sftpd: add support for upload resume

we support resume only if the client sets the correct offset while resuming
the upload.
Based on the specs the offset is optional for resume, but all the tested
clients sets a right offset.
If an invalid offset is given we interrupt the transfer with the error
"Invalid write offset ..."

See https://github.com/pkg/sftp/issues/295

This commit add a new upload mode: "atomic with resume support", this acts
as atomic but if there is an upload error the temporary file is renamed
to the requested path and not deleted, this way a client can reconnect
and resume the upload

sftpd/sftpd.go

@@ -37,6 +37,12 @@ const (
 	protocolSCP       = "SCP"
 )
 
+const (
+	uploadModeStandard = iota
+	uploadModeAtomic
+	uploadModeAtomicWithResume
+)
+
 var (
 	mutex                sync.RWMutex
 	openConnections      map[string]Connection
@@ -356,6 +362,10 @@ func updateConnectionActivity(id string) {
 	}
 }
 
+func isAtomicUploadEnabled() bool {
+	return uploadMode == uploadModeAtomic || uploadMode == uploadModeAtomicWithResume
+}
+
 func executeAction(operation string, username string, path string, target string) error {
 	if !utils.IsStringInSlice(operation, actions.ExecuteOn) {
 		return nil