Allow customization of max WS connections. (#8257)

* Allow customization of max WS connections.

* remove superflous line.

* Add test for CLI argument parsing.

parity/cli/mod.rs

@@ -499,6 +499,10 @@ usage! {
 			"--ws-hosts=[HOSTS]",
 			"List of allowed Host header values. This option will validate the Host header sent by the browser, it is additional security against some attack vectors. Special options: \"all\", \"none\".",
 
+			ARG arg_ws_max_connections: (usize) = 100usize, or |c: &Config| c.websockets.as_ref()?.max_connections,
+			"--ws-max-connections=[CONN]",
+			"Maximal number of allowed concurrent WS connections.",
+
 		["API and console options – IPC"]
 			FLAG flag_no_ipc: (bool) = false, or |c: &Config| c.ipc.as_ref()?.disable.clone(),
 			"--no-ipc",
@@ -1087,6 +1091,7 @@ struct Ws {
 	apis: Option<Vec<String>>,
 	origins: Option<Vec<String>>,
 	hosts: Option<Vec<String>>,
+	max_connections: Option<usize>,
 }
 
 #[derive(Default, Debug, PartialEq, Deserialize)]
@@ -1520,6 +1525,7 @@ mod tests {
 			arg_ws_apis: "web3,eth,net,parity,traces,rpc,secretstore".into(),
 			arg_ws_origins: "none".into(),
 			arg_ws_hosts: "none".into(),
+			arg_ws_max_connections: 100,
 
 			// IPC
 			flag_no_ipc: false,
@@ -1759,6 +1765,7 @@ mod tests {
 				apis: None,
 				origins: Some(vec!["none".into()]),
 				hosts: None,
+				max_connections: None,
 			}),
 			rpc: Some(Rpc {
 				disable: Some(true),