PacketFee asserts relayers must be nil causing invalid PacketFee types after json marshaling/unmarshaling

[colin-axner]

Summary of Bug

ICS29 PacketFee validation asserts that the relayers field must be nil. This causes packet fees with empty slices to fail validation.

Problem

We create PacketFee's with nil relayer fields (uninitialized slice), but JSON marshaling a PacketFee and unmarshaling it causes an empty slice to be created. Adding:

pf := types.NewPacketFees(packetFees)
bz := types.ModuleCdc.MustMarshalJSON(&pf)
newPf := types.PacketFees{}
types.ModuleCdc.MustUnmarshalJSON(bz, &newPf)
suite.Require().Equal(pf, newPf)

to 29-fee/keeper/keeper_test.go#TestFeesInEscrow causes:

--- FAIL: TestKeeperTestSuite (11.55s)
    --- FAIL: TestKeeperTestSuite/TestFeesInEscrow (0.14s)
        keeper_test.go:107: 
            	Error Trace:	keeper_test.go:107
            	Error:      	Not equal: 
            	            	expected: types.PacketFees{PacketFees:[]types.PacketFee{types.PacketFee{Fee:types.Fee{RecvFee:types.Coins{types.Coin{Denom:"stake", Amount:types.Int{i:(*big.Int)(0xc00051dda0)}}}, AckFee:types.Coins{types.Coin{Denom:"stake", Amount:types.Int{i:(*big.Int)(0xc00051ddc0)}}}, TimeoutFee:types.Coins{types.Coin{Denom:"stake", Amount:types.Int{i:(*big.Int)(0xc00051dde0)}}}}, RefundAddress:"cosmos16dt3rr3757spdyc3q2ctlx5fz03purt0ffn3t8", Relayers:[]string(nil)}, types.PacketFee{Fee:types.Fee{RecvFee:types.Coins{types.Coin{Denom:"stake", Amount:types.Int{i:(*big.Int)(0xc00051dda0)}}}, AckFee:types.Coins{types.Coin{Denom:"stake", Amount:types.Int{i:(*big.Int)(0xc00051ddc0)}}}, TimeoutFee:types.Coins{types.Coin{Denom:"stake", Amount:types.Int{i:(*big.Int)(0xc00051dde0)}}}}, RefundAddress:"cosmos16dt3rr3757spdyc3q2ctlx5fz03purt0ffn3t8", Relayers:[]string(nil)}, types.PacketFee{Fee:types.Fee{RecvFee:types.Coins{types.Coin{Denom:"stake", Amount:types.Int{i:(*big.Int)(0xc00051dda0)}}}, AckFee:types.Coins{types.Coin{Denom:"stake", Amount:types.Int{i:(*big.Int)(0xc00051ddc0)}}}, TimeoutFee:types.Coins{types.Coin{Denom:"stake", Amount:types.Int{i:(*big.Int)(0xc00051dde0)}}}}, RefundAddress:"cosmos16dt3rr3757spdyc3q2ctlx5fz03purt0ffn3t8", Relayers:[]string(nil)}, types.PacketFee{Fee:types.Fee{RecvFee:types.Coins{types.Coin{Denom:"stake", Amount:types.Int{i:(*big.Int)(0xc00051dda0)}}}, AckFee:types.Coins{types.Coin{Denom:"stake", Amount:types.Int{i:(*big.Int)(0xc00051ddc0)}}}, TimeoutFee:types.Coins{types.Coin{Denom:"stake", Amount:types.Int{i:(*big.Int)(0xc00051dde0)}}}}, RefundAddress:"cosmos16dt3rr3757spdyc3q2ctlx5fz03purt0ffn3t8", Relayers:[]string(nil)}, types.PacketFee{Fee:types.Fee{RecvFee:types.Coins{types.Coin{Denom:"stake", Amount:types.Int{i:(*big.Int)(0xc00051dda0)}}}, AckFee:types.Coins{types.Coin{Denom:"stake", Amount:types.Int{i:(*big.Int)(0xc00051ddc0)}}}, TimeoutFee:types.Coins{types.Coin{Denom:"stake", Amount:types.Int{i:(*big.Int)(0xc00051dde0)}}}}, RefundAddress:"cosmos16dt3rr3757spdyc3q2ctlx5fz03purt0ffn3t8", Relayers:[]string(nil)}}}
            	            	actual  : types.PacketFees{PacketFees:[]types.PacketFee{types.PacketFee{Fee:types.Fee{RecvFee:types.Coins{types.Coin{Denom:"stake", Amount:types.Int{i:(*big.Int)(0xc001d729a0)}}}, AckFee:types.Coins{types.Coin{Denom:"stake", Amount:types.Int{i:(*big.Int)(0xc001d729c0)}}}, TimeoutFee:types.Coins{types.Coin{Denom:"stake", Amount:types.Int{i:(*big.Int)(0xc001d72a00)}}}}, RefundAddress:"cosmos16dt3rr3757spdyc3q2ctlx5fz03purt0ffn3t8", Relayers:[]string{}}, types.PacketFee{Fee:types.Fee{RecvFee:types.Coins{types.Coin{Denom:"stake", Amount:types.Int{i:(*big.Int)(0xc001d72a20)}}}, AckFee:types.Coins{types.Coin{Denom:"stake", Amount:types.Int{i:(*big.Int)(0xc001d72a60)}}}, TimeoutFee:types.Coins{types.Coin{Denom:"stake", Amount:types.Int{i:(*big.Int)(0xc001d72a80)}}}}, RefundAddress:"cosmos16dt3rr3757spdyc3q2ctlx5fz03purt0ffn3t8", Relayers:[]string{}}, types.PacketFee{Fee:types.Fee{RecvFee:types.Coins{types.Coin{Denom:"stake", Amount:types.Int{i:(*big.Int)(0xc001d72ac0)}}}, AckFee:types.Coins{types.Coin{Denom:"stake", Amount:types.Int{i:(*big.Int)(0xc001d72ae0)}}}, TimeoutFee:types.Coins{types.Coin{Denom:"stake", Amount:types.Int{i:(*big.Int)(0xc001d72b20)}}}}, RefundAddress:"cosmos16dt3rr3757spdyc3q2ctlx5fz03purt0ffn3t8", Relayers:[]string{}}, types.PacketFee{Fee:types.Fee{RecvFee:types.Coins{types.Coin{Denom:"stake", Amount:types.Int{i:(*big.Int)(0xc001d72b40)}}}, AckFee:types.Coins{types.Coin{Denom:"stake", Amount:types.Int{i:(*big.Int)(0xc001d72b80)}}}, TimeoutFee:types.Coins{types.Coin{Denom:"stake", Amount:types.Int{i:(*big.Int)(0xc001d72ba0)}}}}, RefundAddress:"cosmos16dt3rr3757spdyc3q2ctlx5fz03purt0ffn3t8", Relayers:[]string{}}, types.PacketFee{Fee:types.Fee{RecvFee:types.Coins{types.Coin{Denom:"stake", Amount:types.Int{i:(*big.Int)(0xc001d72be0)}}}, AckFee:types.Coins{types.Coin{Denom:"stake", Amount:types.Int{i:(*big.Int)(0xc001d72c00)}}}, TimeoutFee:types.Coins{types.Coin{Denom:"stake", Amount:types.Int{i:(*big.Int)(0xc001d72c40)}}}}, RefundAddress:"cosmos16dt3rr3757spdyc3q2ctlx5fz03purt0ffn3t8", Relayers:[]string{}}}}

We can see the relayers field differs in each, one is Relayers:[]string(nil)} and the other is Relayers:[]string{}

I found this issue testing exporting/importing as importing failed basic validation since the packet fees exported were no longer considered valid

Solution

Treat empty Relayers slices as valid

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