webdav: fix directory creation with 4shared - fixes #4428

When we run MKCOL on 4shared on a directory that already exists, this
returns a 409/Conflict error. However this error code usually means
that the intermediate collections need creating.

The actual error code to return when trying to create a directory that
already exists isn't specified in the RFC, only that an error MUST be
returned and there are already 3 statuses checked in the code.

However using 409 makes rclone's usual strategy for making directories
fail and return the 409 error.

This patch tries the MKCOL and if it returns an unrecognised error
code, then calls PROPFIND on the directory to discover whether the
directory really exists or not.

This should also cover other WebDAV servers returning other error
messages we haven't accounted for in the code yet.

backend/webdav/webdav.go

@@ -686,8 +686,8 @@ func (f *Fs) PutStream(ctx context.Context, in io.Reader, src fs.ObjectInfo, opt
 
 // mkParentDir makes the parent of the native path dirPath if
 // necessary and any directories above that
-func (f *Fs) mkParentDir(ctx context.Context, dirPath string) error {
+func (f *Fs) mkParentDir(ctx context.Context, dirPath string) (err error) {
-	// defer log.Trace(dirPath, "")("")
+	// defer log.Trace(dirPath, "")("err=%v", &err)
 	// chop off trailing / if it exists
 	if strings.HasSuffix(dirPath, "/") {
 		dirPath = dirPath[:len(dirPath)-1]
@@ -699,6 +699,27 @@ func (f *Fs) mkParentDir(ctx context.Context, dirPath string) error {
 	return f.mkdir(ctx, parent)
 }
 
+// _dirExists - list dirPath to see if it exists
+//
+// dirPath should be a native path ending in a /
+func (f *Fs) _dirExists(ctx context.Context, dirPath string) (exists bool) {
+	opts := rest.Opts{
+		Method: "PROPFIND",
+		Path:   dirPath,
+		ExtraHeaders: map[string]string{
+			"Depth": "0",
+		},
+	}
+	var result api.Multistatus
+	var resp *http.Response
+	var err error
+	err = f.pacer.Call(func() (bool, error) {
+		resp, err = f.srv.CallXML(ctx, &opts, nil, &result)
+		return f.shouldRetry(resp, err)
+	})
+	return err == nil
+}
+
 // low level mkdir, only makes the directory, doesn't attempt to create parents
 func (f *Fs) _mkdir(ctx context.Context, dirPath string) error {
 	// We assume the root is already created
@@ -719,19 +740,29 @@ func (f *Fs) _mkdir(ctx context.Context, dirPath string) error {
 		return f.shouldRetry(resp, err)
 	})
 	if apiErr, ok := err.(*api.Error); ok {
-		// already exists
+		// Check if it already exists. The response code for this isn't
+		// defined in the RFC so the implementations vary wildly.
+		//
 		// owncloud returns 423/StatusLocked if the create is already in progress
 		if apiErr.StatusCode == http.StatusMethodNotAllowed || apiErr.StatusCode == http.StatusNotAcceptable || apiErr.StatusCode == http.StatusLocked {
 			return nil
 		}
+		// 4shared returns a 409/StatusConflict here which clashes
+		// horribly with the intermediate paths don't exist meaning. So
+		// check to see if actually exists. This will correct other
+		// error codes too.
+		if f._dirExists(ctx, dirPath) {
+			return nil
+		}
+
 	}
 	return err
 }
 
 // mkdir makes the directory and parents using native paths
-func (f *Fs) mkdir(ctx context.Context, dirPath string) error {
+func (f *Fs) mkdir(ctx context.Context, dirPath string) (err error) {
-	// defer log.Trace(dirPath, "")("")
+	// defer log.Trace(dirPath, "")("err=%v", &err)
-	err := f._mkdir(ctx, dirPath)
+	err = f._mkdir(ctx, dirPath)
 	if apiErr, ok := err.(*api.Error); ok {
 		// parent does not exist so create it first then try again
 		if apiErr.StatusCode == http.StatusConflict {