CanDeactivate guard is triggered again by its own UrlTree return

🐞 bug report

Affected Package

The issue is caused by package @angular/router

Is this a regression?

Unclear, we didn't use this feature in previous angular versions.

Description

When returning a UrlTree from a CanDeactivate guard, the same guard is executed again

🔬 Minimal Reproduction

https://stackblitz.com/edit/angular-8-candeactivate-guard-error-infinite-loop

Steps for reproduction:

• Ignore anything in the UI
• Click on the Go to login-2 anchor
• Infinite navigation loop is triggered

🔥 Exception or Error

∞ loop

🌍 Your Environment

Angular Version:

8.0.0

Anything else relevant?

In my opinion returned UrlTree should not trigger again the guard returning it, otherwise that will force to keep logic (or even state) into the guard to control when to return UrlTree, which might lead into coupling.

Apologies if this is the desired behavior, if that is the case maybe the documentation should state that.

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[jvquarck]

I don’t think that the documentation clearly states that returning a urlTree might retrigger this guard and possibly create an infinite loop. This usage case of the canDeactivate guard is like an interceptor. You are modifying this request/navigation based on certain conditions. If we translate this current behavior to interceptors it would mean that you’d need to keep state in every interceptor in the stack, since every interception would trigger again the entire interceptor stack and you’d need to check if you have modified the request already or not.