Concurrent downloads & installation

[djc]

This is what I had in mind. It seems to compile but I've done no testing or benchmarking.

@FranciscoTGouveia maybe that's something you can have a look at?

Alternative to

• Interleave downloads with installations when downloading a toolchain #4471

Obsoletes

• Remove lifetime parameters to allow multi-threaded installations #4567

[FranciscoTGouveia]

It compiles, but I believe there is an infinite loop; at least, I could not get any toolchain installation to finish as it seemed to hang on the download of the last component.

Additionally, I would like to note that when a component is being installed, although ongoing downloads continue normally, new component downloads cannot start.
As a result, during an installation (assuming RUSTUP_CONCURRENT_DOWNLOADS=2, which is the default), no more than one download is active at a time.

This isn’t an issue when RUSTUP_CONCURRENT_DOWNLOADS is configured to maximize concurrency, but otherwise, it can have a noticeable impact on performance (I wasn’t able to benchmark this due to the infinite loop).

Finally, I would like to point out that running both downloads and installations on the same thread prevents the progress bars for other downloads from updating (see here).

[djc]

I've fixed this up so that it can at least pass basic manual tests. CI is failing because I changed the log output -- happy to fix this up if we decide that we want to move forward with this.

[rami3l]

I think this works out pretty well during both code inspection and manual tests, so I consider this to be a somewhat faithful reproduction of the first step of #4471. Given that, I agree with merging this despite the fact that the project will suffer from temporary UX regressions.

I'd like to invite @FranciscoTGouveia do another round of review from his side to make sure everything is retained from the original solution.

[djc]

Here's what I think we could do next to solve the remaining UX problem and even further improve the UX:

• Update the progress indicator during unpacking, using a ProgressBarIter wrapper
• Update the progress indicator during installing, by making installation "async", maybe by calling yield_now() every 250ms or so from DirectoryPackage::install()

(A more principled approach might be to use tokio::fs but because there's a lot of layers in our install process that's probably a bunch of work, if not exactly hard.)