webdav: fix "XML syntax error" on paths which don't exist with ownCloud 10.16

ownCloud 10.16 answers a PROPFIND for a path which doesn't exist with
a 207 Multi-Status response, rather than a 404, with a body which
starts a multistatus document then appends a Sabre NotFound error to
it. This isn't valid XML so rclone failed with

    read metadata failed: XML syntax error on line 2: expected attribute name in element

whenever it was pointed at a directory which didn't exist yet.

Detect the NotFound error in the response and treat it as a 404.

backend/webdav/webdav.go

@@ -345,6 +345,41 @@ func itemIsDir(item *api.Response) bool {
 	return false
 }
 
+// sabreNotFound is the exception SabreDAV based servers report for a
+// path which doesn't exist
+var sabreNotFound = []byte(`Sabre\DAV\Exception\NotFound`)
+
+// callPropfind does the PROPFIND in opts and decodes the response into result
+//
+// ownCloud 10.16 starts a 207 Multi-Status response before it finds
+// that the path doesn't exist, then appends a Sabre NotFound error to
+// it which makes it invalid XML. This is returned as the 404 *api.Error
+// which servers normally give for a path which doesn't exist.
+func (f *Fs) callPropfind(ctx context.Context, opts *rest.Opts, result *api.Multistatus) (resp *http.Response, err error) {
+	resp, err = f.srv.Call(ctx, opts)
+	if err != nil {
+		return resp, err
+	}
+	// Keep the start of the body to look for the error in
+	start := make([]byte, 4096)
+	n, _ := io.ReadFull(resp.Body, start)
+	start = start[:n]
+	body := resp.Body
+	resp.Body = struct {
+		io.Reader
+		io.Closer
+	}{io.MultiReader(bytes.NewReader(start), body), body}
+	err = rest.DecodeXML(resp, result)
+	if err != nil && bytes.Contains(start, sabreNotFound) {
+		return resp, &api.Error{
+			Exception:  string(sabreNotFound),
+			Status:     http.StatusText(http.StatusNotFound),
+			StatusCode: http.StatusNotFound,
+		}
+	}
+	return resp, err
+}
+
 // readMetaDataForPath reads the metadata from the path
 func (f *Fs) readMetaDataForPath(ctx context.Context, path string) (info *api.Prop, err error) {
 	// FIXME how do we read back additional properties?
@@ -366,7 +401,7 @@ func (f *Fs) readMetaDataForPath(ctx context.Context, path string) (info *api.Pr
 	var resp *http.Response
 	err = f.pacer.Call(func() (bool, error) {
 		var attempt api.Multistatus
-		resp, err = f.srv.CallXML(ctx, &opts, nil, &attempt)
+		resp, err = f.callPropfind(ctx, &opts, &attempt)
 		if err == nil {
 			result = attempt
 		}
@@ -820,7 +855,7 @@ func (f *Fs) listAll(ctx context.Context, dir string, directoriesOnly bool, file
 	var resp *http.Response
 	err = f.pacer.Call(func() (bool, error) {
 		var attempt api.Multistatus
-		resp, err = f.srv.CallXML(ctx, &opts, nil, &attempt)
+		resp, err = f.callPropfind(ctx, &opts, &attempt)
 		if err == nil {
 			result = attempt
 		}

backend/webdav/webdav_internal_test.go

@@ -354,3 +354,49 @@ func TestListAllRetryDoesNotConcatenate(t *testing.T) {
 	}
 	assert.ElementsMatch(t, want, remotes)
 }
+
+// TestNotFoundInMultistatus checks that a missing path is detected when the
+// server reports it by appending a Sabre NotFound error to a 207 response it
+// has already started, as ownCloud 10.16 does, rather than by returning 404.
+func TestNotFoundInMultistatus(t *testing.T) {
+	head := `<?xml version="1.0"?>` + "\n" + `<d:multistatus xmlns:d="DAV:" xmlns:s="http://sabredav.org/ns" xmlns:oc="http://owncloud.org/ns"`
+	root := head + `><d:response><d:href>/</d:href><d:propstat><d:prop><d:resourcetype><d:collection/></d:resourcetype></d:prop><d:status>HTTP/1.1 200 OK</d:status></d:propstat></d:response></d:multistatus>`
+	notFound := head + `<?xml version="1.0" encoding="utf-8"?>
+<d:error xmlns:d="DAV:" xmlns:s="http://sabredav.org/ns">
+  <s:exception>Sabre\DAV\Exception\NotFound</s:exception>
+  <s:message>File with name missing could not be located</s:message>
+</d:error>
+`
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		require.Equal(t, "PROPFIND", r.Method)
+		w.Header().Set("Content-Type", "application/xml; charset=utf-8")
+		w.WriteHeader(207)
+		body := notFound
+		if r.URL.Path == "/" {
+			body = root
+		}
+		_, err := fmt.Fprint(w, body)
+		require.NoError(t, err)
+	}))
+	defer ts.Close()
+
+	configfile.Install()
+	m := configmap.Simple{
+		"type": "webdav",
+		"url":  ts.URL,
+	}
+	ctx := context.Background()
+
+	// A root which doesn't exist yet
+	_, err := webdav.NewFs(ctx, remoteName, "missing", m)
+	require.NoError(t, err)
+
+	f, err := webdav.NewFs(ctx, remoteName, "", m)
+	require.NoError(t, err)
+
+	_, err = f.NewObject(ctx, "missing")
+	assert.Equal(t, fs.ErrorObjectNotFound, err)
+
+	_, err = f.List(ctx, "missing")
+	assert.Equal(t, fs.ErrorDirNotFound, err)
+}