lerna run --parallel not working for rollup watch - rollupjs

Question

Background:
I have a lerna monorepo with yarn workspaces with two packages. I am using rollup as the bundler.
packages/module1/package.json:
{
scripts: {
"watch": "rollup -c rollup.config.js --watch",
"build": "NODE_ENV=production && rollup -c rollup.config.js"
}
}
packages/module2/package.json:
{
scripts: {
"watch": "rollup -c rollup.config.js --watch",
"build": "NODE_ENV=production && rollup -c rollup.config.js"
}
}
Expected Behavior:
lerna run build will run the build scripts for each package.
lerna run watch will run the watch scripts for each package in watch mode.
Current Behavior:
lerna run build works as expected. The build script runs properly for both packages.
lerna run watch just hangs there:
lerna notice cli v3.13.1
lerna info Executing command in 2 packages: "yarn run watch"
[[just hangs here]]
I have tried lerna run --parallel watch, and this only runs once. It exits after rollup completes. In other words, it never seems to be watching.

Answer 1

I believe the command you are looking for is lerna exec. This will run whatever command is passed to it over every package in your Monorepo.
lerna exec --parallel -- yarn build
If each package has the same build step, you could abstract it to the top level package.json like so:
lerna exec --parallel -- rollup -c=rollup.config.js
Which will go into each package and run that rollup command.
Sources:
Adding Rollup to a Monorepo
Creating a Monorepo with Lerna & Yarn Workspaces

Answer 2

It will need some tweaks to enable rollup to watch in parallel for the lerna monorepo.
lerna run --parallel watch
The code above will only run for one package and block the rest of the packages, and here is the code for the inner workings of the rollup. The following code snippet is the Watcher class constructor from the rollup github code base. As you can see, the watcher actually accept an array of configs. So now you only need to write some wrapper code to incorporate all your configs into one and then run the watch from the same config for all packages.
constructor(configs: GenericConfigObject[] | GenericConfigObject) {
this.emitter = new (class extends EventEmitter implements RollupWatcher {
close: () => void;
constructor(close: () => void) {
super();
this.close = close;
// Allows more than 10 bundles to be watched without
// showing the `MaxListenersExceededWarning` to the user.
this.setMaxListeners(Infinity);
}
})(this.close.bind(this));
this.tasks = (Array.isArray(configs) ? configs : configs ? [configs] : []).map(
config => new Task(this, config)
);
this.running = true;
process.nextTick(() => this.run());
}