Upgrade from 0.2.175 to 0.2.176 breaks s390x build

[saschagrunert]

When upgrading libc from 0.2.175 to 0.2.176 in containers/conmon-rs#2770, then I got a build error for s390x builds:

error[E0308]: mismatched types
   --> /build/cargo-vendor-dir/nix-0.30.1/src/sys/statfs.rs:306:16
    |
306 |         FsType(self.0.f_type)
    |         ------ ^^^^^^^^^^^^^ expected `u64`, found `u32`
    |         |
    |         arguments to this struct are incorrect
    |
note: tuple struct defined here
   --> /build/cargo-vendor-dir/nix-0.30.1/src/sys/statfs.rs:94:12
    |
94  | pub struct FsType(pub fs_type_t);
    |            ^^^^^^
help: you can convert a `u32` to a `u64`
    |
306 |         FsType(self.0.f_type.into())
    |                             +++++++

error[E0308]: mismatched types
   --> /build/cargo-vendor-dir/nix-0.30.1/src/sys/statfs.rs:345:9
    |
344 |     pub fn optimal_transfer_size(&self) -> libc::c_ulong {
    |                                            ------------- expected `u64` because of return type
345 |         self.0.f_bsize
    |         ^^^^^^^^^^^^^^ expected `u64`, found `u32`
    |
help: you can convert a `u32` to a `u64`
    |
345 |         self.0.f_bsize.into()
    |                       +++++++

error[E0308]: mismatched types
   --> /build/cargo-vendor-dir/nix-0.30.1/src/sys/statfs.rs:401:9
    |
400 |     pub fn block_size(&self) -> libc::c_ulong {
    |                                 ------------- expected `u64` because of return type
401 |         self.0.f_bsize
    |         ^^^^^^^^^^^^^^ expected `u64`, found `u32`
    |
help: you can convert a `u32` to a `u64`
    |
401 |         self.0.f_bsize.into()
    |                       +++++++

error[E0308]: mismatched types
   --> /build/cargo-vendor-dir/nix-0.30.1/src/sys/statfs.rs:485:9
    |
484 |     pub fn maximum_name_length(&self) -> libc::c_ulong {
    |                                          ------------- expected `u64` because of return type
485 |         self.0.f_namelen
    |         ^^^^^^^^^^^^^^^^ expected `u64`, found `u32`
    |
help: you can convert a `u32` to a `u64`
    |
485 |         self.0.f_namelen.into()
    |                         +++++++

Could be related to #4549

[tgross35]

What is your target triple? Cc @neuschaefer

[Gelbpunkt]

cargoBuildHook flags: -j 4 --target s390x-unknown-linux-musl --offline --profile release

Looks like they're using the musl triple. The PR fixed the definitions of statfs to match musl, which is an acceptable breaking change given that otherwise you'd be using different field offsets and types than the ones in musl, which would very likely break at runtime.

[tgross35]

Ah yeah thanks, I didn't follow the links.

Indeed: the <= 0.2.175 version is incorrect and there's no way it's working right now, every field except f_type is at the wrong offset. Correctness fixes like this is allowed breakage, especially so since this is a tier 3 target. Nix will just need to update.

Note that change brings the musl struct in line with the glibc version so updating config should be straightforward.

[Gelbpunkt]

I'm currently patching nix and will PR a fix for the definitions there, just letting you know here so that the work isn't duplicated :) We'll probably encounter the fallout of this in Alpine once people update their dependencies.

[Gelbpunkt]

See nix-rust/nix#2678