`rustup update` should run self update first

[killercup]

Currently, running rustup update updates all toolchains and then runs self update.

It might be wise to run self update first, to pick up

• possible changes in the metadata format,
• security fixes for downloading,
• new compression options,
• and other improvements

as soon as possible and use them for updating the toolchain.

One difficulty might be that rustup will need to restart itself/start the new version when there was an update before running the toolchain updates.

[brson]

I agree that it would be better to do the self-update first, but it's very tricky. On windows, we can't replace the rustup executable while rustup is running, so the last thing rustup update does is exit successfully, then a separate process overwrites rustup.exe. So there's a chance the update actually fails but doesn't get reported. If we were to do incompatible metadata changes before overwriting the exe, and then the self-update fails, rustup could be left completely broken.

[Diggsey]

You can rename a running process though, so we can have rustup download the new version, rename itself to a temp directory, call the new rustup with the original update command line (piping stdin/stdout through the old rustup), and then delete itself. If the self deletion fails we get a left-over executable in your temp directory rather than a broken installation.

[brson]

Ah yes that does seem doable.

[vks]

Not running rustup self update first seems to have caused upgrades to fail for a lot of people.