_pickle.UnpicklingError: Memo value not found at index 1

Bug report

Bug description:

When pickling a container object. The pickler class will store child-objects to its self.memo even if the container could not be fully pickled (and not written to the output).
This can cause references to memo objects that are not in the output stream.

import sys
import pickle
import io

buf = io.BytesIO()
pickler = pickle.Pickler(buf)
i_will_be_memorized = 'qqqqqqqqqqqq'
i_cant_be_pickled = sys

try:
    pickler.dump([i_will_be_memorized, i_cant_be_pickled])
except TypeError:
    # i_will_be_memorized is now in pickler memory despite the exception
    pass

# this dump will use the memo
pickler.dump(i_will_be_memorized)

data = buf.getbuffer().tobytes()

# so this will fail with:
#   _pickle.UnpicklingError: Memo value not found at index 1
pickle.loads(data)

CPython versions tested on:

3.12

Operating systems tested on:

Windows

Linked PRs

• gh-125762
• gh-130231
• gh-130232

issue にある Python pickle と io.BytesIO のスニペットで失敗を再現し、その後、失敗したコンテナダンプに関連する pickle.Pickler の memo 処理を調査します。後続のダンプが出力ストリームに存在しない memo エントリを参照せず、示されたシーケンスに対する回帰テストのカバレッジがあることを完了条件とします。