logging: `SMTPHandler.emit()` leaks the SMTP connection when sending fails

Bug report

Bug description:

SMTPHandler.emit() only closes its SMTP connection on the success path: smtp.quit() is the last statement of the try block (Lib/logging/handlers.py#L1109-L1154).

If starttls(), login() or send_message() raises, control jumps to except Exception: self.handleError(record) and the connection is never closed — cleanup is left to the garbage collector.

Reproducer

import gc, logging, logging.handlers, socket, threading, time

logging.raiseExceptions = False
open_conns = []

def handle(conn):
    conn.sendall(b"220 fake ESMTP\r\n")
    for line in conn.makefile("rb"):
        cmd = line.strip().upper()
        if cmd.startswith(b"EHLO"):
            conn.sendall(b"250-fake\r\n250 AUTH PLAIN LOGIN\r\n")
        elif cmd.startswith(b"AUTH"):
            conn.sendall(b"535 authentication failed\r\n")
        else:
            conn.sendall(b"250 ok\r\n")
    open_conns.remove(conn)  # reached when the client closes the connection
    conn.close()

def serve(listener):
    while True:
        conn, _ = listener.accept()
        open_conns.append(conn)
        threading.Thread(target=handle, args=(conn,), daemon=True).start()

listener = socket.create_server(("127.0.0.1", 0))
threading.Thread(target=serve, args=(listener,), daemon=True).start()

h = logging.handlers.SMTPHandler(("127.0.0.1", listener.getsockname()[1]),
                                 "me@example.com", "you@example.com", "subject",
                                 credentials=("user", "wrong-password"))
for i in range(5):
    h.emit(logging.makeLogRecord({"msg": "hello"}))  # SMTPAuthenticationError
time.sleep(0.5)
print("open connections after 5 failed emits:", len(open_conns))
gc.collect(); time.sleep(0.5)
print("after gc.collect():", len(open_conns))

Output on main:

open connections after 5 failed emits: 5
after gc.collect(): 0

CPython versions tested on:

CPython main branch

Operating systems tested on:

macOS

Linked PRs

• gh-155950

從 Lib/logging/handlers.py 中的 SMTPHandler.emit() 開始，並執行提供的驗證失敗重現程式。完成的標準是，starttls()、login() 或 send_message() 失敗的路徑不再讓 SMTP 連線保持開啟狀態，並根據重現程式檢查行為；連結的 PR gh-155950 表示相關工作已經在進行中。