Release remaining dataset locks when file editor stops

[John-A-Davies]

The purpose of this item is to release locks the WEBUI user has taken on datasets in the cases when the user has not specifically released them.

Parent epic: #498

Dataset locks will persist unless explicitly released. The WEBUI user can do this by closing the file editor tab for that dataset. The problem to solve is that if

1. the file editor window is closed
2. the browser is closed
3. the user session times out
4. the client becomes inoperative

then the remaining locks will not be released, so the locked dataset can't be accessed.

[John-A-Davies]

Workaround: If you re-open the file editor tab for the locked dataset, then close that tab, the lock will be released.

[John-A-Davies]

Locks are taken and released by the server on command from the client. If the client stops for any reason, the server must release the locks on its behalf. So the server must monitor the client to see if it is still active. This will be implemented by a keep-alive heartbeat sent from the client to the server. If the server detects a missed heartbeat, it will release all locks requested by the client.

A server can have many clients. Each client session is started by a user. One user can start many sessions. The server must release only those locks taken by that client session.

Implementation.

Add 2 fields to the lock table in the server

1. session ID
2. flag, to indicate whether a heartbeat has been received in the past time window

Logic.

1. Whenever the server detects the heartbeat, it sets the flag

2. the server periodically checks the flag at the end of each time window
   3. If the flag is set, the session is alive; unset the flag
   4. If the flag is not set, the session is dead; release all locks for that session.

[John-A-Davies]

#498

[John-A-Davies]

I looked in zss for is some identifier connected with the client process that's running the file editor window, for example on the HTTP request. However, Irek informed me that zss doesn't store any client info. It only verifies the cookie by decoding it using a symmetric key.