src/vs/workbench/contrib/tasks/common/problemCollectors.ts/WatchingProblemCollector/tryBegin
class WatchingProblemCollector:	private async tryBegin(line: string): Promise<boolean> {
		let result = false;
		for (const background of this.backgroundPatterns) {
			let matches = background.begin.regexp.exec(line);
			if (matches) {
				if (this._activeBackgroundMatchers.has(background.key)) {
					continue;
				}
				this._activeBackgroundMatchers.add(background.key);
				result = true;
				this.lines = [];
				this.lines.push(line);
				this._onDidStateChange.fire(ProblemCollectorEvent.create(ProblemCollectorEventKind.BackgroundProcessingBegins));
				this.cleanMarkerCaches();
				this.resetCurrentResource();
				let owner = background.matcher.owner;
				let file = matches[background.begin.file!];
				if (file) {
					let resource = getResource(file, background.matcher);
					this.recordResourceToClean(owner, await resource);
				} else {
					this.recordResourcesToClean(owner);
				}
			}
		}
		return result;
	}