[sui-system] skip stake subsidy distribution if epoch is short (Myste…

…nLabs#10500)

## Description 

If the length of an epoch is shorter than our set duration then skip
stake subsidy distribution.

## Test Plan 

added a test
---
If your changes are not user-facing and not a breaking change, you can
skip the following section. Otherwise, please indicate what changed, and
then add to the Release Notes section as highlighted during the release
process.

### Type of Change (Check all that apply)

- [ ] user-visible impact
- [ ] breaking change for a client SDKs
- [x] breaking change for FNs (FN binary must upgrade)
- [x] breaking change for validators or node operators (must upgrade
binaries)
- [ ] breaking change for on-chain data layout
- [ ] necessitate either a data wipe or data migration

### Release notes

crates/sui-framework/docs/sui_system_state_inner.md

@@ -2090,6 +2090,7 @@ gas coins.
     epoch_start_timestamp_ms: u64, // Timestamp of the epoch start
     ctx: &<b>mut</b> TxContext,
 ) : Balance&lt;SUI&gt; {
+    <b>let</b> prev_epoch_start_timestamp = self.epoch_start_timestamp_ms;
     self.epoch_start_timestamp_ms = epoch_start_timestamp_ms;
 
     <b>let</b> bps_denominator_u64 = (<a href="sui_system_state_inner.md#0x3_sui_system_state_inner_BASIS_POINT_DENOMINATOR">BASIS_POINT_DENOMINATOR</a> <b>as</b> u64);
@@ -2119,11 +2120,15 @@ gas coins.
 
     // Include stake subsidy in the rewards given out <b>to</b> validators and stakers.
     // Delay distributing any stake subsidies until after `stake_subsidy_start_epoch`.
-    <b>let</b> <a href="stake_subsidy.md#0x3_stake_subsidy">stake_subsidy</a> = <b>if</b> (<a href="_epoch">tx_context::epoch</a>(ctx) &gt;= self.parameters.stake_subsidy_start_epoch) {
-        <a href="stake_subsidy.md#0x3_stake_subsidy_advance_epoch">stake_subsidy::advance_epoch</a>(&<b>mut</b> self.<a href="stake_subsidy.md#0x3_stake_subsidy">stake_subsidy</a>)
-    } <b>else</b> {
-        <a href="_zero">balance::zero</a>()
-    };
+    // And <b>if</b> this epoch is shorter than the regular epoch duration, don't distribute any stake subsidy.
+    <b>let</b> <a href="stake_subsidy.md#0x3_stake_subsidy">stake_subsidy</a> =
+        <b>if</b> (<a href="_epoch">tx_context::epoch</a>(ctx) &gt;= self.parameters.stake_subsidy_start_epoch  &&
+            epoch_start_timestamp_ms &gt;= prev_epoch_start_timestamp + self.parameters.epoch_duration_ms)
+        {
+            <a href="stake_subsidy.md#0x3_stake_subsidy_advance_epoch">stake_subsidy::advance_epoch</a>(&<b>mut</b> self.<a href="stake_subsidy.md#0x3_stake_subsidy">stake_subsidy</a>)
+        } <b>else</b> {
+            <a href="_zero">balance::zero</a>()
+        };
 
     <b>let</b> stake_subsidy_amount = <a href="_value">balance::value</a>(&<a href="stake_subsidy.md#0x3_stake_subsidy">stake_subsidy</a>);
     <a href="_join">balance::join</a>(&<b>mut</b> computation_reward, <a href="stake_subsidy.md#0x3_stake_subsidy">stake_subsidy</a>);

crates/sui-framework/packages/sui-system/sources/stake_subsidy.move

@@ -90,4 +90,10 @@ module sui_system::stake_subsidy {
     public fun current_epoch_subsidy_amount(self: &StakeSubsidy): u64 {
         math::min(self.current_distribution_amount, balance::value(&self.balance))
     }
+
+    #[test_only]
+    /// Returns the number of distributions that have occurred.
+    public(friend) fun get_distribution_counter(self: &StakeSubsidy): u64 {
+        self.distribution_counter
+    }
 }

crates/sui-framework/packages/sui-system/sources/sui_system.move

@@ -65,6 +65,8 @@ module sui_system::sui_system {
 
     #[test_only]
     friend sui_system::governance_test_utils;
+    #[test_only]
+    friend sui_system::sui_system_tests;
 
     struct SuiSystemState has key {
         id: UID,
@@ -653,6 +655,12 @@ module sui_system::sui_system {
         sui_system_state_inner::get_storage_fund_object_rebates(self)
     }
 
+    #[test_only]
+    public fun get_stake_subsidy_distribution_counter(wrapper: &mut SuiSystemState): u64 {
+        let self = load_system_state(wrapper);
+        sui_system_state_inner::get_stake_subsidy_distribution_counter(self)
+    }
+
     // CAUTION: THIS CODE IS ONLY FOR TESTING AND THIS MACRO MUST NEVER EVER BE REMOVED.  Creates a
     // candidate validator - bypassing the proof of possession check and other metadata validation
     // in the process.

crates/sui-framework/packages/sui-system/sources/sui_system_state_inner.move

@@ -842,6 +842,7 @@ module sui_system::sui_system_state_inner {
         epoch_start_timestamp_ms: u64, // Timestamp of the epoch start
         ctx: &mut TxContext,
     ) : Balance<SUI> {
+        let prev_epoch_start_timestamp = self.epoch_start_timestamp_ms;
         self.epoch_start_timestamp_ms = epoch_start_timestamp_ms;
 
         let bps_denominator_u64 = (BASIS_POINT_DENOMINATOR as u64);
@@ -871,11 +872,15 @@ module sui_system::sui_system_state_inner {
 
         // Include stake subsidy in the rewards given out to validators and stakers.
         // Delay distributing any stake subsidies until after `stake_subsidy_start_epoch`.
-        let stake_subsidy = if (tx_context::epoch(ctx) >= self.parameters.stake_subsidy_start_epoch) {
-            stake_subsidy::advance_epoch(&mut self.stake_subsidy)
-        } else {
-            balance::zero()
-        };
+        // And if this epoch is shorter than the regular epoch duration, don't distribute any stake subsidy.
+        let stake_subsidy =
+            if (tx_context::epoch(ctx) >= self.parameters.stake_subsidy_start_epoch  &&
+                epoch_start_timestamp_ms >= prev_epoch_start_timestamp + self.parameters.epoch_duration_ms)
+            {
+                stake_subsidy::advance_epoch(&mut self.stake_subsidy)
+            } else {
+                balance::zero()
+            };
 
         let stake_subsidy_amount = balance::value(&stake_subsidy);
         balance::join(&mut computation_reward, stake_subsidy);
@@ -1074,6 +1079,11 @@ module sui_system::sui_system_state_inner {
         validator_set::get_candidate_validator_ref(validators(self), validator_address)
     }
 
+    #[test_only]
+    public(friend) fun get_stake_subsidy_distribution_counter(self: &SuiSystemStateInnerV2): u64 {
+        stake_subsidy::get_distribution_counter(&self.stake_subsidy)
+    }
+
     #[test_only]
     public(friend) fun set_epoch_for_testing(self: &mut SuiSystemStateInnerV2, epoch_num: u64) {
         self.epoch = epoch_num

crates/sui-framework/packages/sui-system/tests/sui_system_tests.move

@@ -20,11 +20,12 @@ module sui_system::sui_system_tests {
     use sui::table;
     use std::vector;
     use sui::balance;
-    use sui::test_utils::assert_eq;
+    use sui::test_utils::{assert_eq, destroy};
     use std::option::Self;
     use sui::url;
     use std::string;
     use std::ascii;
+    use sui::tx_context;
 
     #[test]
     fun test_report_validator() {
@@ -957,4 +958,34 @@ module sui_system::sui_system_tests {
         test_scenario::return_shared(system_state);
         test_scenario::end(scenario_val);
     }
+
+    #[test]
+    fun test_skip_stake_subsidy() {
+        let scenario_val = test_scenario::begin(@0x0);
+        let scenario = &mut scenario_val;
+        // Epoch duration is set to be 42 here.
+        set_up_sui_system_state(vector[@0x1, @0x2]);
+
+        // If the epoch length is less than 42 then the stake subsidy distribution counter should not be incremented. Otherwise it should.
+        advance_epoch_and_check_distribution_counter(scenario, 42, true);
+        advance_epoch_and_check_distribution_counter(scenario, 32, false);
+        advance_epoch_and_check_distribution_counter(scenario, 52, true);
+        test_scenario::end(scenario_val);
+    }
+
+    fun advance_epoch_and_check_distribution_counter(scenario: &mut Scenario, epoch_length: u64, should_increment_counter: bool) {
+        test_scenario::next_tx(scenario, @0x0);
+        let new_epoch = tx_context::epoch(test_scenario::ctx(scenario)) + 1;
+        let system_state = test_scenario::take_shared<SuiSystemState>(scenario);
+        let prev_epoch_time = sui_system::epoch_start_timestamp_ms(&mut system_state);
+        let prev_counter = sui_system::get_stake_subsidy_distribution_counter(&mut system_state);
+
+        let rebate = sui_system::advance_epoch_for_testing(
+            &mut system_state, new_epoch, 1, 0, 0, 0, 0, 0, 0, prev_epoch_time + epoch_length, test_scenario::ctx(scenario)
+        );
+        destroy(rebate);
+        assert_eq(sui_system::get_stake_subsidy_distribution_counter(&mut system_state), prev_counter + (if (should_increment_counter) 1 else 0));
+        test_scenario::return_shared(system_state);
+        test_scenario::next_epoch(scenario, @0x0);
+    }
 }

crates/sui-protocol-config/src/lib.rs

@@ -19,7 +19,8 @@ const MAX_PROTOCOL_VERSION: u64 = 4;
 // Version 3: gas model v2, including all sui conservation fixes. Fix for loaded child object
 //            changes, enable package upgrades, add limits on `max_size_written_objects`,
 //            `max_size_written_objects_system_tx`
-// Version 4: New reward slashing rate.
+// Version 4: New reward slashing rate. Framework changes to skip stake susbidy when the epoch
+//            length is short.
 
 #[derive(
     Copy, Clone, Debug, Hash, Serialize, Deserialize, PartialEq, Eq, PartialOrd, Ord, JsonSchema,