Ladybird: WebContent remains as a zombie process in certain scenarios

[kuchikuu]

Issue: WebContent may remain present after closing Ladybird.

Reproducing difficulty: Very easy with certain websites

HEAD at testing time: commit 41cc8e75f2087d61bb56930136e268e32ddb1bf0 (HEAD -> master, origin/master, origin/HEAD) (As as a reference when was this tested)

When it does not happen:

• Doing CTRL+C in the terminal (seems to) always kill spawned processes (WebContent does not become a zombie process)

When it does happen:

• Pressing the X button or doing File -> Quit when closing the browser

Suggested conditions required for the bug to happen:

1. The browser must be busy with something
2. The webpage must be very complex or broken

In order for 1) to happen: 1) Open a webpage 2) Wait for it to load + give it a few seconds to stay idle 3) Start clicking everywhere, spamming F5 etc 4) Quickly close the browser using File -> Quit or the X button 5) Ladybird closes, but WebContent becomes a zombie process

Demonstration: (Disclaimer: Mojeek is a relatively lightweight page. It is hard to make Ladybird busy with it, but it does work)

https://github.com/SerenityOS/serenity/assets/48770967/6e03d183-05b8-4e67-bb60-e7a3c891b7e9

In order for 2) to happen: 1) Load something broken, like for example https://duolingo.com or complex like https://adilene.net/ 2) Wait like 10-15 seconds (You don't even need to spam anything, just wait) 3) So far 99,999% of the time closing the browser at this stage will result in WebContent becoming a zombie process.

Exaggerated demonstration:

https://github.com/SerenityOS/serenity/assets/48770967/14ef8eb0-f009-4e6e-8786-23d5dc9b8920

And here is a small comparison: One browser is closed with File -> Quit and leaves a zombie process, the other is closed using CTRL+C in the terminal and that does not leave any zombie process.

https://github.com/SerenityOS/serenity/assets/48770967/6025f6c8-420e-4939-a902-0139d2a31eb8

[ADKaster]

I ran into this while implementing a process viewer/task manager for Ladybird. It seems like we have some really invalid assumptions about what will happen to the other side of unix sockets when we close the UI process.

[Phantop]

Can affirm this happening to me quite often—the vestigial WebContent process always will fully utilize a CPU core which is somewhat annoying on a laptop alas.