Current component should (probably?) not be available inside onMount

[Conduitry]

Describe the bug
Things that internally use get_current_component() should probably fail when they are called from an onMount callback.

Logs
None.

To Reproduce

<script>
	import { getContext, onMount } from 'svelte';
	import { get_current_component } from 'svelte/internal';
	onMount(() => {
		console.log(get_current_component());
	});
</script>

Expected behavior
I'd expect this to crash, but it doesn't. There is an exception if you use setTimeout instead of onMount.

Stacktraces
n/a

Information about your Svelte project:

• Svelte 3.17.0

Severity
I'm not sure. I'm not actually positive this is a bug, but it feels like it. I don't think we want people to be able to call other lifecycle hooks or access the context within onMount.

Additional context
Came up in chat by way of a question about calling setContext in a component and then calling getContext during that component's onMount callback. It feels like this shouldn't actually be allowed.

[dopry]

I asked the original question on discord. The case I had was on https://github.com/dopry/svelte-auth0/blob/b11d46db898c2fea0015ae7ce0e56d7a6e9d1ab2/src/components/Auth0Context.svelte#L41

if I replace
const auth0 = await auth0Promise;
with
const auth0 = await getContext(AUTH0_CONTEXT_CLIENT_PROMISE);
auth0 is undefined.

You can test by running npm run showcase:dev

[Conduitry]

Something else that just came up on chat is someone trying to call setContext() during a reactive code block. This won't provide the reactivity they were looking for - but also should this perhaps be a runtime error? Should the current_component also be unavailable during reactive declarations and blocks?

[Conduitry]

Confirmed with Rich that this is not an intended feature, and that if it is easy to prevent this then we should.

[dopry]

Starting with documentation may be a good idea, if the compile time errors will take a moment to implement. My poor understanding is that OnMount is basically running at construction time and that the component isn't in a stable state for modification. And that rather than interacting directly with the component I should be interacting with in-scope variables that are used by the component instead.

[Conduitry]

The above component now throws an error at runtime in 3.25.0.

[rmunn]

Confirmed with Rich that this is not an intended feature, and that if it is easy to prevent this then we should.

@Conduitry - Do you remember this discussion from a year and a half ago? I ask because I'm trying to fix #6919 with #6920, and making the current component explicitly unavailable during onMount is what's causing #6919 (it makes bind:this not work during onMount).

Thing is, this is the example code from the bind:this docs:

<script>
	import { onMount } from 'svelte';

	let canvasElement;

	onMount(() => {
		const ctx = canvasElement.getContext('2d');
		drawStuff(ctx);
	});
</script>

<canvas bind:this={canvasElement}></canvas>

Those docs certainly imply that bind:this should work during onMount.

If you have any memory of the conversation with Rich that you mentioned on Feb 4, 2020 — or if you have any way to retrieve chat logs of that conversation from over 18 months ago — I'd love to understand the expected state of the component during onMount. Because right now, the Svelte docs seem to be promising something about bind:this that the library doesn't actually deliver, and I'd like to know whether the docs should be updated or whether I should pursue #6920 further.