Re: [w3c/ServiceWorker] consider allowing multiple worker thread instances for a single registration (#756)

__My use case:__ App has multiple languages. Each can optionally be downloaded for offline use. Therefore, do not want to download all languages during SW install. App uses `postMessage` to tell SW when a new language should be added to the offline list. SW both downloads the new language and does a full update of all cached responses (in case API has changed and new language is using the new API).

__Where global variable comes in:__ I need a locking mechanism to prevent two updates happening at the same time. I could store a value in IDB, but what if that lock value is set to `true` and the update process fails without setting it back to `false`? Then no other update will happen as SW thinks one is still in progress. Instead, I plan to have a global `UPDATE_IN_PROGRESS` in SW, defaulting to `false`. Then, even if the update process dies, the value will reset to `false` when the SW is killed.

Is this a good or bad idea? And is there an alternative if SWs will allow multiple instances?

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