perf: Support request cancellation

When multiple requests of the same type are sent in quick succession, the
LSP client will automatically issue `$/cancelRequest` notification to cancel
the earlier requests.

For example, when the mouse is moving around, multiple `hover` requests could
be fired, but only the last one is meaningful to users. In this case, we
should handle the cancelled requests and not do any work.

This will significantly speed up our server's ability to process incoming
requests / notifications.

Author: kyliau

integration/lsp/ivy_spec.ts

@@ -92,6 +92,24 @@ describe('Angular Ivy language server', () => {
         .toBe(`Property 'doesnotexist' does not exist on type 'FooComponent'.`);
   });
 
+  it('should support request cancellation', async () => {
+    openTextDocument(client, APP_COMPONENT);
+    const languageServiceEnabled = await waitForNgcc(client);
+    expect(languageServiceEnabled).toBeTrue();
+    // Send a request and immediately cancel it
+    const promise = client.sendRequest(lsp.HoverRequest.type, {
+      textDocument: {
+        uri: `file://${APP_COMPONENT}`,
+      },
+      position: {line: 4, character: 25},
+    });
+    // Request above is tagged with ID = 1
+    client.sendNotification('$/cancelRequest', {id: 1});
+    await expectAsync(promise).toBeRejectedWith(jasmine.objectContaining({
+      code: lsp.LSPErrorCodes.RequestCancelled,
+    }));
+  });
+
   it('does not break after opening `.d.ts` file from external template', async () => {
     client.sendNotification(lsp.DidOpenTextDocumentNotification.type, {
       textDocument: {

server/src/session.ts

@@ -67,7 +67,25 @@ export class Session {
     this.ivy = options.ivy;
     this.logToConsole = options.logToConsole;
     // Create a connection for the server. The connection uses Node's IPC as a transport.
-    this.connection = lsp.createConnection();
+    this.connection = lsp.createConnection({
+      // cancelUndispatched is a "middleware" to handle all cancellation requests.
+      // LSP spec requires every request to send a response back, even if it is
+      // cancelled. See
+      // https://microsoft.github.io/language-server-protocol/specifications/specification-current/#cancelRequest
+      cancelUndispatched(message: lsp.Message): lsp.ResponseMessage |
+      undefined {
+        return {
+          jsonrpc: message.jsonrpc,
+          // This ID is just a placeholder to satisfy the ResponseMessage type.
+          // `vscode-jsonrpc` will replace the ID with the ID of the message to
+          // be cancelled. See
+          // https://github.com/microsoft/vscode-languageserver-node/blob/193f06bf602ee1120afda8f0bac33c5161cab18e/jsonrpc/src/common/connection.ts#L619
+          id: -1,
+          error: new lsp.ResponseError(lsp.LSPErrorCodes.RequestCancelled, 'Request cancelled'),
+        };
+      }
+    });
+
     this.addProtocolHandlers(this.connection);
     this.projectService = this.createProjectService(options);
   }