From 34c2900a647e7ac34c2384a5875e02e8dad96654 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Thu, 5 Jun 2025 23:10:07 -0400
Subject: [PATCH] WIP

---
 .../src/internal/client/error-handling.js     | 22 ++-----
 .../svelte/src/internal/client/runtime.js     | 63 +++++++++++--------
 .../samples/error-boundary-12/_config.js      |  7 +--
 .../samples/error-boundary-12/main.svelte     | 10 ++-
 .../samples/error-boundary-13/Child.svelte    | 13 +++-
 .../samples/error-boundary-13/_config.js      |  2 +-
 .../samples/error-boundary-13/main.svelte     | 11 +---
 .../samples/error-boundary-18/_config.js      |  8 +--
 .../samples/error-boundary-18/main.svelte     | 12 ++--
 9 files changed, 72 insertions(+), 76 deletions(-)

diff --git a/packages/svelte/src/internal/client/error-handling.js b/packages/svelte/src/internal/client/error-handling.js
index 754267184d..ead5e0646f 100644
--- a/packages/svelte/src/internal/client/error-handling.js
+++ b/packages/svelte/src/internal/client/error-handling.js
@@ -15,37 +15,25 @@ export function reset_is_throwing_error() {
 	is_throwing_error = false;
 }
 
+/** @type {null | unknown} */
+let current_error = null;
+
 /**
  * @param {unknown} error
  * @param {Effect} effect
  * @param {Effect | null} [previous_effect]
  */
 export function handle_error(error, effect, previous_effect = null) {
-	if (is_throwing_error) {
-		if (previous_effect === null) {
-			is_throwing_error = false;
-		}
-
-		if (should_rethrow_error(effect)) {
-			throw error;
-		}
-
+	if (error === current_error) {
+		// TODO is this necessary?
 		return;
 	}
 
-	if (previous_effect !== null) {
-		is_throwing_error = true;
-	}
-
 	if (DEV && error instanceof Error) {
 		adjust_error(error, effect);
 	}
 
 	invoke_error_boundary(error, effect);
-
-	if (should_rethrow_error(effect)) {
-		throw error;
-	}
 }
 
 /**
diff --git a/packages/svelte/src/internal/client/runtime.js b/packages/svelte/src/internal/client/runtime.js
index 2ac3125a0f..4eddd7d3bf 100644
--- a/packages/svelte/src/internal/client/runtime.js
+++ b/packages/svelte/src/internal/client/runtime.js
@@ -343,6 +343,13 @@ export function update_reaction(reaction) {
 		}
 
 		return result;
+	} catch (error) {
+		var effect = get_effect(reaction);
+		if (effect) {
+			handle_error(error, effect);
+		} else {
+			throw error;
+		}
 	} finally {
 		new_deps = previous_deps;
 		skipped_deps = previous_skipped_deps;
@@ -357,6 +364,18 @@ export function update_reaction(reaction) {
 	}
 }
 
+/** @param {Reaction} reaction */
+function get_effect(reaction) {
+	/** @type {Reaction | null;} */
+	var r = reaction;
+
+	while (r !== null && (r.f & DERIVED) !== 0) {
+		r = /** @type {Derived} */ (r).parent;
+	}
+
+	return /** @type {Effect | null} */ (r);
+}
+
 /**
  * @template V
  * @param {Reaction} signal
@@ -470,8 +489,6 @@ export function update_effect(effect) {
 		if (DEV) {
 			dev_effect_stack.push(effect);
 		}
-	} catch (error) {
-		handle_error(error, effect, previous_effect);
 	} finally {
 		is_updating_effect = was_updating_effect;
 		active_effect = previous_effect;
@@ -570,27 +587,23 @@ function flush_queued_effects(effects) {
 		var effect = effects[i];
 
 		if ((effect.f & (DESTROYED | INERT)) === 0) {
-			try {
-				if (check_dirtiness(effect)) {
-					update_effect(effect);
-
-					// Effects with no dependencies or teardown do not get added to the effect tree.
-					// Deferred effects (e.g. `$effect(...)`) _are_ added to the tree because we
-					// don't know if we need to keep them until they are executed. Doing the check
-					// here (rather than in `update_effect`) allows us to skip the work for
-					// immediate effects.
-					if (effect.deps === null && effect.first === null && effect.nodes_start === null) {
-						if (effect.teardown === null) {
-							// remove this effect from the graph
-							unlink_effect(effect);
-						} else {
-							// keep the effect in the graph, but free up some memory
-							effect.fn = null;
-						}
+			if (check_dirtiness(effect)) {
+				update_effect(effect);
+
+				// Effects with no dependencies or teardown do not get added to the effect tree.
+				// Deferred effects (e.g. `$effect(...)`) _are_ added to the tree because we
+				// don't know if we need to keep them until they are executed. Doing the check
+				// here (rather than in `update_effect`) allows us to skip the work for
+				// immediate effects.
+				if (effect.deps === null && effect.first === null && effect.nodes_start === null) {
+					if (effect.teardown === null) {
+						// remove this effect from the graph
+						unlink_effect(effect);
+					} else {
+						// keep the effect in the graph, but free up some memory
+						effect.fn = null;
 					}
 				}
-			} catch (error) {
-				handle_error(error, effect);
 			}
 		}
 	}
@@ -649,12 +662,8 @@ function process_effects(root) {
 			} else if (is_branch) {
 				effect.f ^= CLEAN;
 			} else {
-				try {
-					if (check_dirtiness(effect)) {
-						update_effect(effect);
-					}
-				} catch (error) {
-					handle_error(error, effect);
+				if (check_dirtiness(effect)) {
+					update_effect(effect);
 				}
 			}
 
diff --git a/packages/svelte/tests/runtime-runes/samples/error-boundary-12/_config.js b/packages/svelte/tests/runtime-runes/samples/error-boundary-12/_config.js
index b3edfefa20..4e771dfafa 100644
--- a/packages/svelte/tests/runtime-runes/samples/error-boundary-12/_config.js
+++ b/packages/svelte/tests/runtime-runes/samples/error-boundary-12/_config.js
@@ -5,9 +5,8 @@ export default test({
 	test({ assert, target }) {
 		const btn = target.querySelector('button');
 
-		btn?.click();
-		flushSync();
-
-		assert.htmlEqual(target.innerHTML, `<button>change</button><p>Error occured</p>`);
+		assert.throws(() => {
+			flushSync(() => btn?.click());
+		}, /kaboom/);
 	}
 });
diff --git a/packages/svelte/tests/runtime-runes/samples/error-boundary-12/main.svelte b/packages/svelte/tests/runtime-runes/samples/error-boundary-12/main.svelte
index bae46c2590..d9dee1e2b0 100644
--- a/packages/svelte/tests/runtime-runes/samples/error-boundary-12/main.svelte
+++ b/packages/svelte/tests/runtime-runes/samples/error-boundary-12/main.svelte
@@ -1,20 +1,18 @@
 <script>
 	let count = $state(0);
 
-	const things = $derived.by(() => {
+	const d = $derived.by(() => {
 		if (count === 1) {
-			throw new Error('123')
+			throw new Error('kaboom')
 		}
-		return [1, 2 ,3]
+		return count
 	})
 </script>
 
 <button onclick={() => count++}>change</button>
 
 <svelte:boundary>
-	{#each things as thing}
-		<p>{thing}</p>
-	{/each}
+	{d}
 
 	{#snippet failed()}
 		<p>Error occured</p>
diff --git a/packages/svelte/tests/runtime-runes/samples/error-boundary-13/Child.svelte b/packages/svelte/tests/runtime-runes/samples/error-boundary-13/Child.svelte
index 6e607871d3..de482da985 100644
--- a/packages/svelte/tests/runtime-runes/samples/error-boundary-13/Child.svelte
+++ b/packages/svelte/tests/runtime-runes/samples/error-boundary-13/Child.svelte
@@ -1,7 +1,14 @@
 <script>
-	const { things } = $props();
+	const { count } = $props();
+
+	const d = $derived.by(() => {
+		if (count === 1) {
+			throw new Error('kaboom')
+		}
+		return count
+	});
 
 	$effect(() => {
-		things
-	})
+		d;
+	});
 </script>
diff --git a/packages/svelte/tests/runtime-runes/samples/error-boundary-13/_config.js b/packages/svelte/tests/runtime-runes/samples/error-boundary-13/_config.js
index b3edfefa20..3825dc7811 100644
--- a/packages/svelte/tests/runtime-runes/samples/error-boundary-13/_config.js
+++ b/packages/svelte/tests/runtime-runes/samples/error-boundary-13/_config.js
@@ -8,6 +8,6 @@ export default test({
 		btn?.click();
 		flushSync();
 
-		assert.htmlEqual(target.innerHTML, `<button>change</button><p>Error occured</p>`);
+		assert.htmlEqual(target.innerHTML, `<button>change</button><p>Error occurred</p>`);
 	}
 });
diff --git a/packages/svelte/tests/runtime-runes/samples/error-boundary-13/main.svelte b/packages/svelte/tests/runtime-runes/samples/error-boundary-13/main.svelte
index 65b71106fa..56e4846675 100644
--- a/packages/svelte/tests/runtime-runes/samples/error-boundary-13/main.svelte
+++ b/packages/svelte/tests/runtime-runes/samples/error-boundary-13/main.svelte
@@ -2,21 +2,14 @@
 	import Child from './Child.svelte';
 
 	let count = $state(0);
-
-	const things = $derived.by(() => {
-		if (count === 1) {
-			throw new Error('123')
-		}
-		return [1, 2 ,3]
-	})
 </script>
 
 <button onclick={() => count++}>change</button>
 
 <svelte:boundary>
-	<Child {things} />
+	<Child {count} />
 
 	{#snippet failed()}
-		<p>Error occured</p>
+		<p>Error occurred</p>
 	{/snippet}
 </svelte:boundary>
diff --git a/packages/svelte/tests/runtime-runes/samples/error-boundary-18/_config.js b/packages/svelte/tests/runtime-runes/samples/error-boundary-18/_config.js
index fedaaa9ad1..e092d0e7c7 100644
--- a/packages/svelte/tests/runtime-runes/samples/error-boundary-18/_config.js
+++ b/packages/svelte/tests/runtime-runes/samples/error-boundary-18/_config.js
@@ -5,11 +5,11 @@ export default test({
 	test({ assert, target }) {
 		let btn = target.querySelector('button');
 
-		btn?.click();
-		btn?.click();
-
 		assert.throws(() => {
-			flushSync();
+			flushSync(() => {
+				btn?.click();
+				btn?.click();
+			});
 		}, /test\n\n\tin {expression}\n/);
 	}
 });
diff --git a/packages/svelte/tests/runtime-runes/samples/error-boundary-18/main.svelte b/packages/svelte/tests/runtime-runes/samples/error-boundary-18/main.svelte
index b9de7126db..e73dcacc8a 100644
--- a/packages/svelte/tests/runtime-runes/samples/error-boundary-18/main.svelte
+++ b/packages/svelte/tests/runtime-runes/samples/error-boundary-18/main.svelte
@@ -1,16 +1,18 @@
 <script>
 	let count = $state(0);
 
-	let test = $derived.by(() => {
+	function maybe_throw() {
 		if (count > 1) {
 			throw new Error('test');
 		}
-	});
+
+		return count;
+	}
 </script>
 
 <svelte:boundary onerror={(e) => { throw(e) }}>
-		<div>Count: {count}</div>
-		<button onclick={() => count++}>Increment</button>
-		{count} / {test}
+	<div>Count: {count}</div>
+	<button onclick={() => count++}>Increment</button>
+	{count} / {maybe_throw()}
 </svelte:boundary>