Allow the shielded reward precision for tokens to be explicitly set.

.changelog/unreleased/improvements/4423-configurable-masp-precision.md

@@ -0,0 +1,2 @@
+- Allow the shielded reward precision for tokens to be explicitly set via
+  governance proposals ([\#4423](https://github.com/anoma/namada/pull/4423))

Cargo.toml

@@ -189,8 +189,8 @@ libfuzzer-sys = "0.4"
 libloading = "0.8"
 linkme = "0.3"
 madato = "0.7"
-masp_primitives = { version = "1.2" }
-masp_proofs = { version = "1.2", default-features = false, features = ["local-prover"] }
+masp_primitives = { git = "https://github.com/anoma/masp", rev = "d38ab0e9bbf5b7161f8e951e5302b87da867aa4d" }
+masp_proofs = { git = "https://github.com/anoma/masp", rev = "d38ab0e9bbf5b7161f8e951e5302b87da867aa4d", default-features = false, features = ["local-prover"] }
 num256 = "0.6"
 num_cpus = "1.13"
 num_enum = "0.7"

crates/shielded_token/src/conversion.rs

@@ -34,7 +34,7 @@ use crate::storage_key::{masp_assets_hash_key, masp_token_map_key};
 use crate::storage_key::{
     masp_kd_gain_key, masp_kp_gain_key, masp_last_inflation_key,
     masp_last_locked_amount_key, masp_locked_amount_target_key,
-    masp_max_reward_rate_key,
+    masp_max_reward_rate_key, masp_reward_precision_key,
 };
 #[cfg(any(feature = "multicore", test))]
 use crate::{ConversionLeaf, Error, OptionExt, ResultExt};
@@ -87,21 +87,44 @@ pub fn calculate_masp_rewards_precision<S, TransToken>(
 ) -> Result<(u128, Denomination)>
 where
     S: StorageWrite + StorageRead,
-    TransToken: trans_token::Read<S>,
+    TransToken: trans_token::Keys + trans_token::Read<S>,
 {
     let denomination = TransToken::read_denom(storage, addr)?
         .expect("failed to read token denomination");
+
     // Inflation is implicitly denominated by this value. The lower this
     // figure, the less precise inflation computations are. This is especially
     // problematic when inflation is coming from a token with much higher
     // denomination than the native token. The higher this figure, the higher
     // the threshold of holdings required in order to receive non-zero rewards.
-    // This value should be fixed constant for each asset type. Here we choose
-    // a thousandth of the given asset.
-    let precision_denom = std::cmp::max(u32::from(denomination.0), 3)
-        .checked_sub(3)
-        .expect("Cannot underflow");
-    Ok((checked!(10u128 ^ precision_denom)?, denomination))
+    // This value should be fixed constant for each asset type. Here we read a
+    // value from storage and failing that we choose a thousandth of the given
+    // asset.
+    // Key to read/write reward precision from
+    let reward_precision_key = masp_reward_precision_key::<TransToken>(addr);
+    // Now read the desired reward precision for this token address
+    let reward_precision: u128 =
+        storage.read(&reward_precision_key)?.map_or_else(
+            || -> Result<u128> {
+                // Since reading reward precision has failed, choose a
+                // thousandth of the given token. But clamp the precision above
+                // by 10^38, the maximum power of 10 that can be contained by a
+                // u128.
+                let precision_denom =
+                    u32::from(denomination.0).saturating_sub(3).clamp(0, 38);
+                let reward_precision = checked!(10u128 ^ precision_denom)?;
+                // Record the precision that is now being used so that it does
+                // not have to be recomputed each time, and to
+                // ensure that this value is not accidentally
+                // changed even by a change to this initialization
+                // algorithm.
+                storage.write(&reward_precision_key, reward_precision)?;
+                Ok(reward_precision)
+            },
+            Ok,
+        )?;
+
+    Ok((reward_precision, denomination))
 }
 
 /// Compute the MASP rewards by applying the PD-controller to the genesis

crates/shielded_token/src/masp/shielded_wallet.rs

@@ -25,7 +25,7 @@ use masp_primitives::transaction::fees::fixed::FeeRule;
 use masp_primitives::transaction::{builder, Transaction};
 use masp_primitives::zip32::{ExtendedKey, PseudoExtendedKey};
 use namada_core::address::Address;
-use namada_core::arith::checked;
+use namada_core::arith::{checked, CheckedAdd, CheckedSub};
 use namada_core::borsh::{BorshDeserialize, BorshSerialize};
 use namada_core::chain::BlockHeight;
 use namada_core::collections::{HashMap, HashSet};
@@ -348,11 +348,24 @@ impl<U: ShieldedUtils + MaybeSend + MaybeSync> ShieldedWallet<U> {
         // Forget about the trace amount left over because we cannot
         // realize its value
         let trace = I128Sum::from_pair(asset_type, value % threshold);
-        // Record how much more of the given conversion has been used
-        *usage += required;
-        // Apply the conversions to input and move the trace amount to output
-        *input += conv * required - trace.clone();
-        *output += trace;
+        match checked!(input + &(conv * required) - &trace) {
+            // If applying the conversion does not overflow or result in
+            // negative input
+            Ok(new_input) if new_input >= I128Sum::zero() => {
+                // Record how much more of the given conversion has been used
+                *usage += required;
+                // Apply conversions to input and move trace amount to output
+                *input = new_input;
+                *output += trace;
+            }
+            _ => {
+                // Otherwise don't apply the conversion and simply move value
+                // over to output
+                let comp = I128Sum::from_pair(asset_type, value);
+                *output += comp.clone();
+                *input -= comp;
+            }
+        }
         Ok(())
     }
 

crates/shielded_token/src/storage_key.rs

@@ -36,6 +36,8 @@ pub const MASP_LOCKED_AMOUNT_TARGET_KEY: &str = "locked_amount_target";
 pub const MASP_MAX_REWARD_RATE_KEY: &str = "max_reward_rate";
 /// The key for the total inflation rewards minted by MASP
 pub const MASP_TOTAL_REWARDS: &str = "max_total_rewards";
+/// The key for the reward precision for a given asset
+pub const MASP_REWARD_PRECISION_KEY: &str = "reward_precision";
 
 /// Obtain the nominal proportional key for the given token
 pub fn masp_kp_gain_key<TransToken: trans_token::Keys>(
@@ -61,6 +63,14 @@ pub fn masp_max_reward_rate_key<TransToken: trans_token::Keys>(
         .with_segment(MASP_MAX_REWARD_RATE_KEY.to_owned())
 }
 
+/// The precision of rewards for the given token
+pub fn masp_reward_precision_key<TransToken: trans_token::Keys>(
+    token_addr: &Address,
+) -> storage::Key {
+    TransToken::parameter_prefix(token_addr)
+        .with_segment(MASP_REWARD_PRECISION_KEY.to_owned())
+}
+
 /// Obtain the locked target amount key for the given token
 pub fn masp_locked_amount_target_key<TransToken: trans_token::Keys>(
     token_addr: &Address,