From c111273365361f68ddb12938baf2ffaddf79e423 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Mateusz=20Burzy=C5=84ski?= <mateuszburzynski@gmail.com>
Date: Thu, 7 Dec 2023 17:49:49 +0100
Subject: [PATCH] Notify the `error` listener when subscribing to an errored
 actor (#4570)

---
 .changeset/rich-ears-grow.md           |  5 +++
 packages/core/src/interpreter.ts       | 25 ++++++++++--
 packages/core/test/interpreter.test.ts | 53 ++++++++++++++++++++++++++
 3 files changed, 79 insertions(+), 4 deletions(-)
 create mode 100644 .changeset/rich-ears-grow.md

diff --git a/.changeset/rich-ears-grow.md b/.changeset/rich-ears-grow.md
new file mode 100644
index 0000000000..7ed2adbe7d
--- /dev/null
+++ b/.changeset/rich-ears-grow.md
@@ -0,0 +1,5 @@
+---
+'xstate': patch
+---
+
+Fixed an issue that caused a `complete` listener to be called instead of the `error` one when the actor was subscribed after being stopped.
diff --git a/packages/core/src/interpreter.ts b/packages/core/src/interpreter.ts
index a1138b703b..46728387ba 100644
--- a/packages/core/src/interpreter.ts
+++ b/packages/core/src/interpreter.ts
@@ -365,10 +365,27 @@ export class Actor<TLogic extends AnyActorLogic>
     if (this._processingStatus !== ProcessingStatus.Stopped) {
       this.observers.add(observer);
     } else {
-      try {
-        observer.complete?.();
-      } catch (err) {
-        reportUnhandledError(err);
+      switch ((this._snapshot as any).status) {
+        case 'done':
+          try {
+            observer.complete?.();
+          } catch (err) {
+            reportUnhandledError(err);
+          }
+          break;
+        case 'error': {
+          const err = (this._snapshot as any).error;
+          if (!observer.error) {
+            reportUnhandledError(err);
+          } else {
+            try {
+              observer.error(err);
+            } catch (err) {
+              reportUnhandledError(err);
+            }
+          }
+          break;
+        }
       }
     }
 
diff --git a/packages/core/test/interpreter.test.ts b/packages/core/test/interpreter.test.ts
index 593eb87528..e66da8a123 100644
--- a/packages/core/test/interpreter.test.ts
+++ b/packages/core/test/interpreter.test.ts
@@ -1849,3 +1849,56 @@ it('should not process events sent directly to own actor ref before initial entr
     'EV transition'
   ]);
 });
+
+it('should not notify the completion observer for an active logic when it gets subscribed before starting', () => {
+  const spy = jest.fn();
+
+  const machine = createMachine({});
+  createActor(machine).subscribe({ complete: spy });
+
+  expect(spy).not.toHaveBeenCalled();
+});
+
+it('should not notify the completion observer for an errored logic when it gets subscribed after it errors', () => {
+  const spy = jest.fn();
+
+  const machine = createMachine({
+    entry: () => {
+      throw new Error('error');
+    }
+  });
+  const actorRef = createActor(machine);
+  actorRef.subscribe({ error: () => {} });
+  actorRef.start();
+
+  actorRef.subscribe({
+    complete: spy
+  });
+
+  expect(spy).not.toHaveBeenCalled();
+});
+
+it('should notify the error observer for an errored logic when it gets subscribed after it errors', () => {
+  const spy = jest.fn();
+
+  const machine = createMachine({
+    entry: () => {
+      throw new Error('error');
+    }
+  });
+  const actorRef = createActor(machine);
+  actorRef.subscribe({ error: () => {} });
+  actorRef.start();
+
+  actorRef.subscribe({
+    error: spy
+  });
+
+  expect(spy).toMatchMockCallsInlineSnapshot(`
+    [
+      [
+        [Error: error],
+      ],
+    ]
+  `);
+});