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Safe desired targets call #12826

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Dec 10, 2022
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Safe desired targets call #12826

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d93cb65
checked call for desired targets
Ank4n Nov 29, 2022
62d6461
fix compile
Ank4n Dec 2, 2022
7a689a2
fmt
Ank4n Dec 2, 2022
bbc807e
fix tests
Ank4n Dec 2, 2022
f672412
cleaner with and_then
Ank4n Dec 8, 2022
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10 changes: 5 additions & 5 deletions frame/election-provider-multi-phase/src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1409,12 +1409,12 @@ impl<T: Config> Pallet<T> {
return Err(ElectionError::DataProvider("Snapshot too big for submission."))
}

let mut desired_targets =
T::DataProvider::desired_targets().map_err(ElectionError::DataProvider)?;
let mut desired_targets = <Pallet<T> as ElectionProviderBase>::desired_targets_checked()
.map_err(|e| ElectionError::DataProvider(e))?;

// If `desired_targets` > `targets.len()`, cap `desired_targets` to that
// level and emit a warning
let max_desired_targets: u32 = (targets.len() as u32).min(T::MaxWinners::get());
// If `desired_targets` > `targets.len()`, cap `desired_targets` to that level and emit a
// warning
let max_desired_targets: u32 = targets.len() as u32;
if desired_targets > max_desired_targets {
log!(
warn,
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10 changes: 6 additions & 4 deletions frame/election-provider-multi-phase/src/signed.rs
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Expand Up @@ -594,10 +594,12 @@ mod tests {
DesiredTargets::set(4);
MaxWinners::set(3);

let (_, _, actual_desired_targets) = MultiPhase::create_snapshot_external().unwrap();

// snapshot is created with min of desired_targets and MaxWinners
assert_eq!(actual_desired_targets, 3);
// snapshot not created because data provider returned an unexpected number of
// desired_targets
assert_noop!(
MultiPhase::create_snapshot_external(),
ElectionError::DataProvider("desired_targets must not be greater than MaxWinners."),
);
})
}

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2 changes: 1 addition & 1 deletion frame/election-provider-support/src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -391,7 +391,7 @@ pub trait ElectionProviderBase {
if desired_targets <= Self::MaxWinners::get() {
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Ok(desired_targets)
} else {
Err("desired_targets should not be greater than MaxWinners")
Err("desired_targets must not be greater than MaxWinners.")
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I am leaning towards throwing an error if desired_targets < MaxWinners. Alternatively, we can cap the desired_targets to MaxWinners silently.

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My feeling is that error-ing the snapshot creation because desired_targets < MaxWinners is probably too harsh. I think that capping the desired_targets to MaxWinners would be a good option, but I'd do it when in fn snapshot_creation_external() when handling the error from this call. And add a warning too. Something along the lines of:

fn create_snapshot_external(
  // ...
  let mut desired_targets = <Pallet<T> as ElectionProviderBase>::desired_targets_checked().or_else(|e| 
     match e {
        Error::DesiredTargetsGreaterThanMaxWinners => {
             // warning: capping the desired targets
             Ok(<Pallet<T> as ElectionProviderBase>::MaxWinners::get())
          }
          _ => Err(ElectionError::DataProvider(e)),
  })?;

  // ...

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@Ank4n Ank4n Dec 8, 2022 •
edited
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I have been split about this. We are already capping desired_targets to electable_targets so in that way it makes sense to cap desired_targets to MaxWinners as well.

But to present another perspective why this is a different case, electable_targets is a dynamic value that depends on total number of validator intentions while MaxWinners and desired_targets are static value configured in the runtime. If we consider this as a configuration issue, we should ideally fail till this configuration is fixed. We also have try_state checks to make sure we are not making this configuration error before it becomes part of the runtime.

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MaxWinners is the cap that should NEVER be reached, I think returning an error is better.

},
Err(e) => Err(e),
}
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