[3.13] gh-89544: Fix IDLE hang after Restart Shell while receiving output (GH-157644) (#157972)

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 b0dda15)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

Author: miss-islington

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):
 

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:

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.