If I unlock an encrypted wallet for 359711509437336256 or less seconds, I can sign transactions with it:

    $ bitcoin-cli walletpassphrase "$x" 359711509437336256
    $ echo $?
    0
    $ bitcoin-cli signrawtransaction $tx | grep complete
      "complete": true

But if I unlock it for 359711509437336257 or more seconds, it appears to have unlocked but I can't sign any transactions:

    $ bitcoin-cli walletpassphrase "$x" 359711509437336257
    $ echo $?
    0
    $ bitcoin-cli signrawtransaction $tx | grep complete
    error code: -13
    error message:
    Error: Please enter the wallet passphrase with walletpassphrase first.

I don't know the significance of that number. It's nowhere near a power of 2.

I was trying to unlock the wallet effectively "forever", so put a big number. If I use too big a number (2^63 or more) I get an error message `JSON integer out of range` but for numbers between 359711509437336257 and 2^63-1 inclusive I get no error, but no actual unlocking.
throw std::runtime_error(
            "abandontransaction \"txid\"\n"
            "\nMark in-wallet transaction <txid> as abandoned\n"
            "This will mark this transaction and all its in-wallet descendants as abandoned which will allow\n"
            "for their inputs to be respent.  It can be used to replace \"stuck\" or evicted transactions.\n"
            "It only works on transactions which are not included in a block and are not currently in the mempool.\n"
            "It has no effect on transactions which are already conflicted or abandoned.\n"
            "\nArguments:\n"
            "1. \"txid\"    (string, required) The transaction id\n"
            "\nResult:\n"
            "\nExamples:\n"
            + HelpExampleCli("abandontransaction", "\"1075db55d416d3ca199f55b6084e2115b9345e16c5cf302fc80e9d5fbf5d48d\"")
            + HelpExampleRpc("abandontransaction", "\"1075db55d416d3ca199f55b6084e2115b9345e16c5cf302fc80e9d5fbf5d48d\"")
        );
    ObserveSafeMode();
    
    
    pwallet->BlockUntilSyncedToCurrentChain();
    LOCK2(cs_main, pwallet->cs_wallet);
    uint256 hash;
    hash.SetHex(request.params[0].get_str());
    if (!pwallet->mapWallet.count(hash)) {
        throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Invalid or non-wallet transaction id");
    }
    if (!pwallet->AbandonTransaction(hash)) {
        throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Transaction not eligible for abandonment");
    }
    return NullUniValue;
}
UniValue backupwallet(const JSONRPCRequest& request)