Initial GSN support (beta) (#1844)

* Add base Context contract

* Add GSNContext and tests

* Add RelayHub deployment to tests

* Add RelayProvider integration, complete GSNContext tests

* Switch dependency to openzeppelin-gsn-provider

* Add default txfee to provider

* Add basic signing recipient

* Sign more values

* Add comment clarifying RelayHub's msg.data

* Make context constructors internal

* Rename SigningRecipient to GSNRecipientSignedData

* Add ERC20Charge recipients

* Harcode RelayHub address into GSNContext

* Fix Solidity linter errors

* Run server from binary, use gsn-helpers to fund it

* Migrate to published @openzeppelin/gsn-helpers

* Silence false-positive compiler warning

* Use GSN helper assertions

* Rename meta-tx to gsn, take out of drafts

* Merge ERC20 charge recipients into a single one

* Rename GSNRecipients to Bouncers

* Add GSNBouncerUtils to decouple the bouncers from GSNRecipient

* Add _upgradeRelayHub

* Store RelayHub address using unstructored storage

* Add IRelayHub

* Add _withdrawDeposits to GSNRecipient

* Add relayHub version to recipient

* Make _acceptRelayedCall and _declineRelayedCall easier to use

* Rename GSNBouncerUtils to GSNBouncerBase, make it IRelayRecipient

* Improve GSNBouncerBase, make pre and post sender-protected and optional

* Fix GSNBouncerERC20Fee, add tests

* Add missing GSNBouncerSignature test

* Override transferFrom in __unstable__ERC20PrimaryAdmin

* Fix gsn dependencies in package.json

* Rhub address slot reduced by 1

* Rename relay hub changed event

* Use released gsn-provider

* Run relayer with short sleep of 1s instead of 100ms

* update package-lock.json

* clear circle cache

* use optimized gsn-provider

* update to latest @openzeppelin/gsn-provider

* replace with gsn dev provider

* remove relay server

* rename arguments in approveFunction

* fix GSNBouncerSignature test

* change gsn txfee

* initialize development provider only once

* update RelayHub interface

* adapt to new IRelayHub.withdraw

* update @openzeppelin/gsn-helpers

* update relayhub singleton address

* fix helper name

* set up gsn provider for coverage too

* lint

* Revert "set up gsn provider for coverage too"

This reverts commit 8a7b5be5f9.

* remove unused code

* add gsn provider to coverage

* move truffle contract options back out

* increase gas limit for coverage

* remove unreachable code

* add more gas for GSNContext test

* fix test suite name

* rename GSNBouncerBase internal API

* remove onlyRelayHub modifier

* add explicit inheritance

* remove redundant event

* update name of bouncers error codes enums

* add basic docs page for gsn contracts

* make gsn directory all caps

* add changelog entry

* lint

* enable test run to fail in coverage

test/GSN/GSNContext.test.js

@@ -0,0 +1,78 @@
+const { BN, constants, expectEvent, expectRevert } = require('openzeppelin-test-helpers');
+const { ZERO_ADDRESS } = constants;
+const gsn = require('@openzeppelin/gsn-helpers');
+
+const GSNContextMock = artifacts.require('GSNContextMock');
+const ContextMockCaller = artifacts.require('ContextMockCaller');
+
+const { shouldBehaveLikeRegularContext } = require('./Context.behavior');
+
+contract('GSNContext', function ([_, deployer, sender, newRelayHub]) {
+  beforeEach(async function () {
+    this.context = await GSNContextMock.new();
+    this.caller = await ContextMockCaller.new();
+  });
+
+  describe('get/set RelayHub', function () {
+    const singletonRelayHub = '0xD216153c06E857cD7f72665E0aF1d7D82172F494';
+
+    it('initially returns the singleton instance address', async function () {
+      expect(await this.context.getRelayHub()).to.equal(singletonRelayHub);
+    });
+
+    it('can be upgraded to a new RelayHub', async function () {
+      const { logs } = await this.context.upgradeRelayHub(newRelayHub);
+      expectEvent.inLogs(logs, 'RelayHubChanged', { oldRelayHub: singletonRelayHub, newRelayHub });
+    });
+
+    it('cannot upgrade to the same RelayHub', async function () {
+      await expectRevert(
+        this.context.upgradeRelayHub(singletonRelayHub),
+        'GSNContext: new RelayHub is the current one'
+      );
+    });
+
+    it('cannot upgrade to the zero address', async function () {
+      await expectRevert(this.context.upgradeRelayHub(ZERO_ADDRESS), 'GSNContext: new RelayHub is the zero address');
+    });
+
+    context('with new RelayHub', function () {
+      beforeEach(async function () {
+        await this.context.upgradeRelayHub(newRelayHub);
+      });
+
+      it('returns the new instance address', async function () {
+        expect(await this.context.getRelayHub()).to.equal(newRelayHub);
+      });
+    });
+  });
+
+  context('when called directly', function () {
+    shouldBehaveLikeRegularContext(sender);
+  });
+
+  context('when receiving a relayed call', function () {
+    beforeEach(async function () {
+      await gsn.fundRecipient(web3, { recipient: this.context.address });
+    });
+
+    describe('msgSender', function () {
+      it('returns the relayed transaction original sender', async function () {
+        const { tx } = await this.context.msgSender({ from: sender, useGSN: true });
+        await expectEvent.inTransaction(tx, GSNContextMock, 'Sender', { sender });
+      });
+    });
+
+    describe('msgData', function () {
+      it('returns the relayed transaction original data', async function () {
+        const integerValue = new BN('42');
+        const stringValue = 'OpenZeppelin';
+        const callData = this.context.contract.methods.msgData(integerValue.toString(), stringValue).encodeABI();
+
+        // The provider doesn't properly estimate gas for a relayed call, so we need to manually set a higher value
+        const { tx } = await this.context.msgData(integerValue, stringValue, { gas: 1000000, useGSN: true });
+        await expectEvent.inTransaction(tx, GSNContextMock, 'Data', { data: callData, integerValue, stringValue });
+      });
+    });
+  });
+});