perf: don't reschedule the whole batch on writes

This makes flushing effects much faster in scenarios where there's a lot of effects and many of the write to state.

We added this "bail and reschedule" mechanism mainly to prevent subsequent effects from being called when a prior effect execution did e.g. cause an if block to become false and the effect about to run next therefore being destroyed.

This change addresses that issue by flushing the newly created batch right away, but only executing its branches. Through this we also get rid of the false-positive loop protection error.

The one downside is that effect execution ordering is slightly different: Only things scheduled within the current batch are executed in order, all new (pre-)effects will run afterwards. The worst thing that can happen as a result is rerunning render or user effects more often than they need to, but I expect this to be minimal in comparison to the perf wins.

Fixes #16548

packages/svelte/src/internal/client/reactivity/batch.js

@@ -173,10 +173,10 @@ export class Batch {
 	skipped_effects = new Set();
 
 	/**
-	 *
 	 * @param {Effect[]} root_effects
+	 * @param {boolean} block_effects_only
 	 */
-	process(root_effects) {
+	process(root_effects, block_effects_only = false) {
 		queued_root_effects = [];
 
 		previous_batch = null;
@@ -187,7 +187,7 @@ export class Batch {
 		// if there are multiple batches, we are 'time travelling' —
 		// we need to undo the changes belonging to any batch
 		// other than the current one
-		if (batches.size > 1) {
+		if (batches.size > (block_effects_only ? 2 : 1)) {
 			current_values = new Map();
 			batch_deriveds = new Map();
 
@@ -209,7 +209,12 @@ export class Batch {
 		}
 
 		for (const root of root_effects) {
-			this.#traverse_effect_tree(root);
+			this.#traverse_effect_tree(root, block_effects_only);
+		}
+
+		if (block_effects_only) {
+			queued_root_effects = root_effects;
+			return;
 		}
 
 		// if we didn't start any new async work, and no async work
@@ -276,9 +281,12 @@ export class Batch {
 	 * Traverse the effect tree, executing effects or stashing
 	 * them for later execution as appropriate
 	 * @param {Effect} root
+	 * @param {boolean} block_effects_only
 	 */
-	#traverse_effect_tree(root) {
+	#traverse_effect_tree(root, block_effects_only) {
+		if (!block_effects_only) {
 			root.f ^= CLEAN;
+		}
 
 		var effect = root.first;
 
@@ -290,7 +298,11 @@ export class Batch {
 			var skip = is_skippable_branch || (flags & INERT) !== 0 || this.skipped_effects.has(effect);
 
 			if (!skip && effect.fn !== null) {
-				if (is_branch) {
+				if (block_effects_only) {
+					if ((flags & BLOCK_EFFECT) !== 0 && (flags & CLEAN) === 0) {
+						update_effect(effect);
+					}
+				} else if (is_branch) {
 					effect.f ^= CLEAN;
 				} else if ((flags & CLEAN) === 0) {
 					if ((flags & EFFECT) !== 0) {
@@ -396,6 +408,12 @@ export class Batch {
 		this.deactivate();
 	}
 
+	flush_block_effects() {
+		if (queued_root_effects.length > 0) {
+			this.process(queued_root_effects, true);
+		}
+	}
+
 	/**
 	 * Append and remove branches to/from the DOM
 	 */
@@ -626,14 +644,10 @@ function flush_queued_effects(effects) {
 				current_batch.current.size > n &&
 				(effect.f & USER_EFFECT) !== 0
 			) {
-				break;
+				current_batch.flush_block_effects();
 			}
 		}
 	}
-
-	while (i < length) {
-		schedule_effect(effects[i++]);
-	}
 }
 
 /**