This patch changes the CallbackManager to use WeakMethods (weakrefs) to
break the ref cycle and allow the GC to clean up the wallet objects.
unregister_callbacks() will also get called automatically, from
EventListener.__del__, to clean up the CallbackManager.
I also added a few unit tests for this.
fixes https://github.com/spesmilo/electrum/issues/10427
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original problem:
In many subclasses of `EventListener`, such as `Abstract_Wallet`, `LNWatcher`,
`LNPeerManager`, we call `register_callbacks()` in `__init__`.
`unregister_callbacks()` is usually called in the `stop()` method.
Example - consider the wallet object:
- `Abstract_Wallet.__init__()` calls `register_callbacks()`
- there is a `start_network()` method
- there is a `stop()` method, which calls `unregister_callbacks()`
- typically the wallet API user only calls `stop()` if they also called
`start_network()`.
This means the callbacks are often left registered, leading to the wallet
objects not getting GC-ed. The GC won't clean them up as
`util.callback_mgr.callbacks` stores strong refs to instance methods
of `Abstract_Wallet`, hence strong refs to the `Abstract_Wallet` objects.
An annoying example is `daemon.check_password_for_directory`, which
potentially creates wallet objects for all wallet files in the datadir.
It simply constructs the wallets, does not call `start_network()` and
neither does it call `stop()`.
When creating a new wallet in a Electrum instance with existing wallets
this change forces the user to reuse a password of any existing wallet
if `SimpleConfig.WALLET_USE_SINGLE_PASSWORD` is True.
This prevents the amount of different passwords from increasing and
guides the user towards a single wallet password (the intended default).
- fix: qml gui errors when trying to open a wallets with only keystore-encryption
- fixes https://github.com/spesmilo/electrum/issues/10171
- qml gui to prompt for password on wallet open even if wallet is not storage-encrypted
