ROS2 Service Call in a Publisher Timer Callback

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Running ROS2 Foxy on an RPI4 w/ Ubuntu

I have implemented an SPI service node that provides shared bus access across multiple nodes. This works as expected; I can send a byte array as a request, and the response returns the full duplex recieved data.

However, I am now trying to create another node that publishes data that must be accessed via the SPI service, but I am getting a runtime error:

terminate called after throwing an instance of 'std::runtime_error'
  what():  Node has already been added to an executor.

After researching, I found the various comments online about not being able to call a service from another callback due to deadlocks.

So my question is: what is the 'correct' way to architect this, given that I want to have multipler peripherals on the SPI bus that could be time shared to access different components for different nodes, and I need to grab data via SPI for publishing (or upon recipt if a subscriber).

Thank you, Scott

UPDATE:

Thank you for the response; I have found this link as well with an async call that allows for an automatic callback:

https://github.com/ros2/demos/blob/40...

Here is the example code:

    // We give the async_send_request() method a callback that will get executed once the response
    // is received.
    // This way we can return immediately from this method and allow other work to be done by the
    // executor in `spin` while waiting for the response.
    using ServiceResponseFuture =
      rclcpp::Client<example_interfaces::srv::AddTwoInts>::SharedFuture;
    auto response_received_callback = [this](ServiceResponseFuture future) {
        auto result = future.get();
        RCLCPP_INFO(this->get_logger(), "Result of add_two_ints: %" PRId64, result->sum)
        rclcpp::shutdown();
      };
    auto future_result = client_->async_send_request(request, response_received_callback);
  }

I get the concept that this implementation will therfore permit a callback execution upon receipt of the response, unblocking execution.

However, given my limitied C++ experience, it is not clear how I can wrap my needed function and pass a callback as an argument.

For example, my current implementation is this:

int32_t TMC5130::SpiServiceClient::RegisterRead( uint8_t addr )
{
//Per the datasheet, reading a register requires:
//  1) Each datagram consists of 5 bytes; the first byte contains the target
//      address and a read/write flag as the MS bit.

std::vector<uint8_t> pkt1 = {addr & 0x7F, 0, 0, 0, 0 };
std::vector<uint8_t> pkt2 = {addr & 0x7F, 0, 0, 0, 0 };

auto request = std::make_shared<cpp_rpispi::srv::Spixfer::Request>();

request->tx = BytesToHexList( pkt1 );
request->tx += ",";
request->tx += BytesToHexList( pkt2 );

auto result = this->client_->async_send_request( request );

if( rclcpp::spin_until_future_complete( this->node_, result) == rclcpp::FutureReturnCode::SUCCESS )
{
    //RCLCPP_INFO(rclcpp::get_logger("rclcpp"), "rx: {%s}", result.get()->rx.c_str() );
}
else
{
    RCLCPP_ERROR(rclcpp::get_logger("rclcpp"), "failed");
}

//assume rx is a dual 5 byte datagram
//  [00,11,22,33,44],[55,66,77,88,99]
//  012345678901234567890123456789012
//  0         1         2         3
int32_t data3 = std::stoi( result.get()->rx.substr(21,2), nullptr, 16 );
int32_t data2 = std::stoi( result.get()->rx.substr(24,2), nullptr, 16 );
int32_t data1 = std::stoi( result.get()->rx.substr(27,2), nullptr, 16 );
int32_t data0 = std::stoi( result.get()->rx.substr(30,2), nullptr, 16 );

return (data3<<24 | data2<<16 | data1<<8 | data0 );
}

UPDATE ... (more)