Meson re-builds with system rustc on `sudo ninja install`

[gfxstrand]

Describe the bug
A clear and concise description of what the bug is.

To Reproduce

1. Install rust via rustup
2. Clone Mesa
3. Configure with -Dvulkan-drivers=nouveau
4. ninja -C _build
5. sudo ninja -C _build install

Expected behavior
The install shouldn't build anything and should just install

Actual behavior
Something goes searching for rustc, finds the one in /usr/bin, detects that it's different, and tries to re-build some of the rust components. This has two ill effects:

1. Your build directory has now been touched by root and you have to chown -R the whole thing if you want to touch it as a regular user again
2. The Rust build fails because the system rustc is not the same as the one you installed via rustup which is what the first build used.

[bonzini]

Something is ninja itself, though I'm not sure of what actually causes it to notice the change in PATH. But anyway, the best solution is to use meson install or ninja install without sudo, as it's not needed.

Maybe Meson should note down the uid it was run with, and warn if meson install is run as root but meson setup wasn't.

[eli-schwartz]

Something goes searching for rustc, finds the one in /usr/bin, detects that it's different, and tries to re-build some of the rust components.

Can you try running ninja with the -d explain arguments? It should tell you why it attempted to rebuild any given target.

But anyway, the best solution is to use meson install or ninja install without sudo, as it's not needed.

Yup, indeed, see https://mesonbuild.com/Installing.html#installing-as-the-superuser for details.

But note additionally that sudo meson install will actually do the right thing since meson 1.1.0, and print "Dropping privileges to $SUDO_USER before running ninja..." (and not only does it print that it is doing it, it actually does it! :D)

[eli-schwartz]

Maybe Meson should note down the uid it was run with, and warn if meson install is run as root but meson setup wasn't.

I'm not sure this makes sense these days, since meson officially supports any combination other than "sudo + ninja", and we don't control ninja. We'd have to somehow check inside meson install to see whether the parent process was ninja, as well as check the uid of said parent process.