LSP: No-op client/registerCapability requests

Some language servers send this request even though we don't enable
dynamic capability registration. In some language servers that use the
vscode-languageserver-node dependency, replying with an error can cause
the server to exit because an un-caught rejected promise is fatal.

Replying with a JSONRPC error that the method is not handled is the
correct thing to do but given that it causes some language servers in
the wild to exit and that ignoring it doesn't cause any harm, we can
workaround the exits by just sending an OK and doing nothing. This is
similar to a workaround done by neovim.

helix-lsp/src/lib.rs

@@ -530,6 +530,7 @@ pub enum MethodCall {
     ApplyWorkspaceEdit(lsp::ApplyWorkspaceEditParams),
     WorkspaceFolders,
     WorkspaceConfiguration(lsp::ConfigurationParams),
+    RegisterCapability(lsp::RegistrationParams),
 }
 
 impl MethodCall {
@@ -549,6 +550,10 @@ impl MethodCall {
                 let params: lsp::ConfigurationParams = params.parse()?;
                 Self::WorkspaceConfiguration(params)
             }
+            lsp::request::RegisterCapability::METHOD => {
+                let params: lsp::RegistrationParams = params.parse()?;
+                Self::RegisterCapability(params)
+            }
             _ => {
                 return Err(Error::Unhandled);
             }

helix-term/src/application.rs

@@ -1014,6 +1014,17 @@ impl Application {
                             .collect();
                         Ok(json!(result))
                     }
+                    Ok(MethodCall::RegisterCapability(_params)) => {
+                        log::warn!("Ignoring a client/registerCapability request because dynamic capability registration is not enabled");
+                        // Language Servers based on the `vscode-languageserver-node` library often send
+                        // client/registerCapability even though we do not enable dynamic registration
+                        // for any capabilities. We should send a MethodNotFound JSONRPC error in this
+                        // case but that rejects the registration promise in the server which causes an
+                        // exit. So we work around this by ignoring the request and sending back an OK
+                        // response.
+
+                        Ok(serde_json::Value::Null)
+                    }
                 };
 
                 let language_server = match self.editor.language_servers.get_by_id(server_id) {