feat: implement subnet EMA reset mechanism

This commit implements a mechanism for subnet owners to reset negative
EMA values that prevent their subnet from receiving emissions.

Changes:
- Add MaxEmaResetCost storage item (default: 100 TAO)
- Add reset_subnet_ema extrinsic (call_index: 125)
- Add get_ema_reset_cost helper function
- Add EmaNotInitialized, SubnetEmaNotNegative, NotEnoughBalanceToPayEmaResetCost errors
- Add SubnetEmaReset event with netuid, who, cost, previous_ema
- Add comprehensive tests for the new functionality

The reset cost is calculated as |EMA| × (1/α), capped at MaxEmaResetCost.
The burned TAO is recycled to reduce total issuance.

Closes #2228

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: calm329

pallets/subtensor/src/coinbase/subnet_emissions.rs

@@ -1,8 +1,10 @@
 use super::*;
 use alloc::collections::BTreeMap;
+use frame_support::dispatch::DispatchResult;
 use safe_math::FixedExt;
 use substrate_fixed::transcendental::{exp, ln};
 use substrate_fixed::types::{I32F32, I64F64, U64F64, U96F32};
+use subtensor_runtime_common::TaoCurrency;
 
 impl<T: Config> Pallet<T> {
     pub fn get_subnets_to_emit_to(subnets: &[NetUid]) -> Vec<NetUid> {
@@ -246,4 +248,104 @@ impl<T: Config> Pallet<T> {
             })
             .collect::<BTreeMap<NetUid, U64F64>>()
     }
+
+    /// Calculates the cost to reset a subnet's EMA to zero.
+    ///
+    /// The cost formula is: |EMA| × (1/α), capped at MaxEmaResetCost.
+    /// Where α is the FlowEmaSmoothingFactor normalized by i64::MAX.
+    ///
+    /// Returns the cost in RAO (TaoCurrency), or None if EMA is not negative.
+    pub fn get_ema_reset_cost(netuid: NetUid) -> Option<TaoCurrency> {
+        // Get the current EMA value
+        let (_, ema) = SubnetEmaTaoFlow::<T>::get(netuid)?;
+
+        // Only allow reset if EMA is negative
+        if ema >= I64F64::saturating_from_num(0) {
+            return None;
+        }
+
+        // Get the absolute value of EMA
+        let abs_ema = ema.saturating_abs();
+
+        // Get the smoothing factor (alpha) and normalize it
+        // FlowEmaSmoothingFactor is stored as u64 normalized by i64::MAX (2^63)
+        let alpha_normalized = FlowEmaSmoothingFactor::<T>::get();
+
+        // Cost = |EMA| × (1/α) = |EMA| × (i64::MAX / alpha_normalized)
+        // This can overflow, so we need to be careful with the calculation
+        let i64_max = I64F64::saturating_from_num(i64::MAX);
+        let alpha = I64F64::saturating_from_num(alpha_normalized).safe_div(i64_max);
+
+        // Calculate cost = |EMA| / alpha
+        let cost_raw = abs_ema.safe_div(alpha);
+
+        // Convert to u64 (RAO)
+        let cost_rao = cost_raw
+            .checked_to_num::<u64>()
+            .unwrap_or(u64::MAX);
+
+        // Cap at MaxEmaResetCost
+        let max_cost = MaxEmaResetCost::<T>::get();
+        let cost = TaoCurrency::from(cost_rao).min(max_cost);
+
+        Some(cost)
+    }
+
+    /// Resets the subnet EMA to zero by burning TAO.
+    ///
+    /// This function allows subnet owners to reset negative EMA values that
+    /// prevent their subnet from receiving emissions.
+    pub fn do_reset_subnet_ema(origin: T::RuntimeOrigin, netuid: NetUid) -> DispatchResult {
+        // Ensure the subnet exists
+        ensure!(Self::if_subnet_exist(netuid), Error::<T>::SubnetNotExists);
+
+        // Ensure the caller is the subnet owner
+        let who = Self::ensure_subnet_owner(origin, netuid)?;
+
+        // Get the current EMA value - check if initialized
+        let (_, previous_ema) = SubnetEmaTaoFlow::<T>::get(netuid)
+            .ok_or(Error::<T>::EmaNotInitialized)?;
+
+        // Ensure EMA is negative
+        ensure!(
+            previous_ema < I64F64::saturating_from_num(0),
+            Error::<T>::SubnetEmaNotNegative
+        );
+
+        // Get the reset cost
+        let cost = Self::get_ema_reset_cost(netuid)
+            .ok_or(Error::<T>::SubnetEmaNotNegative)?;
+
+        // Ensure the owner has enough balance
+        ensure!(
+            Self::can_remove_balance_from_coldkey_account(&who, cost.into()),
+            Error::<T>::NotEnoughBalanceToPayEmaResetCost
+        );
+
+        // Remove the balance from the owner's account
+        let actual_cost = Self::remove_balance_from_coldkey_account(&who, cost.into())?;
+
+        // Burn the TAO (reduce total issuance)
+        Self::recycle_tao(actual_cost);
+
+        // Reset the EMA to zero
+        let current_block = Self::get_current_block_as_u64();
+        SubnetEmaTaoFlow::<T>::insert(netuid, (current_block, I64F64::saturating_from_num(0)));
+
+        // Also reset the accumulated flow
+        Self::reset_tao_outflow(netuid);
+
+        // Convert previous_ema to i128 for the event (I64F64 is 128 bits total)
+        let previous_ema_bits = previous_ema.to_bits();
+
+        // Emit the event
+        Self::deposit_event(Event::SubnetEmaReset {
+            netuid,
+            who,
+            cost: actual_cost,
+            previous_ema: previous_ema_bits,
+        });
+
+        Ok(())
+    }
 }

pallets/subtensor/src/lib.rs

@@ -1493,6 +1493,16 @@ pub mod pallet {
     pub type FlowEmaSmoothingFactor<T: Config> =
         StorageValue<_, u64, ValueQuery, DefaultFlowEmaSmoothingFactor<T>>;
 
+    #[pallet::type_value]
+    /// Default maximum cost for resetting subnet EMA (100 TAO in RAO).
+    pub fn DefaultMaxEmaResetCost<T: Config>() -> TaoCurrency {
+        TaoCurrency::from(100_000_000_000u64) // 100 TAO
+    }
+    #[pallet::storage]
+    /// --- ITEM --> Maximum cost for resetting subnet EMA
+    pub type MaxEmaResetCost<T: Config> =
+        StorageValue<_, TaoCurrency, ValueQuery, DefaultMaxEmaResetCost<T>>;
+
     /// ============================
     /// ==== Global Parameters =====
     /// ============================

pallets/subtensor/src/macros/dispatches.rs

@@ -2432,5 +2432,35 @@ mod dispatches {
 
             Ok(())
         }
+
+        /// --- Resets the subnet EMA to zero by burning TAO.
+        ///
+        /// This extrinsic allows subnet owners to reset negative EMA values that
+        /// prevent their subnet from receiving emissions. The cost is proportional
+        /// to |EMA| × (1/α), capped at MaxEmaResetCost.
+        ///
+        /// # Arguments
+        /// * `origin` - The origin of the call, must be the subnet owner.
+        /// * `netuid` - The network identifier of the subnet to reset.
+        ///
+        /// # Errors
+        /// * `SubnetNotExists` - The subnet does not exist.
+        /// * `EmaNotInitialized` - The subnet EMA has not been initialized.
+        /// * `SubnetEmaNotNegative` - The subnet EMA is not negative, reset not needed.
+        /// * `NotEnoughBalanceToPayEmaResetCost` - Insufficient balance to pay reset cost.
+        ///
+        /// # Events
+        /// Emits a `SubnetEmaReset` event on success.
+        #[pallet::call_index(125)]
+        #[pallet::weight((
+            Weight::from_parts(35_000_000, 0)
+                .saturating_add(T::DbWeight::get().reads(5_u64))
+                .saturating_add(T::DbWeight::get().writes(3_u64)),
+            DispatchClass::Normal,
+            Pays::Yes
+        ))]
+        pub fn reset_subnet_ema(origin: OriginFor<T>, netuid: NetUid) -> DispatchResult {
+            Self::do_reset_subnet_ema(origin, netuid)
+        }
     }
 }

pallets/subtensor/src/macros/errors.rs

@@ -268,5 +268,11 @@ mod errors {
         InvalidSubnetNumber,
         /// Unintended precision loss when unstaking alpha
         PrecisionLoss,
+        /// EMA has not been initialized for this subnet.
+        EmaNotInitialized,
+        /// Subnet EMA is not negative, reset not needed.
+        SubnetEmaNotNegative,
+        /// Not enough balance to pay the EMA reset cost.
+        NotEnoughBalanceToPayEmaResetCost,
     }
 }

pallets/subtensor/src/macros/events.rs

@@ -479,5 +479,17 @@ mod events {
             /// The amount of alpha distributed
             alpha: AlphaCurrency,
         },
+
+        /// Subnet EMA has been reset by the owner.
+        SubnetEmaReset {
+            /// The subnet ID
+            netuid: NetUid,
+            /// The account that executed the reset
+            who: T::AccountId,
+            /// The cost paid for the reset in TAO
+            cost: TaoCurrency,
+            /// The previous EMA value before reset (as i128 bits representation of I64F64)
+            previous_ema: i128,
+        },
     }
 }