From eb8914a194c1c2bb51e49d9e3ef564c7150d2c46 Mon Sep 17 00:00:00 2001
From: Serhiy Storchaka <storchaka@gmail.com>
Date: Wed, 16 Sep 2026 23:25:32 +0300
Subject: [PATCH] gh-89544: Fix IDLE hang after Restart Shell while receiving
 output

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.
---
 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.