Run NodeJS event loop / wait for child process to finish

Question

I first tried a general description of the problem, then some more detail why the usual approaches don\'t work. If you would like to read these abstracted explanations go on

Answer 1

Adding a third ( :) ) solution to your problem after you clarified what behavior you seek I suggest using Fibers.

Fibers let you do co-routines in nodejs. Coroutines are functions that allow multiple entry/exit points. This means you will be able to yield control and resume it as you please.

Here is a sleep function from the official documentation that does exactly that, sleep for a given amount of time and perform actions.

function sleep(ms) {
    var fiber = Fiber.current;
    setTimeout(function() {
        fiber.run();
    }, ms);
    Fiber.yield();
}

Fiber(function() {
    console.log('wait... ' + new Date);
    sleep(1000);
    console.log('ok... ' + new Date);
}).run();
console.log('back in main');

You can place the code that does the waiting for the resource in a function, causing it to yield and then run again when the task is done.

For example, adapting your example from the question:

var pausedExecution, importantData;
function getImportantData() {
    while (importantData === undefined) {
        pausedExecution = Fiber.current;
        Fiber.yield();
        pausedExecution = undefined;
    }

    if (importantData === null) {
        throw new Error("Data could not be generated.");
    } else {
        // we should have proper data now
        return importantData;
    }
}

function callback(partialDataMessage) {
    if (partialDataMessage.needsCorrection) {
        var theData = getImportantData();
        // use data to correct message
        process.send(correctedMessage); // send corrected result to main process
    } else {
        process.send(partialDataMessage); // send unmodified result to main process
    }
}

function executeCode(code) {
    // setup child process to calculate the data
    importantDataCalculator = fork("./runtime");
    importantDataCalculator.on("message", function (msg) {
        if (msg.type === "result") {
            importantData = msg.data;
        } else if (msg.type === "error") {
            importantData = null;
        } else {
            throw new Error("Unknown message from dataGenerator!");
        }

        if (pausedExecution) {
            // execution is waiting for the data
            pausedExecution.run();
        }
    });


    // wrap the execution of the code in a Fiber, so it can be paused
    Fiber(function () {
        runCodeWithCallback(code, callback); // the callback will be called from time to time when the code produces new data
        // this callback is synchronous and blocking,
        // but it will yield control to the event loop if it has to wait for the child-process to finish
    }).run();
}

Good luck! I always say it is better to solve one problem in 3 ways than solving 3 problems the same way. I'm glad we were able to work out something that worked for you. Admittingly, this was a pretty interesting question.