allow to refuse an upload from a sync upload hook

Fixes #880

Signed-off-by: Nicola Murino <nicola.murino@gmail.com>

docs/custom-actions.md

@@ -75,6 +75,8 @@ If the `hook` defines an HTTP URL then this URL will be invoked as HTTP POST. Th
 The HTTP hook will use the global configuration for HTTP clients and will respect the retry configurations.
 
 The `pre-*` actions are always executed synchronously while the other ones are asynchronous. You can specify the actions to run synchronously via the `execute_sync` configuration key. Executing an action synchronously means that SFTPGo will not return a result code to the client (which is waiting for it) until your hook have completed its execution. If your hook takes a long time to complete this could cause a timeout on the client side, which wouldn't receive the server response in a timely manner and eventually drop the connection.
+If you add the `upload` action to the `execute_sync` configuration key, SFTPGo will try to delete the uploaded file and return an error to the client if the hook fails. A hook is considered failed if the external command completes with a non-zero exit status or the HTTP notification response code is other than `200` (or the HTTP endpoint cannot be reached or times out).
+After a hook failure, the uploaded size is removed from the quota if SFTPGo is able to remove the file.
 
 ## Provider events