Error handling: Duplicated components after an error is thrown in a view

[raykyri]

This is somewhat a duplicate of #2273, but I also haven't seen it mentioned in the thread or anywhere else (or maybe I missed it):

When an error is thrown in a view in Mithril, the diffing engine will often get confused and start rendering multiple copies of the same component to the page.

This seems unexpected, and I haven't seen it mentioned anywhere else. Am I doing something wrong?

[osban]

@raykyri not sure why you'd get an error thrown in a view, but maybe you could post some simplified code (ideally in a flems, possibly reproducing the error)?

[lucastorri]

Hi All!

I believe I have an example. It is quite non-sensical, but we had something similar in real code:

https://flems.io/#0=N4IgZglgNgpgziAXAbVAOwIYFsZJAOgAsAXLKEAGhAGMB7NYmBvAHgBMIA3AAgjYF4AOiAwAHUcID0APkog4MWNWIR6CRCAAMiAIwBWEAF8K6bLg34AVgip0GTYnm7OXzu3GLdOGKAFd43PzcyJr4AEwU3KEAnJGhepEAchiJALoA3IJorq7eAE5ePv6BhX7wIRlZOS753HS+DCWamdnV3FVt7p4AguIlwB1tXhAwAO6I3AAUAJSB0twDrUPOeTDEvnnZWJODy87CHJzCFLt726d77fKKMMrHF3uLl2301IQYaADmMBOTMLP8eaTbxlEowfDEDB5b7EfAg-zTE5LZ7cYwPZbw+D4LBiSbAgHzc4gWiiFT0Y5eabTdHVRE0nLnZHPYS+KD3JmXTFwbG4-FzBb0oYQMBTeoMADU4u48wAzNwAGTy3hwZKJfHUjko7irdabbiMrVDYRQCDsw1tImiPK0T6rOBwCnAfUYAAeEx0mk0kUxEx4ACpuB7NKjvRrza5pi1w9xFAptWsNtkdFHDYYwyj03tI-S0ZrUS0sl1tbRaJ4gmxaNRfDgGPgYQBRWA14gAIQAngBJNg7ETiYTZtBYbG0BrESbW0uRXqiAdyBRKMlodQgGWIAAcRhMIEwODw+Go9rkdkYzA0RlSVBNaAA1upUNuzHgsBBiIQ8tA5BtyBoSMRRHBEEkSQGlEa9Pn3WgsEkZ9X3fKAAAEwnwUIABZoJfN9oGxCA0CsGwQGINtRHMeRqHfUlz0MIA

If you change the option in the select a few times, you'll see that new items in the list are being added and the old ones are not removed. The stack trace is not helpful, because it happens inside mithril and not in the user's code.

Thank you!

[ArthurClemens]

It seems to me that the issue is that key is missing in the li items.

[kfule]

If an exception is thrown, the update of the root.vnodes is skipped.
If a new dom was created before that exception, the vnode of that new dom will not exist in the root.vnodes that were not updated. Therefore, the new dom becomes an unmanaged zombie. 🧟
If the same thing happens on every redraw, the number of zombies may grow. 🧟 🧟 🧟

I have created smaller flems. The more buttons you click, the more zombies you will have.

Of course, this can also occur if an error is thrown in the component view(). (flems, cf. #1937). In this example, however, it appears that the number of zombies is only increased to two. This seems to be related to reentrantLock.

Like flems above, if the error message is on the console, you may not need to be too afraid of the zombies.