From 98f02c29555d0a92c8d4c6e733fb9125d6eb7aa1 Mon Sep 17 00:00:00 2001
From: Tim <hello@timsmart.co>
Date: Fri, 17 Jan 2025 15:31:51 +1300
Subject: [PATCH] improve performance of Effect.forkIn

---
 .changeset/yellow-keys-lick.md                |  5 +++
 .../effect/src/internal/effect/circular.ts    | 36 ++++++++++---------
 packages/effect/src/internal/fiberRuntime.ts  |  3 +-
 3 files changed, 26 insertions(+), 18 deletions(-)
 create mode 100644 .changeset/yellow-keys-lick.md

diff --git a/.changeset/yellow-keys-lick.md b/.changeset/yellow-keys-lick.md
new file mode 100644
index 00000000000..33a88f4a63c
--- /dev/null
+++ b/.changeset/yellow-keys-lick.md
@@ -0,0 +1,5 @@
+---
+"effect": patch
+---
+
+improve performance of Effect.forkIn
diff --git a/packages/effect/src/internal/effect/circular.ts b/packages/effect/src/internal/effect/circular.ts
index 17cfd98973a..6b95040224a 100644
--- a/packages/effect/src/internal/effect/circular.ts
+++ b/packages/effect/src/internal/effect/circular.ts
@@ -27,7 +27,6 @@ import type * as Types from "../../Types.js"
 import * as internalCause from "../cause.js"
 import * as effect from "../core-effect.js"
 import * as core from "../core.js"
-import * as executionStrategy from "../executionStrategy.js"
 import * as internalFiber from "../fiber.js"
 import * as fiberRuntime from "../fiberRuntime.js"
 import { globalScope } from "../fiberScope.js"
@@ -362,23 +361,26 @@ export const forkIn = dual<
 >(
   2,
   (self, scope) =>
-    core.uninterruptibleMask((restore) =>
-      core.flatMap(scope.fork(executionStrategy.sequential), (child) =>
-        pipe(
-          restore(self),
-          core.onExit((exit) => child.close(exit)),
-          fiberRuntime.forkDaemon,
-          core.tap((fiber) =>
-            child.addFinalizer(() =>
-              core.fiberIdWith((fiberId) =>
-                Equal.equals(fiberId, fiber.id()) ?
-                  core.void :
-                  core.asVoid(core.interruptFiber(fiber))
-              )
-            )
+    core.withFiberRuntime((parent, parentStatus) => {
+      const scopeImpl = scope as fiberRuntime.ScopeImpl
+      const fiber = fiberRuntime.unsafeFork(self, parent, parentStatus.runtimeFlags, globalScope)
+      if (scopeImpl.state._tag === "Open") {
+        const finalizer = () =>
+          core.fiberIdWith((fiberId) =>
+            Equal.equals(fiberId, fiber.id()) ?
+              core.void :
+              core.asVoid(core.interruptFiber(fiber))
           )
-        ))
-    )
+        scopeImpl.state.finalizers.add(finalizer)
+        fiber.addObserver(() => {
+          if (scopeImpl.state._tag === "Closed") return
+          scopeImpl.state.finalizers.delete(finalizer)
+        })
+      } else {
+        fiber.unsafeInterruptAsFork(parent.id())
+      }
+      return core.succeed(fiber)
+    })
 )
 
 /** @internal */
diff --git a/packages/effect/src/internal/fiberRuntime.ts b/packages/effect/src/internal/fiberRuntime.ts
index f89c7dfc852..baec9ad2083 100644
--- a/packages/effect/src/internal/fiberRuntime.ts
+++ b/packages/effect/src/internal/fiberRuntime.ts
@@ -3207,7 +3207,8 @@ export const scopeTag = Context.GenericTag<Scope.Scope>("effect/Scope")
 /* @internal */
 export const scope: Effect.Effect<Scope.Scope, never, Scope.Scope> = scopeTag
 
-interface ScopeImpl extends Scope.CloseableScope {
+/** @internal */
+export interface ScopeImpl extends Scope.CloseableScope {
   state: {
     readonly _tag: "Open"
     readonly finalizers: Set<Scope.Scope.Finalizer>