Fix #7640: tally calculation precision error (#7641)

Solution:
- change `(a / b) * c` to `a * b / c`

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

CHANGELOG.md

@@ -40,7 +40,7 @@ Ref: https://keepachangelog.com/en/1.0.0/
 
 * (x/staking) [\#7499](https://github.com/cosmos/cosmos-sdk/pull/7499) `BondStatus` is now a protobuf `enum` instead of an `int32`, and JSON serialized using its protobuf name, so expect names like `BOND_STATUS_UNBONDING` as opposed to `Unbonding`.
 * (x/evidence) [\#7538](https://github.com/cosmos/cosmos-sdk/pull/7538) The ABCI's `Result.Data` field of `MsgSubmitEvidence` does not contain the raw evidence's hash, but the encoded `MsgSubmitEvidenceResponse` struct.
-* (x/upgrade) [#7697](https://github.com/cosmos/cosmos-sdk/pull/7697) Rename flag name "--time" to "--upgrade-time", "--info" to "--upgrade-info", to keep it consistent with help message.
+* (x/upgrade) [#7697](https://github.com/cosmos/cosmos-sdk/pull/7697) Rename flag name "--time" to "--upgrade-time", "--info" to "--upgrade-info", to keep it consistent with help message.
 
 ### API Breaking
 
@@ -63,6 +63,7 @@ Ref: https://keepachangelog.com/en/1.0.0/
 
 * (kvstore) [\#7415](https://github.com/cosmos/cosmos-sdk/pull/7415) Allow new stores to be registered during on-chain upgrades.
 * (client) [\#7699](https://github.com/cosmos/cosmos-sdk/pull/7699) Fix panic in context when setting invalid nodeURI. `WithNodeURI` does not set the `Client` in the context.
+* (x/gov) [#7641](https://github.com/cosmos/cosmos-sdk/pull/7641) Fix tally calculation precision error.
 
 
 ## [v0.40.0-rc0](https://github.com/cosmos/cosmos-sdk/releases/tag/v0.40.0-rc0) - 2020-10-13
@@ -653,6 +654,7 @@ generalized genesis accounts through the `GenesisAccount` interface.
 * (sdk) [\#4758](https://github.com/cosmos/cosmos-sdk/issues/4758) update `x/genaccounts` to match module spec
 * (simulation) [\#4824](https://github.com/cosmos/cosmos-sdk/issues/4824) `PrintAllInvariants` flag will print all failed invariants
 * (simulation) [\#4490](https://github.com/cosmos/cosmos-sdk/issues/4490) add `InitialBlockHeight` flag to resume a simulation from a given block
+  
   * Support exporting the simulation stats to a given JSON file
 * (simulation) [\#4847](https://github.com/cosmos/cosmos-sdk/issues/4847), [\#4838](https://github.com/cosmos/cosmos-sdk/pull/4838) and [\#4869](https://github.com/cosmos/cosmos-sdk/pull/4869) `SimApp` and simulation refactors:
   * Implement `SimulationManager` for executing modules' simulation functionalities in a modularized way
@@ -966,6 +968,7 @@ that error is that the account doesn't exist.
 * (simulation) PrintAllInvariants flag will print all failed invariants
 * (simulation) Add `InitialBlockHeight` flag to resume a simulation from a given block
 * (simulation) [\#4670](https://github.com/cosmos/cosmos-sdk/issues/4670) Update simulation statistics to JSON format
+  
   - Support exporting the simulation stats to a given JSON file
 * [\#4775](https://github.com/cosmos/cosmos-sdk/issues/4775) Refactor CI config
 * Upgrade IAVL to v0.12.4
@@ -1571,7 +1574,8 @@ BREAKING CHANGES
 FEATURES
 
 * Gaia REST API
-  * [\#2358](https://github.com/cosmos/cosmos-sdk/issues/2358) Add distribution module REST interface
+  
+* [\#2358](https://github.com/cosmos/cosmos-sdk/issues/2358) Add distribution module REST interface
   
 * Gaia CLI  (`gaiacli`)
   * [\#3429](https://github.com/cosmos/cosmos-sdk/issues/3429) Support querying

types/decimal_test.go

@@ -478,6 +478,14 @@ func (s *decimalTestSuite) TestDecEncoding() {
 	}
 }
 
+// Showcase that different orders of operations causes different results.
+func (s *decimalTestSuite) TestOperationOrders() {
+	n1 := sdk.NewDec(10)
+	n2 := sdk.NewDec(1000000010)
+	s.Require().Equal(n1.Mul(n2).Quo(n2), sdk.NewDec(10))
+	s.Require().NotEqual(n1.Mul(n2).Quo(n2), n1.Quo(n2).Mul(n2))
+}
+
 func BenchmarkMarshalTo(b *testing.B) {
 	bis := []struct {
 		in   sdk.Dec

x/gov/keeper/tally.go

@@ -57,8 +57,8 @@ func (keeper Keeper) Tally(ctx sdk.Context, proposal types.Proposal) (passes boo
 				val.DelegatorDeductions = val.DelegatorDeductions.Add(delegation.GetShares())
 				currValidators[valAddrStr] = val
 
-				delegatorShare := delegation.GetShares().Quo(val.DelegatorShares)
+				// delegation shares * bonded / total shares
-				votingPower := delegatorShare.MulInt(val.BondedTokens)
+				votingPower := delegation.GetShares().MulInt(val.BondedTokens).Quo(val.DelegatorShares)
 
 				results[vote.Option] = results[vote.Option].Add(votingPower)
 				totalVotingPower = totalVotingPower.Add(votingPower)
@@ -78,8 +78,7 @@ func (keeper Keeper) Tally(ctx sdk.Context, proposal types.Proposal) (passes boo
 		}
 
 		sharesAfterDeductions := val.DelegatorShares.Sub(val.DelegatorDeductions)
-		fractionAfterDeductions := sharesAfterDeductions.Quo(val.DelegatorShares)
+		votingPower := sharesAfterDeductions.MulInt(val.BondedTokens).Quo(val.DelegatorShares)
-		votingPower := fractionAfterDeductions.MulInt(val.BondedTokens)
 
 		results[val.Vote] = results[val.Vote].Add(votingPower)
 		totalVotingPower = totalVotingPower.Add(votingPower)