introducing new vbank assets results in O(n) work to update smart wallets

[dckc]

Describe the bug

As noted in a Dec 5 comment on #6625, when proposal 17 to add DAI passed and 2 new vbank issues were added, each smart wallet woke up, added a purse, and published its new state to vstorage:

agoric-sdk/packages/smart-wallet/src/smartWallet.js

Lines 523 to 524 in 2c812d2

	// watch the bank for new issuers to make purses out of
	void observeIteration(E(bank).getAssetSubscription(), {

This is clearly not consistent with our Limit work by message size design constraint.

To Reproduce

Steps to reproduce the behavior:

1. Add several hundred wallets
2. call E(vbank).addAsset(...)
3. see hundreds of purses created and hundreds of vstorage updates

Expected behavior

TBD, but something that respects our Limit work by message size design constraint.

RPC clients should be able to get the list of vbank brands without relying on updates to individual wallets:

• expose VBANK brand list via chainStorage RPC #5762

design discussion: IBIS: How to introduce new VBank Issuers to Wallets - Google Docs

Platform Environment

mainnet. pismoA

[dckc]

Failing test

ea697a5 cc @turadg @arirubinstein

~/projects/agoric-sdk/packages/smart-wallet$ yarn test test/test-addAsset.js 
...
  ✔ [expected fail] avoid O(wallets) storage writes for a new asset
    ℹ {
        base: {
          wallets: 2,
          writes: 4,
        },
        test: {
          wallets: 8,
          writes: 16,
        },
      }

[dckc]

I coded up lazy purse creation in #6774 . But another option that's starting to look good is

Skip chainStorage writes for vbank purse balances

Now that we have agoricNames.vbankAssets (#5762 ), clients can just subscribe to queries from x/bank and join with vbankAssets on denom to get brand, displayInfo.decimalPlaces, etc. The smartWallet need not track purse balances; it can just use purses for the duration of handling an offer.

This would update our approach to

• wallet history: on chain storage vs. update size vs. off-chain searching #6184

It would be a significant change to the wallet front end. And we would need another approach when we add non-vbank assets (LP tokens etc.)

I'm not sure how the REPL wallet API would work either.

[dckc]

Let's call this fixed in #5412
and consider #6807 part of #6788