From 1217da745277e598b9538c2d2be08d4181593fec Mon Sep 17 00:00:00 2001
From: Serhiy Storchaka <storchaka@gmail.com>
Date: Wed, 23 Sep 2026 04:32:25 +0300
Subject: [PATCH] gh-89544: Fix IDLE hang after Restart Shell while receiving
 output (GH-157644)

RPCClient.accept() reinitializes the SocketIO for the new connection,
but the receive buffer kept the partially received packet from the old
connection, so no message from the new user process was ever complete.
(cherry picked from commit b0dda158aad00f0064e220f882792a5bd16ba0a0)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
---
 Lib/idlelib/idle_test/test_rpc.py             | 19 +++++++++++++++++++
 Lib/idlelib/rpc.py                            |  9 +++++----
 ...6-09-17-01-00-00.gh-issue-89544.rstbuf.rst |  2 ++
 3 files changed, 26 insertions(+), 4 deletions(-)
 create mode 100644 Misc/NEWS.d/next/IDLE/2026-09-17-01-00-00.gh-issue-89544.rstbuf.rst

diff --git a/Lib/idlelib/idle_test/test_rpc.py b/Lib/idlelib/idle_test/test_rpc.py
index 81eff398c72f45b..15c3ed14b8f6f3e 100644
--- a/Lib/idlelib/idle_test/test_rpc.py
+++ b/Lib/idlelib/idle_test/test_rpc.py
@@ -1,9 +1,28 @@
 "Test rpc, coverage 20%."
 
 from idlelib import rpc
+import socket
+import struct
 import unittest
 
 
+class SocketIOTest(unittest.TestCase):
+
+    def test_reconnect_discards_partial_packet(self):
+        # gh-89544: RPCClient.accept() reinitializes the SocketIO after
+        # a restart; a partially received packet must not be kept.
+        old_sock, old_peer = socket.socketpair()
+        new_sock, new_peer = socket.socketpair()
+        with old_sock, old_peer, new_sock, new_peer:
+            sockio = rpc.SocketIO(old_sock)
+            old_peer.sendall(struct.pack('<i', 100) + b'x' * 10)
+            self.assertIsNone(sockio.pollpacket(1))
+            sockio.close()
+            rpc.SocketIO.__init__(sockio, new_sock)
+            new_peer.sendall(struct.pack('<i', 3) + b'abc')
+            self.assertEqual(sockio.pollpacket(1), b'abc')
+
+
 
 class CodePicklerTest(unittest.TestCase):
 
diff --git a/Lib/idlelib/rpc.py b/Lib/idlelib/rpc.py
index 3f0b2230dd185d7..9af9863d74e0b6a 100644
--- a/Lib/idlelib/rpc.py
+++ b/Lib/idlelib/rpc.py
@@ -139,6 +139,11 @@ def __init__(self, sock, objtable=None, debugging=None):
         self.objtable = objtable
         self.responses = {}
         self.cvars = {}
+        # Receive buffer state.  A new connection must not inherit a
+        # partially received packet from the old one (gh-89544).
+        self.buff = b''
+        self.bufneed = 4
+        self.bufstate = 0 # meaning: 0 => reading count; 1 => reading data
 
     def close(self):
         sock = self.sock
@@ -345,10 +350,6 @@ def putmessage(self, message):
                 raise OSError("socket no longer exists")
             s = s[n:]
 
-    buff = b''
-    bufneed = 4
-    bufstate = 0 # meaning: 0 => reading count; 1 => reading data
-
     def pollpacket(self, wait):
         self._stage0()
         if len(self.buff) < self.bufneed:
diff --git a/Misc/NEWS.d/next/IDLE/2026-09-17-01-00-00.gh-issue-89544.rstbuf.rst b/Misc/NEWS.d/next/IDLE/2026-09-17-01-00-00.gh-issue-89544.rstbuf.rst
new file mode 100644
index 000000000000000..4a4c885eff96818
--- /dev/null
+++ b/Misc/NEWS.d/next/IDLE/2026-09-17-01-00-00.gh-issue-89544.rstbuf.rst
@@ -0,0 +1,2 @@
+Fix IDLE hanging after Restart Shell while receiving a large output from the
+user process.