Sharing dynamically dispatched registry in web framework state

[sd2k]

I may be missing something but it seems like it's not possible to use the Registry with the default M = SendEncodeMetric type as shared state in web frameworks (e.g. tide::with_state), because those require that the state is all Sync but SendEncodeMetric is not Sync. If I update the tide example to use a dynamically dispatched Registry then it fails to compile:

✦ ❯ cargo c --example tide
error[E0277]: `(dyn SendEncodeMetric + 'static)` cannot be shared between threads safely
   --> examples/tide.rs:25:36
    |
25  |       let mut app = tide::with_state(State {
    |  ___________________----------------_^
    | |                   |
    | |                   required by a bound introduced by this call
26  | |         registry: Arc::new(registry),
27  | |     });
    | |_____^ `(dyn SendEncodeMetric + 'static)` cannot be shared between threads safely
    |
    = help: the trait `Sync` is not implemented for `(dyn SendEncodeMetric + 'static)`
    = note: required because of the requirements on the impl of `Sync` for `Unique<(dyn SendEncodeMetric + 'static)>`
    = note: required because it appears within the type `Box<(dyn SendEncodeMetric + 'static)>`
    = note: required because it appears within the type `(Descriptor, Box<(dyn SendEncodeMetric + 'static)>)`
    = note: required because of the requirements on the impl of `Sync` for `Unique<(Descriptor, Box<(dyn SendEncodeMetric + 'static)>)>`
    = note: required because it appears within the type `alloc::raw_vec::RawVec<(Descriptor, Box<(dyn SendEncodeMetric + 'static)>)>`
    = note: required because it appears within the type `Vec<(Descriptor, Box<(dyn SendEncodeMetric + 'static)>)>`
    = note: required because it appears within the type `Registry`
    = note: required because of the requirements on the impl of `std::marker::Send` for `Arc<Registry>`
note: required because it appears within the type `State`
   --> examples/tide.rs:59:8
    |
59  | struct State {
    |        ^^^^^
note: required by a bound in `with_state`
   --> /Users/ben/.cargo/registry/src/github.com-1ecc6299db9ec823/tide-0.16.0/src/lib.rs:153:20
    |
153 |     State: Clone + Send + Sync + 'static,
    |                    ^^^^ required by this bound in `with_state`

<several related errors omitted>

It's fixed by adding a Sync bound to SendEncodeMetric but it feels like this is a common use case so I imagine it's come up before and I'm doing something silly - am I?

Thanks!

[mxinden]

Good catch. Thanks @sd2k. This is related to #55. E.g. when you wrap the Registry in both an Arc and a Mutex, it would compile.

It's fixed by adding a Sync bound to SendEncodeMetric but it feels like this is a common use case so I imagine it's come up before and I'm doing something silly - am I?

I would be OK, extending the super trait SendEncodeMetric to SendSyncEncodeMetric. I expect most folks would run prometheus-client in a multi-threded environment with synchronization on each metric anyways. I still want to give folks the chance to not have to pay in a single threaded setup, though that they can still do overriding the default trait parameter on Registry.

If you agree, want to provide a patch @sd2k?

While wrapping Registry in a Mutex and an Arc (see above) is an option, I would prefer to simpler Arc wrapper only.

[sd2k]

Sure @mxinden, sounds good to me. I've created #58 which should hopefully do the trick.

[Jc2k]

Any chance of getting a release with this fix in? It's also biting me on a Rocket project.

[mxinden]

@Jc2k see #59

[Jc2k]

TYVM!