by_id" + r.URL.Path
-
        fs := client.SiteFileSystem(kc)
        fs.ForwardSlashNameSubstitution(h.Config.cluster.Collections.ForwardSlashNameSubstitution)
 
-       switch r.Method {
-       case "GET":
-               fi, err := fs.Stat(r.URL.Path)
-               if os.IsNotExist(err) {
-                       http.Error(w, err.Error(), http.StatusNotFound)
+       var objectNameGiven bool
+       fspath := "/by_id"
+       if id := parseCollectionIDFromDNSName(r.Host); id != "" {
+               fspath += "/" + id
+               objectNameGiven = strings.Count(strings.TrimSuffix(r.URL.Path, "/"), "/") > 0
+       } else {
+               objectNameGiven = strings.Count(strings.TrimSuffix(r.URL.Path, "/"), "/") > 1
+       }
+       fspath += r.URL.Path
+
+       switch {
+       case r.Method == http.MethodGet && !objectNameGiven:
+               // Path is "/{uuid}" or "/{uuid}/", has no object name
+               if _, ok := r.URL.Query()["versioning"]; ok {
+                       // GetBucketVersioning
+                       w.Header().Set("Content-Type", "application/xml")
+                       io.WriteString(w, xml.Header)
+                       fmt.Fprintln(w, `<VersioningConfiguration xmlns="http://s3.amazonaws.com/doc/2006-03-01/"/>`)
+               } else {
+                       // ListObjects
+                       h.s3list(w, r, fs)
+               }
+               return true
+       case r.Method == http.MethodGet || r.Method == http.MethodHead:
+               fi, err := fs.Stat(fspath)
+               if r.Method == "HEAD" && !objectNameGiven {
+                       // HeadBucket
+                       if err == nil && fi.IsDir() {
+                               w.WriteHeader(http.StatusOK)
+                       } else if os.IsNotExist(err) {
+                               s3ErrorResponse(w, NoSuchBucket, "The specified bucket does not exist.", r.URL.Path, http.StatusNotFound)
+                       } else {
+                               s3ErrorResponse(w, InternalError, err.Error(), r.URL.Path, http.StatusBadGateway)
+                       }
                        return true
-               } else if err != nil {
-                       http.Error(w, err.Error(), http.StatusInternalServerError)
+               }
+               if err == nil && fi.IsDir() && objectNameGiven && strings.HasSuffix(fspath, "/") && h.Config.cluster.Collections.S3FolderObjects {
+                       w.Header().Set("Content-Type", "application/x-directory")
+                       w.WriteHeader(http.StatusOK)
+                       return true
+               }
+               if os.IsNotExist(err) ||
+                       (err != nil && err.Error() == "not a directory") ||
+                       (fi != nil && fi.IsDir()) {
+                       s3ErrorResponse(w, NoSuchKey, "The specified key does not exist.", r.URL.Path, http.StatusNotFound)
                        return true
-               } else if fi.IsDir() {
-                       http.Error(w, "not found", http.StatusNotFound)
                }
-               http.FileServer(fs).ServeHTTP(w, r)
+               // shallow copy r, and change URL path
+               r := *r
+               r.URL.Path = fspath
+               http.FileServer(fs).ServeHTTP(w, &r)
                return true
-       case "PUT":
-               f, err := fs.OpenFile(r.URL.Path, os.O_WRONLY|os.O_TRUNC|os.O_CREATE, 0644)
-               if os.IsNotExist(err) {
-                       // create missing intermediate directories, then try again
-                       for i, c := range r.URL.Path {
-                               if i > 0 && c == '/' {
-                                       dir := r.URL.Path[:i]
-                                       err := fs.Mkdir(dir, 0755)
-                                       if err != nil && err != os.ErrExist {
-                                               err = fmt.Errorf("mkdir %q failed: %w", dir, err)
-                                               http.Error(w, err.Error(), http.StatusInternalServerError)
-                                               return true
-                                       }
+       case r.Method == http.MethodPut:
+               if !objectNameGiven {
+                       s3ErrorResponse(w, InvalidArgument, "Missing object name in PUT request.", r.URL.Path, http.StatusBadRequest)
+                       return true
+               }
+               var objectIsDir bool
+               if strings.HasSuffix(fspath, "/") {
+                       if !h.Config.cluster.Collections.S3FolderObjects {
+                               s3ErrorResponse(w, InvalidArgument, "invalid object name: trailing slash", r.URL.Path, http.StatusBadRequest)
+                               return true
+                       }
+                       n, err := r.Body.Read(make([]byte, 1))
+                       if err != nil && err != io.EOF {
+                               s3ErrorResponse(w, InternalError, fmt.Sprintf("error reading request body: %s", err), r.URL.Path, http.StatusInternalServerError)
+                               return true
+                       } else if n > 0 {
+                               s3ErrorResponse(w, InvalidArgument, "cannot create object with trailing '/' char unless content is empty", r.URL.Path, http.StatusBadRequest)
+                               return true
+                       } else if strings.SplitN(r.Header.Get("Content-Type"), ";", 2)[0] != "application/x-directory" {
+                               s3ErrorResponse(w, InvalidArgument, "cannot create object with trailing '/' char unless Content-Type is 'application/x-directory'", r.URL.Path, http.StatusBadRequest)
+                               return true
+                       }
+                       // Given PUT "foo/bar/", we'll use "foo/bar/."
+                       // in the "ensure parents exist" block below,
+                       // and then we'll be done.
+                       fspath += "."
+                       objectIsDir = true
+               }
+               fi, err := fs.Stat(fspath)
+               if err != nil && err.Error() == "not a directory" {
+                       // requested foo/bar, but foo is a file
+                       s3ErrorResponse(w, InvalidArgument, "object name conflicts with existing object", r.URL.Path, http.StatusBadRequest)
+                       return true
+               }
+               if strings.HasSuffix(r.URL.Path, "/") && err == nil && !fi.IsDir() {
+                       // requested foo/bar/, but foo/bar is a file
+                       s3ErrorResponse(w, InvalidArgument, "object name conflicts with existing object", r.URL.Path, http.StatusBadRequest)
+                       return true
+               }
+               // create missing parent/intermediate directories, if any
+               for i, c := range fspath {
+                       if i > 0 && c == '/' {
+                               dir := fspath[:i]
+                               if strings.HasSuffix(dir, "/") {
+                                       err = errors.New("invalid object name (consecutive '/' chars)")
+                                       s3ErrorResponse(w, InvalidArgument, err.Error(), r.URL.Path, http.StatusBadRequest)
+                                       return true
+                               }
+                               err = fs.Mkdir(dir, 0755)
+                               if err == arvados.ErrInvalidArgument {
+                                       // Cannot create a directory
+                                       // here.
+                                       err = fmt.Errorf("mkdir %q failed: %w", dir, err)
+                                       s3ErrorResponse(w, InvalidArgument, err.Error(), r.URL.Path, http.StatusBadRequest)
+                                       return true
+                               } else if err != nil && !os.IsExist(err) {
+                                       err = fmt.Errorf("mkdir %q failed: %w", dir, err)
+                                       s3ErrorResponse(w, InternalError, err.Error(), r.URL.Path, http.StatusInternalServerError)
+                                       return true
                                }
                        }
-                       f, err = fs.OpenFile(r.URL.Path, os.O_WRONLY|os.O_TRUNC|os.O_CREATE, 0644)
                }
+               if !objectIsDir {
+                       f, err := fs.OpenFile(fspath, os.O_WRONLY|os.O_TRUNC|os.O_CREATE, 0644)
+                       if os.IsNotExist(err) {
+                               f, err = fs.OpenFile(fspath, os.O_WRONLY|os.O_TRUNC|os.O_CREATE, 0644)
+                       }
+                       if err != nil {
+                               err = fmt.Errorf("open %q failed: %w", r.URL.Path, err)
+                               s3ErrorResponse(w, InvalidArgument, err.Error(), r.URL.Path, http.StatusBadRequest)
+                               return true
+                       }
+                       defer f.Close()
+                       _, err = io.Copy(f, r.Body)
+                       if err != nil {
+                               err = fmt.Errorf("write to %q failed: %w", r.URL.Path, err)
+                               s3ErrorResponse(w, InternalError, err.Error(), r.URL.Path, http.StatusBadGateway)
+                               return true
+                       }
+                       err = f.Close()
+                       if err != nil {
+                               err = fmt.Errorf("write to %q failed: close: %w", r.URL.Path, err)
+                               s3ErrorResponse(w, InternalError, err.Error(), r.URL.Path, http.StatusBadGateway)
+                               return true
+                       }
+               }
+               err = fs.Sync()
                if err != nil {
-                       err = fmt.Errorf("open %q failed: %w", r.URL.Path, err)
-                       http.Error(w, err.Error(), http.StatusBadRequest)
+                       err = fmt.Errorf("sync failed: %w", err)
+                       s3ErrorResponse(w, InternalError, err.Error(), r.URL.Path, http.StatusInternalServerError)
                        return true
                }
-               defer f.Close()
-               _, err = io.Copy(f, r.Body)
-               if err != nil {
-                       err = fmt.Errorf("write to %q failed: %w", r.URL.Path, err)
-                       http.Error(w, err.Error(), http.StatusBadGateway)
+               w.WriteHeader(http.StatusOK)
+               return true
+       case r.Method == http.MethodDelete:
+               if !objectNameGiven || r.URL.Path == "/" {
+                       s3ErrorResponse(w, InvalidArgument, "missing object name in DELETE request", r.URL.Path, http.StatusBadRequest)
+                       return true
+               }
+               if strings.HasSuffix(fspath, "/") {
+                       fspath = strings.TrimSuffix(fspath, "/")
+                       fi, err := fs.Stat(fspath)
+                       if os.IsNotExist(err) {
+                               w.WriteHeader(http.StatusNoContent)
+                               return true
+                       } else if err != nil {
+                               s3ErrorResponse(w, InternalError, err.Error(), r.URL.Path, http.StatusInternalServerError)
+                               return true
+                       } else if !fi.IsDir() {
+                               // if "foo" exists and is a file, then
+                               // "foo/" doesn't exist, so we say
+                               // delete was successful.
+                               w.WriteHeader(http.StatusNoContent)
+                               return true
+                       }
+               } else if fi, err := fs.Stat(fspath); err == nil && fi.IsDir() {
+                       // if "foo" is a dir, it is visible via S3
+                       // only as "foo/", not "foo" -- so we leave
+                       // the dir alone and return 204 to indicate
+                       // that "foo" does not exist.
+                       w.WriteHeader(http.StatusNoContent)
+                       return true
+               }
+               err = fs.Remove(fspath)
+               if os.IsNotExist(err) {
+                       w.WriteHeader(http.StatusNoContent)
                        return true
                }
-               err = f.Close()
                if err != nil {
-                       err = fmt.Errorf("write to %q failed: %w", r.URL.Path, err)
-                       http.Error(w, err.Error(), http.StatusBadGateway)
+                       err = fmt.Errorf("rm failed: %w", err)
+                       s3ErrorResponse(w, InvalidArgument, err.Error(), r.URL.Path, http.StatusBadRequest)
                        return true
                }
                err = fs.Sync()
                if err != nil {
                        err = fmt.Errorf("sync failed: %w", err)
-                       http.Error(w, err.Error(), http.StatusInternalServerError)
+                       s3ErrorResponse(w, InternalError, err.Error(), r.URL.Path, http.StatusInternalServerError)
                        return true
                }
-               w.WriteHeader(http.StatusOK)
+               w.WriteHeader(http.StatusNoContent)
                return true
        default:
-               http.Error(w, "method not allowed", http.StatusMethodNotAllowed)
+               s3ErrorResponse(w, InvalidRequest, "method not allowed", r.URL.Path, http.StatusMethodNotAllowed)
+
                return true
        }
 }
+
+// Call fn on the given path (directory) and its contents, in
+// lexicographic order.
+//
+// If isRoot==true and path is not a directory, return nil.
+//
+// If fn returns filepath.SkipDir when called on a directory, don't
+// descend into that directory.
+func walkFS(fs arvados.CustomFileSystem, path string, isRoot bool, fn func(path string, fi os.FileInfo) error) error {
+       if isRoot {
+               fi, err := fs.Stat(path)
+               if os.IsNotExist(err) || (err == nil && !fi.IsDir()) {
+                       return nil
+               } else if err != nil {
+                       return err
+               }
+               err = fn(path, fi)
+               if err == filepath.SkipDir {
+                       return nil
+               } else if err != nil {
+                       return err
+               }
+       }
+       f, err := fs.Open(path)
+       if os.IsNotExist(err) && isRoot {
+               return nil
+       } else if err != nil {
+               return fmt.Errorf("open %q: %w", path, err)
+       }
+       defer f.Close()
+       if path == "/" {
+               path = ""
+       }
+       fis, err := f.Readdir(-1)
+       if err != nil {
+               return err
+       }
+       sort.Slice(fis, func(i, j int) bool { return fis[i].Name() < fis[j].Name() })
+       for _, fi := range fis {
+               err = fn(path+"/"+fi.Name(), fi)
+               if err == filepath.SkipDir {
+                       continue
+               } else if err != nil {
+                       return err
+               }
+               if fi.IsDir() {
+                       err = walkFS(fs, path+"/"+fi.Name(), false, fn)
+                       if err != nil {
+                               return err
+                       }
+               }
+       }
+       return nil
+}
+
+var errDone = errors.New("done")
+
+func (h *handler) s3list(w http.ResponseWriter, r *http.Request, fs arvados.CustomFileSystem) {
+       var params struct {
+               bucket    string
+               delimiter string
+               marker    string
+               maxKeys   int
+               prefix    string
+       }
+       params.bucket = strings.SplitN(r.URL.Path[1:], "/", 2)[0]
+       params.delimiter = r.FormValue("delimiter")
+       params.marker = r.FormValue("marker")
+       if mk, _ := strconv.ParseInt(r.FormValue("max-keys"), 10, 64); mk > 0 && mk < s3MaxKeys {
+               params.maxKeys = int(mk)
+       } else {
+               params.maxKeys = s3MaxKeys
+       }
+       params.prefix = r.FormValue("prefix")
+
+       bucketdir := "by_id/" + params.bucket
+       // walkpath is the directory (relative to bucketdir) we need
+       // to walk: the innermost directory that is guaranteed to
+       // contain all paths that have the requested prefix. Examples:
+       // prefix "foo/bar"  => walkpath "foo"
+       // prefix "foo/bar/" => walkpath "foo/bar"
+       // prefix "foo"      => walkpath ""
+       // prefix ""         => walkpath ""
+       walkpath := params.prefix
+       if cut := strings.LastIndex(walkpath, "/"); cut >= 0 {
+               walkpath = walkpath[:cut]
+       } else {
+               walkpath = ""
+       }
+
+       type commonPrefix struct {
+               Prefix string
+       }
+       type listResp struct {
+               XMLName string `xml:"http://s3.amazonaws.com/doc/2006-03-01/ ListBucketResult"`
+               s3.ListResp
+               // s3.ListResp marshals an empty tag when
+               // CommonPrefixes is nil, which confuses some clients.
+               // Fix by using this nested struct instead.
+               CommonPrefixes []commonPrefix
+               // Similarly, we need omitempty here, because an empty
+               // tag confuses some clients (e.g.,
+               // github.com/aws/aws-sdk-net never terminates its
+               // paging loop).
+               NextMarker string `xml:"NextMarker,omitempty"`
+               // ListObjectsV2 has a KeyCount response field.
+               KeyCount int
+       }
+       resp := listResp{
+               ListResp: s3.ListResp{
+                       Name:      strings.SplitN(r.URL.Path[1:], "/", 2)[0],
+                       Prefix:    params.prefix,
+                       Delimiter: params.delimiter,
+                       Marker:    params.marker,
+                       MaxKeys:   params.maxKeys,
+               },
+       }
+       commonPrefixes := map[string]bool{}
+       err := walkFS(fs, strings.TrimSuffix(bucketdir+"/"+walkpath, "/"), true, func(path string, fi os.FileInfo) error {
+               if path == bucketdir {
+                       return nil
+               }
+               path = path[len(bucketdir)+1:]
+               filesize := fi.Size()
+               if fi.IsDir() {
+                       path += "/"
+                       filesize = 0
+               }
+               if len(path) <= len(params.prefix) {
+                       if path > params.prefix[:len(path)] {
+                               // with prefix "foobar", walking "fooz" means we're done
+                               return errDone
+                       }
+                       if path < params.prefix[:len(path)] {
+                               // with prefix "foobar", walking "foobag" is pointless
+                               return filepath.SkipDir
+                       }
+                       if fi.IsDir() && !strings.HasPrefix(params.prefix+"/", path) {
+                               // with prefix "foo/bar", walking "fo"
+                               // is pointless (but walking "foo" or
+                               // "foo/bar" is necessary)
+                               return filepath.SkipDir
+                       }
+                       if len(path) < len(params.prefix) {
+                               // can't skip anything, and this entry
+                               // isn't in the results, so just
+                               // continue descent
+                               return nil
+                       }
+               } else {
+                       if path[:len(params.prefix)] > params.prefix {
+                               // with prefix "foobar", nothing we
+                               // see after "foozzz" is relevant
+                               return errDone
+                       }
+               }
+               if path < params.marker || path < params.prefix {
+                       return nil
+               }
+               if fi.IsDir() && !h.Config.cluster.Collections.S3FolderObjects {
+                       // Note we don't add anything to
+                       // commonPrefixes here even if delimiter is
+                       // "/". We descend into the directory, and
+                       // return a commonPrefix only if we end up
+                       // finding a regular file inside it.
+                       return nil
+               }
+               if params.delimiter != "" {
+                       idx := strings.Index(path[len(params.prefix):], params.delimiter)
+                       if idx >= 0 {
+                               // with prefix "foobar" and delimiter
+                               // "z", when we hit "foobar/baz", we
+                               // add "/baz" to commonPrefixes and
+                               // stop descending.
+                               commonPrefixes[path[:len(params.prefix)+idx+1]] = true
+                               return filepath.SkipDir
+                       }
+               }
+               if len(resp.Contents)+len(commonPrefixes) >= params.maxKeys {
+                       resp.IsTruncated = true
+                       if params.delimiter != "" {
+                               resp.NextMarker = path
+                       }
+                       return errDone
+               }
+               resp.Contents = append(resp.Contents, s3.Key{
+                       Key:          path,
+                       LastModified: fi.ModTime().UTC().Format("2006-01-02T15:04:05.999") + "Z",
+                       Size:         filesize,
+               })
+               return nil
+       })
+       if err != nil && err != errDone {
+               http.Error(w, err.Error(), http.StatusInternalServerError)
+               return
+       }
+       if params.delimiter != "" {
+               resp.CommonPrefixes = make([]commonPrefix, 0, len(commonPrefixes))
+               for prefix := range commonPrefixes {
+                       resp.CommonPrefixes = append(resp.CommonPrefixes, commonPrefix{prefix})
+               }
+               sort.Slice(resp.CommonPrefixes, func(i, j int) bool { return resp.CommonPrefixes[i].Prefix < resp.CommonPrefixes[j].Prefix })
+       }
+       resp.KeyCount = len(resp.Contents)
+       w.Header().Set("Content-Type", "application/xml")
+       io.WriteString(w, xml.Header)
+       if err := xml.NewEncoder(w).Encode(resp); err != nil {
+               ctxlog.FromContext(r.Context()).WithError(err).Error("error writing xml response")
+       }
+}