fix: correct waitForCompletion behavior (#924)

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## Description

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fixes #923

## Related Issue

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#923

## Motivation and Context

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it makes the new feature actually work

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modified test case added by #894

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src/job.ts

@@ -207,7 +207,8 @@ export class CronJob<OC extends CronOnCompleteCommand | null = null, C = null> {
 	}
 
 	async fireOnTick() {
-		if (!this.waitForCompletion && this._isCallbackRunning) return;
+		// skip job if previous callback is still running
+		if (this.waitForCompletion && this._isCallbackRunning) return;
 
 		this._isCallbackRunning = true;
 

tests/cron.test.ts

@@ -1231,10 +1231,12 @@ describe('cron', () => {
 		it('should track multiple running jobs correctly', async () => {
 			const clock = sinon.useFakeTimers();
 			const executionOrder: number[] = [];
+			let started = 0;
 
 			const job = new CronJob(
 				'* * * * * *',
 				async () => {
+					started++;
 					await new Promise<void>(resolve => {
 						setTimeout(() => {
 							callback();
@@ -1256,7 +1258,8 @@ describe('cron', () => {
 
 			await clock.tickAsync(3500);
 
-			expect(executionOrder).toEqual([1, 2]);
+			expect(started).toBe(2);
+			expect(executionOrder).toEqual([1]);
 			job.stop();
 		});