debug: watch expressions are not updated properly

[vince-fugnitto]

Bug Description:

There is an issue with watch expressions not being updated in the debug-view which causes a difference in behavior with vscode and user expectations.

For example:

If we add a watch expression with no debug session present, we correctly get information that the expression is not available.

However, if we start a debug session and hit the breakpoint (making the variable value assigned), the watch expression is not updated:

If we edit the watch expression with the same name (value) then it gets updated:

Steps to Reproduce:

1. start the application using theia as a workspace
2. open the cli.spec.ts file and set a breakpoint at line 38 (after const value)
3. open the debug-view and add a watch expression for value
4. the watch expression should include the message "not available"
5. start the debug session with "run mocha tests"
6. the breakpoint should be hit, but the watch expression is not updated
7. right-click the value watch expression, select "edit expression" and press "enter" (keeping value as a name)
8. the watch expression will be properly updated

[kittaakos]

This is because the very first 'evaluate' happens too early, and the VS Code debug JS extension does not even reject that request: https://github.com/microsoft/vscode-js-debug/blob/d7730405d64e81eb364f76ea38efb7c93a5f877d/src/dap/connection.ts#L172

Since there is a queue of promises to update the watch expression in Theia, and the first neither rejects nor resolves, then the next updates will also wait forever:

theia/packages/debug/src/browser/view/debug-view-model.ts

Lines 218 to 228 in e7619c5

	protected refreshWatchExpressions = debounce(() => {
	this.refreshWatchExpressionsQueue = this.refreshWatchExpressionsQueue.then(async () => {
	try {
	for (const watchExpression of this.watchExpressions) {
	await watchExpression.evaluate();
	}
	} catch (e) {
	console.error('Failed to refresh watch expressions: ', e);
	}
	});
	}, 50);

If the 'evaluate' promises are not queued, the console error can be seen in the backend log, but there is no rejection:

2025-02-07T14:13:38.292Z root INFO REFRESHING WATCH EXPRESSIONS 5
2025-02-07T14:13:38.293Z root INFO EVALUATING WATCH EXPRESSION 5 nestedObject
2025-02-07T14:13:38.293Z root INFO REFRESHING WATCH EXPRESSIONS 5
2025-02-07T14:13:38.293Z root INFO EVALUATING WATCH EXPRESSION 5 nestedObject
2025-02-07T14:13:38.294Z root ERROR [hosted-plugin: 91150] Unknown request: evaluate
Unknown request: evaluate
2025-02-07T14:13:38.480Z root INFO [hosted-plugin: 91150] Creating local connection: f1d3f043-c7f9-4446-8535-22b95602abc1
2025-02-07T14:13:38.483Z root INFO [hosted-plugin: 91150] starting debug adapter session 'f1d3f043-c7f9-4446-8535-22b95602abc1'
2025-02-07T14:13:38.486Z root INFO [hosted-plugin: 91150] Creating plugin connection: f1d3f043-c7f9-4446-8535-22b95602abc1
2025-02-07T14:13:38.607Z root INFO REFRESHING WATCH EXPRESSIONS 6
2025-02-07T14:13:38.607Z root INFO EVALUATING WATCH EXPRESSION 6 nestedObject
2025-02-07T14:13:38.607Z root INFO REFRESHING WATCH EXPRESSIONS 6
2025-02-07T14:13:38.607Z root INFO EVALUATING WATCH EXPRESSION 6 nestedObject
2025-02-07T14:13:38.611Z root INFO DID EVALUATE WATCH EXPRESSION 6 nestedObject
2025-02-07T14:13:38.611Z root INFO DID REFRESH WATCH EXPRESSIONS 6
2025-02-07T14:13:38.611Z root INFO DID EVALUATE WATCH EXPRESSION 6 nestedObject
2025-02-07T14:13:38.611Z root INFO DID REFRESH WATCH EXPRESSIONS 6

watch_part1.mov

watch_part2.mov

Theia can fix it quickly. Do not queue or time out. The more challenging solution is to figure out why JS debug cannot accept the evaluate command, although the adapter has already sent the initialized event to the client.