I can't find a scenario that cannot be solved using node-fibers. The example you provided using node-fibers behaves as expected. The key is to run all the relevant code inside a fiber, so you don't have to start a new fiber in random positions.
Lets see an example: Say you use some framework, which is the entry point of your application (you cannot modify this framework). This framework loads nodejs modules as plugins, and calls some methods on the plugins. Lets say this framework only accepts synchronous functions, and does not use fibers by itself.
There is a library that you want to use in one of your plugins, but this library is async, and you don't want to modify it either.
The main thread cannot be yielded when no fiber is running, but you still can create plugins using fibers! Just create a wrapper entry that starts the whole framework inside a fiber, so you can yield the execution from the plugins.
Downside: If the framework uses setTimeout or Promises internally, then it will escape the fiber context. This can be worked around by mocking setTimeout, Promise.then, and all event handlers.
So this is how you can yield a fiber until a Promise is resolved. This code takes an async (Promise returning) function and resumes the fiber when the promise is resolved:
framework-entry.js
console.log(require("./my-plugin").run());
async-lib.js
exports.getValueAsync = () => {
return new Promise(resolve => {
setTimeout(() => {
resolve("Async Value");
}, 100);
});
};
my-plugin.js
const Fiber = require("fibers");
function fiberWaitFor(promiseOrValue) {
var fiber = Fiber.current, error, value;
Promise.resolve(promiseOrValue).then(v => {
error = false;
value = v;
fiber.run();
}, e => {
error = true;
value = e;
fiber.run();
});
Fiber.yield();
if (error) {
throw value;
} else {
return value;
}
}
const asyncLib = require("./async-lib");
exports.run = () => {
return fiberWaitFor(asyncLib.getValueAsync());
};
my-entry.js
require("fibers")(() => {
require("./framework-entry");
}).run();
When you run node framework-entry.js it will throw an error: Error: yield() called with no fiber running. If you run node my-entry.js it works as expected.