stableswap: Add check for correct input asset denoms to joinPoolSharesInternal and updatePoolForJoin

[AlpinYukseloglu]

Background

Our current implementation of JoinPool for stableswap checks the number of input assets, but does not check what their specific denoms are. This allows for a scenario where arbitrary tokens can be sent into a pool, much like was the case in #1906.

However, unlike in #1906, our stableswap implementation has no implicit denom check when updating pool assets, so anything that is passed in is just directly added to the pool:

osmosis/x/gamm/pool-models/stableswap/pool.go

Lines 163 to 166 in 5879583

	func (p *Pool) updatePoolForJoin(tokensIn sdk.Coins, newShares sdk.Int) {
	p.PoolLiquidity = p.PoolLiquidity.Add(tokensIn...)
	p.TotalShares.Amount = p.TotalShares.Amount.Add(newShares)
	}

If it weren't for a division by zero here incidentally catching denom deposits for denoms not in the pool, this would be a critical bug as it would allow spam/poisoned tokens to be sent into a pool to crowd out and ultimately drain liquidity.

We should add a denom check both in joinPoolSharesInternal and, to cover broader future cases, also add a check to updatePoolForJoin to make sure joining a pool does not change the number of assets in it.

Suggested Design

• Expand the condition below to ensure that p.GetTotalPoolLiquidity() is a DenomsSubsetOf of TokensIn and vice versa (i.e. make sure denoms are equivalent):
  

  osmosis/x/gamm/pool-models/stableswap/amm.go

  Line 382 in 5879583

  } else if len(tokensIn) != p.NumAssets() {

• Add a length check similar to the following to updatePoolForJoin in stableswap/pool.go:
  

  osmosis/x/gamm/pool-models/stableswap/pool.go

  Lines 151 to 153 in 5879583

  	if len(p.PoolLiquidity) != numTokens {
  	panic("updatePoolLiquidityForSwap changed number of tokens in pool")
  	}

Acceptance Criteria

• Existing and new tests pass