derived stores evaluate prematurely / semi-diamond dependency problem

[WHenderson]

Describe the bug

The current implementation of svelte stores have a limitation whereby derived stores may evaluate and trigger their subscribers prematurely.

(@mnrx described it thus in his PR #9458, @mnrx I am taking the liberty of splitting this into its own issue. If you already have an issue tracking this then my apologies - I'll move to yours once I know about it)

Currently, a change to a store's value only causes stores which are immediately dependent on it to be invalidated. Svelte does not propagate invalidation downstream to subsequent derived stores, and this can cause issues when two criteria are met:

• the store dependency graph forks and merges again at least once; and
• the two or more paths between the fork and merge points are of unequal length.

Svelte's current implementation correctly handles dependency diamonds, but such cases do not meet the second criterion; unequal path length.

Example

Consider the following example:

const a = writable(1);
const b = derived(a, a => a*2);
const c = derived([a,b], ([a,b]) => a+b);
c.subscribe(c => console.log(c));
...
<input type=number bind:value={$a} />

This creates a dependency graph something like:

stateDiagram-v2
	direction RL
  
	input
	a
	b
	c
	log

	log --> c
	c --> b
	b --> a
	c --> a
	a --> input

Svelte's Current Implementation

With sveltes current implementation, the derived store c will prematurely evaluate every time store a changes.
In the example above, if we change the input to 2, the current implementation will go through the following sequence:

sequenceDiagram
	autonumber
	
	participant input
	participant a
	participant b
	participant c
	participant log

	note right of a: a == 1
	note right of b: b == a * 2 == 1 * 2 == 2
	note right of c: c == a + b == 1 + 2 == 3

	input ->> a: 2
	note right of a: a == 2
	a -->> c: invalidate
	activate c
	a -->> b: invalidate
	activate b
	a ->> c: 2
	deactivate c
	rect rgb(255, 128, 128)
	note right of c: c == a + b == 2 + 2 == 4
	c ->> log: 4
	end
	a ->> b: 2
	deactivate b
	note right of b: b == a * 2 == 2 * 2 == 4
	b -->> c: invalidate
	activate c
	b ->> c: 4
	deactivate c
	note right of c: c == a + b == 2 + 4 == 6
	c ->> log: 6

Following the diagram, it's clear at point (5) that store c is evaluating prematurely. At point (5) store c believes both its dependencies to be valid, but in fact only a has been resolved at this point.

The correct behaviour would be for invalidations to be "deep" and for resolution to only occur once all dependencies are fully resolved. Thus in the given example, c should only emit once for each change to a.

Why fix it?

Prematurely evaluating a derived store in many situations would result in just a brief glitch - an incorrect calculation immediately followed by the correct one. But in many contexts, the derived store subscriptions result in side effects. Depending on the nature of these side effects, the results of a premature evaluation with incorrect data may be quite pronounced - sending data to a service, permanently modifying data, crashing an application.

Reproduction

See: semi-diamond dependency problem REPL

As a head start for any patch request, here is a test case:

	it('only updates once dependents are resolved', () => {
		const a = writable(1);
		const b = derived(a, a => a*2);
		const c = derived([a,b], ([a,b]) => a+b);

		const values: number[] = [];

		const unsubscribe = c.subscribe(c => {
			values.push(c);
		});

		a.set(2);
		a.set(3);
		assert.deepEqual(values, [3, 6, 9]);
	});

note that the above test case fails with the current implementation

Logs

N/A

System Info

Svelte v4.2.10

Severity

annoyance

[mnrx]

I created #10376 a couple of weeks ago—I think this is the same issue. That said, the sequence diagram and test case you created would be good additions to it.

[WHenderson]

Closing this in favour of the earlier issue @mnrx raised (#10376)