Currently, we only ever download a single update per Firefox session, but we are working on changing that (Bug 353804). One of the barriers is that we will need to create a new Downloader instance in order to download another update. Currently, we only store a single Downloader and we keep using it, even after the download has completed.

This means that to download another update, we either have to accommodate multiple Downloader instances, or get rid of them when we are done with them. Logically, a Downloader should only be needed while downloading, so the latter option seems preferable.

There are a few things that need to be done to allow this:

• Downloader.patchIsStaged needs to be refactored out of Downloader.
• Some usages of UpdateService._downloader will need to be changed to ensure that they still do the right thing when UpdateService._downloader isn't around. I took a quick look through these, and none of them look particularly problematic.
• After download is complete, we'll want to perform UpdateService._downloader = null;

I've identified an additional change that will be required. The download listeners should no longer be stored on the Downloader. Some of them expect to persist across multiple downloads (for example a failed partial patch download that falls back to a complete patch). That expectation would be broken if we delete the download listeners along with the Downloader. Storing them in UpdateService instead will ensure that these expectations continue to be met.

https://hg.mozilla.org/mozilla-central/rev/a1c555ecb161