Possible race condition when adding target while updating with rustup

[spease]

The armv7-unknown-linux-gnueabihf target didn't initially exist, and I wanted to cross-compile a crate.

In one terminal window, I did
rustup update

In another terminal window, I did
rustup target add armv7-unknown-linux-gnueabihf

When both had completed, I attempted
cargo build --target=armv7-unknown-linux-gnueabihf

Resulting in

   Compiling trigger v0.1.0 (file:///home/spease/projects/carlos/trigger)
error[E0514]: found crate `std` compiled by an incompatible version of rustc
  |
  = help: please recompile that crate using this compiler (rustc 1.23.0 (766bd11c8 2018-01-01))
  = note: crate `std` path #1: /home/spease/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/armv7-unknown-linux-gnueabihf/lib/libstd-827ddea93328ae2d.rlib compiled by "rustc 1.22.1 (05e2e1c41 2017-11-22)"
  = note: crate `std` path #2: /home/spease/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/armv7-unknown-linux-gnueabihf/lib/libstd-827ddea93328ae2d.so compiled by "rustc 1.22.1 (05e2e1c41 2017-11-22)"
  = note: crate `std` path #3: /home/spease/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/armv7-unknown-linux-gnueabihf/lib/libstd_unicode-0e1b544c94586415.rlib compiled by "rustc 1.22.1 (05e2e1c41 2017-11-22)"

error: aborting due to previous error

error: Could not compile `trigger`.

Interestingly enough, it wasn't included in the list of available targets or components either even though it seemed to install successfully.

Trying to re-add it gave the following output:

info: downloading component 'rust-std' for 'armv7-unknown-linux-gnueabihf'
info: installing component 'rust-std' for 'armv7-unknown-linux-gnueabihf'
info: rolling back changes
error: failed to install component: 'rust-std-armv7-unknown-linux-gnueabihf', detected conflict: '"lib/rustlib/manifest-rust-std-armv7-unknown-linux-gnueabihf"'

Eventually I was able to reinstall after renaming
~/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/manifest-rust-std-armv7-unknown-linux-gnueabihf

At this point everything seems to work correctly. However my understanding was that rustup / cargo are generally supposed to be safe via a locking mechanism? Or is this just cargo?

[Diggsey]

Dup of #988