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Merge #17719: Document better -keypool as a look-ahead safety mechanism

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f41d589669 Document better -keypool as a look-ahead safety mechanism (Antoine Riard)

Pull request description:

  If after a backup, an address is issued beyond the initial
  keypool range and none of the addresses in this range
  is seen onchain, if a wallet is restored from backup, even in
  case of rescan, funds may be loss due to the look-ahead
  buffer not being incremented and so restored wallet not detecting
  onchain out-of-range address as derived from its seed.

  This scenario is theoretically unavoidable due to the requirement
  of the keypool to have a max size. However, given the default
  keypool size, this is unlikely. Document better keypool size
  implications to avoid user setting a too low value.

  While reviewing #17681, it took me a while to figure out the safety implications of keypool, I find it would be better to document this a bit farther to avoid users shooting themselves in the foot. For further context & discussion, see https://github.com/bitcoin/bitcoin/pull/17681#issuecomment-563613452

ACKs for top commit:
  ryanofsky:
    Code review ACK f41d589669. Just "Warning:" prefix added since the last review
  jonatack:
    ACK f41d589669 code review and build/test. The added `Warning:` since last review is a good addition.

Tree-SHA512: d3d0ee88fcdfc5c8841a2bd4bada0e4eeb412a0dce5054e5fb023643c2fa57206a0f3efb06890c245528dc4431413ed2fd5645b9319d26245d044c490b7f0db0
This commit is contained in:
Samuel Dobson 2020-01-30 09:09:48 +13:00
commit aabec94541
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GPG Key ID: D300116E1C875A3D
2 changed files with 6 additions and 1 deletions
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2
src/wallet/init.cpp
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@ -46,7 +46,7 @@ void WalletInit::AddWalletOptions() const
gArgs.AddArg("-fallbackfee=<amt>", strprintf("A fee rate (in %s/kB) that will be used when fee estimation has insufficient data. 0 to entirely disable the fallbackfee feature. (default: %s)",
CURRENCY_UNIT, FormatMoney(DEFAULT_FALLBACK_FEE)), ArgsManager::ALLOW_ANY, OptionsCategory::WALLET);
gArgs.AddArg("-keypool=<n>", strprintf("Set key pool size to <n> (default: %u)", DEFAULT_KEYPOOL_SIZE), ArgsManager::ALLOW_ANY, OptionsCategory::WALLET);
gArgs.AddArg("-keypool=<n>", strprintf("Set key pool size to <n> (default: %u). Warning: Smaller sizes may increase the risk of losing funds when restoring from an old backup, if none of the addresses in the original keypool have been used.", DEFAULT_KEYPOOL_SIZE), ArgsManager::ALLOW_ANY, OptionsCategory::WALLET);
gArgs.AddArg("-maxtxfee=<amt>", strprintf("Maximum total fees (in %s) to use in a single wallet transaction; setting this too low may abort large transactions (default: %s)",
CURRENCY_UNIT, FormatMoney(DEFAULT_TRANSACTION_MAXFEE)), ArgsManager::ALLOW_ANY, OptionsCategory::DEBUG_TEST);
gArgs.AddArg("-mintxfee=<amt>", strprintf("Fees (in %s/kB) smaller than this are considered zero fee for transaction creation (default: %s)",

5
src/wallet/scriptpubkeyman.h
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@ -74,6 +74,11 @@ std::vector<CKeyID> GetAffectedKeys(const CScript& spk, const SigningProvider& p
* keys (by default 1000) ahead of the last used key and scans for the
* addresses of those keys. This avoids the risk of not seeing transactions
* involving the wallet's addresses, or of re-using the same address.
* In the unlikely case where none of the addresses in the `gap limit` are
* used on-chain, the look-ahead will not be incremented to keep
* a constant size and addresses beyond this range will not be detected by an
* old backup. For this reason, it is not recommended to decrease keypool size
* lower than default value.
*
* The HD-split wallet feature added a second keypool (commit: 02592f4c). There
* is an external keypool (for addresses to hand out) and an internal keypool