Repository: bitcoin/bitcoin
File: src/rpc/blockchain.cpp (C++)

Code before:
        block = latestblock;
        if(timeout)
            cond_blockchange.wait_for(lock, std::chrono::milliseconds(timeout), [&block]() EXCLUSIVE_LOCKS_REQUIRED(cs_blockchange) {return latestblock.height != block.height || latestblock.hash != block.hash || !IsRPCRunning(); });
        else
            cond_blockchange.wait(lock, [&block]() EXCLUSIVE_LOCKS_REQUIRED(cs_blockchange) {return latestblock.height != block.height || latestblock.hash != block.hash || !IsRPCRunning(); });
        block = latestblock;
    }
    UniValue ret(UniValue::VOBJ);
    ret.pushKV("hash", block.hash.GetHex());
    ret.pushKV("height", block.height);
    return ret;
},
    };
}

static RPCHelpMan waitforblock()
{
    return RPCHelpMan{"waitforblock",
                "\nWaits for a specific new block and returns useful info about it.\n"
                "\nReturns the current block on timeout or exit.\n",
                {
                    {"blockhash", RPCArg::Type::STR_HEX, RPCArg::Optional::NO, "Block hash to wait for."},
                    {"timeout", RPCArg::Type::NUM, RPCArg::Default{0}, "Time in milliseconds to wait for a response. 0 indicates no timeout."},
                },
                RPCResult{
                    RPCResult::Type::OBJ, "", "",
                    {
                        {RPCResult::Type::STR_HEX, "hash", "The blockhash"},
                        {RPCResult::Type::NUM, "height", "Block height"},
                    }},
                RPCExamples{
                    HelpExampleCli("waitforblock", "\"0000000000079f8ef3d2c688c244eb7a4570b24c9ed7b4a8c619eb02596f8862\" 1000")
            + HelpExampleRpc("waitforblock", "\"0000000000079f8ef3d2c688c244eb7a4570b24c9ed7b4a8c619eb02596f8862\", 1000")
                },
        [&](const RPCHelpMan& self, const JSONRPCRequest& request) -> UniValue
{
    int timeout = 0;

    uint256 hash(ParseHashV(request.params[0], "blockhash"));

    if (!request.params[1].isNull())
        timeout = request.params[1].getInt<int>();

    CUpdatedBlock block;
    {
        WAIT_LOCK(cs_blockchange, lock);
        if(timeout)
            cond_blockchange.wait_for(lock, std::chrono::milliseconds(timeout), [&hash]() EXCLUSIVE_LOCKS_REQUIRED(cs_blockchange) {return latestblock.hash == hash || !IsRPCRunning();});
        else
            cond_blockchange.wait(lock, [&hash]() EXCLUSIVE_LOCKS_REQUIRED(cs_blockchange) {return latestblock.hash == hash || !IsRPCRunning(); });
        block = latestblock;

Review comment (bug):
In commit "Replace RPCNotifyBlockChange with waitTipChanged()" (9d3b07526110a842c9592425b89785e688a8f46c)

Should pass `block.hash` as `waitTipChanged` argument here to avoid race condition (see earlier suggestion)

Code after:
                "\nWaits for a specific new block and returns useful info about it.\n"
                "\nReturns the current block on timeout or exit.\n"
                "\nMake sure to use no RPC timeout (bitcoin-cli -rpcclienttimeout=0)",
                {
                    {"blockhash", RPCArg::Type::STR_HEX, RPCArg::Optional::NO, "Block hash to wait for."},
                    {"timeout", RPCArg::Type::NUM, RPCArg::Default{0}, "Time in milliseconds to wait for a response. 0 indicates no timeout."},
                },
                RPCResult{
                    RPCResult::Type::OBJ, "", "",
                    {
                        {RPCResult::Type::STR_HEX, "hash", "The blockhash"},
                        {RPCResult::Type::NUM, "height", "Block height"},
                    }},
                RPCExamples{
                    HelpExampleCli("waitforblock", "\"0000000000079f8ef3d2c688c244eb7a4570b24c9ed7b4a8c619eb02596f8862\" 1000")
            + HelpExampleRpc("waitforblock", "\"0000000000079f8ef3d2c688c244eb7a4570b24c9ed7b4a8c619eb02596f8862\", 1000")
                },
        [&](const RPCHelpMan& self, const JSONRPCRequest& request) -> UniValue
{
    int timeout = 0;

    uint256 hash(ParseHashV(request.params[0], "blockhash"));

    if (!request.params[1].isNull())
        timeout = request.params[1].getInt<int>();
    if (timeout < 0) throw JSONRPCError(RPC_MISC_ERROR, "Negative timeout");

    NodeContext& node = EnsureAnyNodeContext(request.context);
    Mining& miner = EnsureMining(node);

    auto block{CHECK_NONFATAL(miner.getTip()).value()};
    const auto deadline{std::chrono::steady_clock::now() + 1ms * timeout};
    while (IsRPCRunning() && block.hash != hash) {
        if (timeout) {
            auto now{std::chrono::steady_clock::now()};
            if (now >= deadline) break;
            const MillisecondsDouble remaining{deadline - now};
            block = miner.waitTipChanged(block.hash, remaining);
        } else {
            block = miner.waitTipChanged(block.hash);
        }
    }

    UniValue ret(UniValue::VOBJ);
    ret.pushKV("hash", block.hash.GetHex());
    ret.pushKV("height", block.height);
    return ret;
},
    };
}