[rami3l]

Part of rust-lang/goals#736.

Related to #3937.

This is the first part of the real "Process Safety/Atomic Transaction" patch, in that it demonstrates where the problems really are when this feature is NOT landed.

It provides a reasonable explanation for recurring issues around #988 in its many incarnations:

• Interrupted transaction could get rustup stuck in loop #4400
• error: failed to install component: 'rust-src', detected conflict: 'lib/rustlib/src/rust/Cargo.lock' #3716
• error: failed to install component: 'cargo-x86_64-unknown-linux-gnu', detected conflict: 'bin/cargo' #4465
• error: the 'cargo' binary, normally provided by the 'cargo' component, is not applicable to the '1.78.0-x86_64-unknown-linux-gnu' toolchain rust-clippy#12763
• Rustup (including proxies) is not safe for concurrent use #988 (comment)
• Corrupt /missing manifests lead to installation conflicts #2417
• rustup-init fails when invoked in parallel #4607

Many thanks to @cachebag's testing framework, without which this would have been much more difficult.

This patch is currently in "draft" state since I am not sure if merging these tests as-is would bring any value. But in any case, the assertions in each of the tests should be flipped at the end of the feature patch.

Assisted by GitHub Copilot on Sonnet 5.5.