*: Add support for MIPS and PowerPC, which lack support for Atomic{U,I}64

[chitoku-k]

Signed-off-by: Chitoku odango@chitoku.jp

---

According to the official documentation, Rust does not provide std::sync::atomic::AtomicU64 and std::sync::atomic::AtomicI64 for MIPS and PowerPC with 32-bit pointers.

PowerPC and MIPS platforms with 32-bit pointers do not have AtomicU64 or AtomicI64 types.

See: https://doc.rust-lang.org/std/sync/atomic/

This means that prometheus-client even doesn't compile for those platforms:

[lychee  client_rust | master  $] cross build --release --target=mipsel-unknown-linux-gnu
   Compiling prometheus-client v0.15.0 (/project)
error[E0432]: unresolved import `std::sync::atomic::AtomicU64`
 --> src/metrics/counter.rs:7:36
  |
7 | use std::sync::atomic::{AtomicU32, AtomicU64, Ordering};
  |                                    ^^^^^^^^^
  |                                    |
  |                                    no `AtomicU64` in `sync::atomic`
  |                                    help: a similar name exists in the module: `AtomicU8`

error[E0432]: unresolved import `std::sync::atomic::AtomicU64`
 --> src/metrics/exemplar.rs:9:5
  |
9 | use std::sync::atomic::AtomicU64;
  |     ^^^^^^^^^^^^^^^^^^^---------
  |     |                  |
  |     |                  help: a similar name exists in the module: `AtomicU8`
  |     no `AtomicU64` in `sync::atomic`

error[E0432]: unresolved import `std::sync::atomic::AtomicU64`
 --> src/metrics/gauge.rs:7:36
  |
7 | use std::sync::atomic::{AtomicU32, AtomicU64, Ordering};
  |                                    ^^^^^^^^^
  |                                    |
  |                                    no `AtomicU64` in `sync::atomic`
  |                                    help: a similar name exists in the module: `AtomicU8`

For more information about this error, try `rustc --explain E0432`.
error: could not compile `prometheus-client` due to 3 previous errors

The atomic-shim crate provides alternative implementation for those platforms if the build target is MIPS or PowerPC, otherwise just re-exports std::sync::atomic::AtomicU64 and std::sync::atomic::AtomicI64. With this patch it successfully compiles.

[lychee  client_rust | support-mips-and-powerpc  $] cross build --release --target=mipsel-unknown-linux-gnu                                                   
    Updating crates.io index                                           
  Downloaded syn v1.0.86                                               
  Downloaded crossbeam-utils v0.8.6                                                                                                            
  Downloaded atomic-shim v0.2.0                                        
  Downloaded 3 crates (284.2 KB) in 0.90s                                                                                                      
   Compiling proc-macro2 v1.0.36                                                                                                               
   Compiling unicode-xid v0.2.2                                                                                                                
   Compiling crossbeam-utils v0.8.6                                    
   Compiling syn v1.0.86                                                                                                                       
   Compiling cfg-if v1.0.0                                             
   Compiling lazy_static v1.4.0                                        
   Compiling stable_deref_trait v1.2.0                                                                                                         
   Compiling itoa v1.0.1                                                                                                                       
   Compiling dtoa v1.0.2                                                                                                                       
   Compiling owning_ref v0.4.1                                                                                                                 
   Compiling atomic-shim v0.2.0                                                                                                                
   Compiling quote v1.0.14                                             
   Compiling prometheus-client-derive-text-encode v0.2.0 (/project/derive-text-encode)                                                         
   Compiling prometheus-client v0.15.0 (/project)                                                                                              
    Finished release [optimized] target(s) in 7.88s

[mxinden]

Hi @chitoku-k,

Thanks for the patch.

---

First of all, would you mind adding the targets that you would like to see supported to the cross-compile matrix in our CI workflow?

client_rust/.github/workflows/rust.yml

Lines 146 to 147 in f7dd611

	- armv7-unknown-linux-gnueabihf
	- powerpc64-unknown-linux-gnu

That way we ensure to never break compatibility.

---

Now to the patch itself:

I wonder whether we should support AtomicU64 through atomic-shim on PowerPC and MIPS platforms with 32-bit pointers. Instead, how about using AtomicU32 on PowerPC and MIPS platforms with 32-bit pointers by default and not offer AtomicU64 at all.

Benefits:

• No performance surprises for users on PowerPC and MIPS platforms with 32-bit pointers.
• No additional dependency and sub-dependencies through atomic-shim.
• Users that do need 64 bit counters can still use Counter with the AtomicU64 provided through atomic-shim.

Drawbacks:

• Potential counter overflows given the restricted 32 bits only.

I would argue, that with enough documentation around the drawback, the benefits outweigh it.

What do you think @chitoku-k?

[chitoku-k]

Thanks @mxinden for your effort made on the cross compilation set up, and your prompt review! I've added mipsel-unknown-linux-gnu that I'm currently dealing with to the target matrix.

---

For what you've pointed out about pros and cons over atomic-shim, it could really depend on what kind of metrics a user tries to expose in their implementation; however, well-known metrics related to UNIX timestamp (such as certification expiries) often want 64-bit integers so it is the best if a user can selectively choose whether to use 32-bits only (reasonable default) or atomic-shim for this use case by the use of Cargo features.

[mxinden]

so it is the best if a user can selectively choose whether to use 32-bits only (reasonable default) or atomic-shim for this use case by the use of Cargo features.

I don't think one should use Cargo features to toggle this behavior. Cargo features are additive across your entire dependency tree. Thus, if some upstream dependency of yours enables the prometheus-client XXX feature, it is enabled across the entire binary.

I would still suggest to use AtomicU32 on architectures that don't support AtomicU64.

In cases where one would still need an AtomicU64 one can provide it themselves, e.g. via atomic-shim.

use std::sync::atomic::Ordering;

struct AtomicU64(atomic_shim::AtomicU64);

impl prometheus_client::metrics::counter::Atomic<u64> for AtomicU64 {
    fn inc(&self) -> u64 {
        self.0.inc_by(1)
    }

    fn inc_by(&self, v: u64) -> u64 {
        self.0.fetch_add(v, Ordering::Relaxed)
    }

    fn get(&self) -> u64 {
        self.0.load(Ordering::Relaxed)
    }
}

fn main() {
    use prometheus_client::metrics::counter::Counter;
    let _counter: Counter<AtomicU64> = Counter::default();
}

@chitoku-k what do you think?

[chitoku-k]

Thank you for responding. As it was difficult to reach a consensus over this approach, I am closing this PR and will create a different PR that simply drops support for Atomic64 on those platforms which don't have its native equivalent type.