http.cookies.BaseCookie.load() applies partial state before raising CookieError

Bug description

BaseCookie.load() parses the cookie string in two phases: parse then apply. The comment says it rejects the string if syntactically invalid. However, _is_legal_key() validation runs during the apply phase (inside Morsel.set()), not the parse phase. Cookies before an illegal key are applied to the jar before CookieError is raised.

from http.cookies import SimpleCookie, CookieError

C = SimpleCookie()
try:
    C.load("a=1; b,c=2; d=3")
except CookieError:
    pass
print(len(C))  # 1, expected 0

Related to gh-84183 but distinct: that issue is about raise-vs-silent for different invalid inputs; this is about partial state when CookieError IS raised.

CPython versions tested on

3.13, 3.14, main

Operating systems tested on

macOS

Linked PRs

• gh-154674

從 http.cookies.BaseCookie.load() 開始，使用報告中顯示的 cookie 字串重現該問題。確認 CookieError 會讓 cookie jar 保持為空；issue 內文連結到 PR gh-154674，因此請先檢查這項工作再繼續。