Incentivize external caller for a contract job using incremental allocation.

registerCallback.sol

pragma solidity ^0.8.1;

contract TimedCallbacks
{
  struct Job { uint time; address addr; bytes data; uint reward; address creator; }
  
  Job[] jobs;
  
  function register(uint _time, address _addr, bytes calldata _data) external payable returns (uint index) {
    index = jobs.length;
    
    Job memory newJob = Job(_time, _addr, _data, msg.value, msg.sender);
    
    jobs.push(newJob);
  }
  
  function invoke(uint index) external returns (bytes memory ret) {
    if (jobs[index].time > 0 && block.timestamp >= jobs[index].time) {
        payable(msg.sender).transfer(jobs[index].reward);
        ( , ret) = jobs[index].addr.call(jobs[index].data);
        delete jobs[index];
    }
  }
  
  function cancel(uint index) external returns (bool res) {
      if (msg.sender == jobs[index].creator) {
          payable(msg.sender).transfer(jobs[index].reward);
          delete jobs[index];
          res = true;
      }
  }
  
  function getNumJobs() external view returns (uint count) {
      count = jobs.length;
  }
  
  function getJob(uint index) external view returns (Job memory j) {
      j = jobs[index];
  }
}