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fix: unset context on stale promises (slightly different approach) #16936

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Merged
merged 2 commits into from
Oct 13, 2025
+6 −20
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fix: unset context on stale promises (slightly different approach) #16936

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e55c882
slightly different approach to #16935
Rich-Harris Oct 13, 2025
769dd7d
move unset_context call
Rich-Harris Oct 13, 2025
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26 changes: 6 additions & 20 deletions packages/svelte/src/internal/client/reactivity/deriveds.js
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Original file line number Diff line number Diff line change
Expand Up @@ -113,32 +113,24 @@ export function async_derived(fn, location) {
// only suspend in async deriveds created on initialisation
var should_suspend = !active_reaction;

/** @type {Map<Batch, ReturnType<typeof deferred<V>> & { rejected?: boolean }>} */
/** @type {Map<Batch, ReturnType<typeof deferred<V>>>} */
var deferreds = new Map();

async_effect(() => {
if (DEV) current_async_effect = active_effect;

/** @type {ReturnType<typeof deferred<V>> & { rejected?: boolean }} */
/** @type {ReturnType<typeof deferred<V>>} */
var d = deferred();
promise = d.promise;

try {
// If this code is changed at some point, make sure to still access the then property
// of fn() to read any signals it might access, so that we track them as dependencies.
Promise.resolve(fn()).then((v) => {
if (d.rejected) {
// If we rejected this stale promise, d.resolve
// is a noop (d.promise.then(handler) below will never run).
// In this case we need to unset the restored context here
// to avoid leaking it (and e.g. cause false-positive mutation errors).
unset_context();
} else {
d.resolve(v);
}
}, d.reject);
// We call `unset_context` to undo any `save` calls that happen inside `fn()`
Promise.resolve(fn()).then(d.resolve, d.reject).then(unset_context);
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@dummdidumm dummdidumm Oct 13, 2025 •
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It would be good to have a comment explaining why we want to have it up here in addition to the handler below, but otherwise lgtm 👍

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actually you know what? we don't need the unset_context below, just here (and in the catch clause)

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@dummdidumm dummdidumm Oct 13, 2025 •
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I don't think so - when you reject a stale promise you'll end up in the handler and need to unset the context there because it doesn't run this line. The previous version was more correct

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Why? The point of #16935 is that context is saved in fn() but may not be correctly unset if the promise is forcibly rejected before fn() resolves. When that happens, there's nothing to unset, surely?

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Most likely yes, if everything's correct elsewhere it should be null going in. So yeah, I guess your change is fine

} catch (error) {
d.reject(error);
unset_context();
}

if (DEV) current_async_effect = null;
Expand All @@ -151,11 +143,7 @@ export function async_derived(fn, location) {
if (!pending) {
batch.increment();

var previous_deferred = deferreds.get(batch);
if (previous_deferred) {
previous_deferred.rejected = true;
previous_deferred.reject(STALE_REACTION);
}
deferreds.get(batch)?.reject(STALE_REACTION);
deferreds.set(batch, d);
}
}
Expand Down Expand Up @@ -199,8 +187,6 @@ export function async_derived(fn, location) {
boundary.update_pending_count(-1);
if (!pending) batch.decrement();
}

unset_context();
};

d.promise.then(handler, (e) => handler(null, e || 'unknown'));
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