Nuget.Server: Add read-through cache, fetching from other feeds (first-time) and caching locally

From @NickCraver on April 24, 2016 12:43

Nuget.Server does most of what we need to build Stack Overflow locally for our packages (mostly produced by TeamCity today). However, it could be much more useful with read-through caching. Here’s the flow:

• Client asks for a specific package version
  • Nuget.Server has it locally
    • Package returned
  • Nuget.Server doesn’t have it
    • If no cache-miss feeds configured, 404 (same as today)
    • If cache-miss feeds configured (new)
      • Nuget.Server fetches (in order, same as nuget.exe) from a configured feed list. (new)
        • If found, Nuget.Server caches it in the local folder and returns it to the client.
        • If not found, 404 (same as today).

It’s really far simpler to show this in code with a prototype. Here’s a fully functional example I made using nuget.org as the cache-miss source: https://github.com/NickCraver/NuGet.Server/commit/452af26550607c56825c38022d464b1422629509

I’d love to upstream a fleshed out version of this feature this because I think it’d be useful to many others: it’d save the world a little bit of bandwidth, dependency depth, and some build failures when the internet goes boom.

A few questions to start with:

• Is there a fundamental problem with the approach?
  • Our uses are simple so maybe we’re blind here.
• What’s the simplest config mechanism we could use for multiple feed sources?
  • appSettings is a bit limited here, but we could make it work.
• Will this be confusing, by nature only working on authenticated feeds (as a cache-miss source)?
  • IMO, auth delegation is confusing, complex, and not worth it here.

Copied from original issue: NuGet/Home#2650

[maartenba]

Recommending BaGet for this scenario.

[ghuntley]

@NickCraver http://stackoverflow.com/a/34959690