This PR removes direct access to process-wide state from ExternalSignerScriptPubKeyMan.
Instead of reading gArgs/Params() from the SPKM, the wallet load/setup path now reads the external signer command and chain once, stores them on CWallet, and passes the narrow values into the external signer SPKM.
This keeps argument and chain selection at the wallet boundary and makes the SPKM less coupled to global state.
ExternalSignerScriptPubKeyMan::GetExternalSigner() reached into global state
(gArgs for -signer, Params() for the chain type) from deep inside the signing
and descriptor-setup paths.
Capture both in LoadWalletArgs -- where the wallet already reads its other
settings from the injected WalletContext -- store them on CWallet, and inject
them into ExternalSignerScriptPubKeyMan at construction. GetExternalSigner()
now takes the command and chain as parameters.
No behavior change: when -signer is unset the command is empty and the same
"restart bitcoind with -signer" error is returned; the chain string is the
value Params().GetChainTypeString() previously provided.
Add a unit test asserting that GetExternalSigner() with an empty command
returns the "needs -signer" error, before any external process is launched.
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For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35429.
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