fix: use unique offer descriptions for swaps

[samsiegart]

Fixes #43
Refs #40

Also removes ,tx.json, will add it to .gitignore in a follow-up PR

[dckc]

go ahead and make it durable?

[dckc]

Since this is an example, we might as well store it durably; adding it isn't any longer than the XXX comment:

Suggested change

	const generateOfferNonce = (() => {
	// XXX: This is not stored durably, so ensure it stays unique after upgrade,
	// perhaps by appending a new character each time.
	let offerNonce = -1;
	return () => {
	offerNonce += 1;
	return `${offerNonce}`;
	};
	})();
	const generateOfferNonce = (() => {
	let offerNonce = provide(baggage, 'offerNonce', () => -1);
	return () => {
	offerNonce += 1;
	baggage.set('offerNonce', offerNonce);
	return `${offerNonce}`;
	};
	})();

along with:

import { provide } from '@agoric/vat-data';

[dckc]

Also, we might as well return offerNonce, since it gets stringified later.

[samsiegart]

Done. Just writing out my discovery process for reference (tried not going straight to your hints):

• Looked for other durable examples in dapp-agoric-basics, found makeDurableGovernorFacet, doesn't seem like quite what I want.
• Go to docs.agoric.com and search "durability", find https://docs.agoric.com/guides/zoe/contract-upgrade.html#durability
• See an example with provide and makeScalarBigMapStore. I just need a number, not a map, not sure if that's allowed, but from your example it apparently is.
• yarn add @agoric/vat-data and use the provide method -> Tests failing, seems like it brought in some conflicting dependency versions.
• Nuke node_modules, remove @agoric/vat-data, do another yarn install and then do yarn why @agoric/vat-data to see which version is already transitively depended on.
• Add that version to package.json, now only one test case failing:

build-proposal › bundles from build:deployer meet 1MB request limit
  test/test-build-proposal.js:18

   17:     const rootPath = undefined; // not needed since bundle is already cached
   18:     const bundle = await bundleCache.load(rootPath, name);
   19:     const fileContents = JSON.stringify(bundle);          

  Rejected promise returned by test. Reason:

  TypeError {
    code: 'ERR_INVALID_ARG_TYPE',
    message: 'The "paths[1]" argument must be of type string. Received undefined',
  }

• Some hint about bundle caching there, run yarn build before yarn test again and now it passes.

[dckc]

... writing out my discovery process...

nice!

• yarn add @agoric/vat-data and use the provide method -> Tests failing, seems like it brought in some conflicting dependency versions.
• Nuke node_modules, remove @agoric/vat-data, do another yarn install and then do yarn why @agoric/vat-data to see which version is already transitively depended on.

Yes, adding dependencies is incredibly fragile. I'm tracking that as...

• bundle contains duplicated modules based on duplicated packages in node_modules agoric-sdk#8621