I tried this with the both 3.0.0-alpha.1 and 3.0.0-alpha.2 and in both the reports are made but the json files are empty.

Could it be because process.exit is sometimes used instead of process.exitCode?

UP, I have the same issue with 16 parallel builds. The one that builds last finishes all build process before webpack-bundle-analyzer starts so I miss bundle analysis report for that last built bundle.

@frvge I think that could help with the issue as recommended by documentation:

https://nodejs.org/api/process.html#process_process_exit_code

So, me and @byara found out that the processes are eagerly terminated without waiting for stuff like plugins to finish.

These particular lines in index.js caused the issue: https://github.com/trivago/parallel-webpack/blob/master/index.js#L141-L144

}).finally(function () {
    workerFarm.end(workers);
    process.removeListener('SIGINT', shutdownCallback);
});

Changing the lines to delay the killing of the workers made it possible for all (but not guaranteed) async operations to complete:

}).finally(function () {
    var __timeout = setTimeout(() => {
        clearTimeout(__timeout)
        workerFarm.end(workers);
        process.removeListener('SIGINT', shutdownCallback);
    }, 10000)
});

There must be a better way to detect when webpack has fully completed its job, then only exit. Maybe with some kind of compilation hook?

Update
I've pinned down the issue to webpack-bundle-analyzer and opened an issue here:
webpack-contrib/webpack-bundle-analyzer#232

I tried to apply what @liangchunn suggested here. Unfortunately, now the last reports are mostly "unfinished". It seems, if there was enough time and the stats file was small enough, they are created properly but if they are huge, webpack-bundle-analyzer makes stats files undone. As @liangchunn pointed out, with my changes, the reports are now made synchronously https://github.com/byara/webpack-bundle-analyzer/blob/master/src/BundleAnalyzerPlugin.js#L58-L69, so there must be other reasons to this behavior.

After the merge of the PR in v2.3.0 we have the option to pass --keep-alive-after-finish flag so parallel-webpack does not end the workers after the build is finished. This solves my problem, therefore I close the issue.