[Performance] Honor "Connection: keep-alive"

[probonopd]

Are we 100% sure that we honor "Connection: keep-alive"?

https://en.wikipedia.org/wiki/HTTP_persistent_connection

[probonopd]

Also see https://www.oreilly.com/library/view/http-the-definitive/1565925092/ch04s05.html, especially the section about "Persistent Versus Parallel Connections". Which strategy are we using?

[TheAssassin]

keep-alive doesn't have anything to do with parallel downloading. Yes, curl internally handles connection re-use, we do use that (I measured that ~ 1 year ago, and we checked the source code of our dependency on that, too).

[probonopd]

keep-alive and parallel are, I think, mutually exclusive. If you download everything in parallel, then you can't reuse any connections. I think we need to not download in parallel, but reuse connections.

[TheAssassin]

No, they're not mutually exclusive. It all depends on what you need to do. AppImageUpdate might have to download chunks from different regions. In that case, I can split up those chunks into smaller one, download those in parallel, but limit the amount of parallel connections and re-use them on further small chunks.

[probonopd]

In any case, we should do some speed testing, using the original zsync_curl and zsync2.