Allow to make new AMB requests while processing other messages (#572)

contracts/upgradeable_contracts/arbitrary_message/BasicAMB.sol

@@ -10,6 +10,7 @@ contract BasicAMB is BasicBridge, VersionableAMB {
     bytes32 internal constant SOURCE_CHAIN_ID_LENGTH = 0xe504ae1fd6471eea80f18b8532a61a9bb91fba4f5b837f80a1cfb6752350af44; // keccak256(abi.encodePacked("sourceChainIdLength"))
     bytes32 internal constant DESTINATION_CHAIN_ID = 0xbbd454018e72a3f6c02bbd785bacc49e46292744f3f6761276723823aa332320; // keccak256(abi.encodePacked("destinationChainId"))
     bytes32 internal constant DESTINATION_CHAIN_ID_LENGTH = 0xfb792ae4ad11102b93f26a51b3749c2b3667f8b561566a4806d4989692811594; // keccak256(abi.encodePacked("destinationChainIdLength"))
+    bytes32 internal constant ALLOW_REENTRANT_REQUESTS = 0xffa3a5a0e192028fc343362a39c5688e5a60819a4dc5ab3ee70c25bc25b78dd6; // keccak256(abi.encodePacked("allowReentrantRequests"))
 
     /**
      * Initializes AMB contract
@@ -82,6 +83,24 @@ contract BasicAMB is BasicBridge, VersionableAMB {
         _setChainIds(_sourceChainId, _destinationChainId);
     }
 
+    /**
+     * Sets the flag to allow passing new AMB requests in the opposite direction,
+     * while other AMB message is being processed.
+     * Only owner can call this method.
+     * @param _enable true, if reentrant requests are allowed.
+     */
+    function setAllowReentrantRequests(bool _enable) external onlyOwner {
+        boolStorage[ALLOW_REENTRANT_REQUESTS] = _enable;
+    }
+
+    /**
+     * Tells if passing reentrant requests is allowed.
+     * @return true, if reentrant requests are allowed.
+     */
+    function allowReentrantRequests() public view returns (bool) {
+        return boolStorage[ALLOW_REENTRANT_REQUESTS];
+    }
+
     /**
      * Internal function for retrieving current nonce value
      * @return nonce value

contracts/upgradeable_contracts/arbitrary_message/MessageDelivery.sol

@@ -30,8 +30,8 @@ contract MessageDelivery is BasicAMB, MessageProcessor {
     */
     function _sendMessage(address _contract, bytes _data, uint256 _gas, uint256 _dataType) public returns (bytes32) {
         // it is not allowed to pass messages while other messages are processed
-        require(messageId() == bytes32(0));
-
+        // if other is not explicitly configured
+        require(messageId() == bytes32(0) || allowReentrantRequests());
         require(_gas >= getMinimumGasUsage(_data) && _gas <= maxGasPerTx());
 
         bytes32 _messageId;

test/arbitrary_message/foreign_bridge.test.js

@@ -736,6 +736,33 @@ contract('ForeignAMB', async accounts => {
       // means that call to requireToPassMessage inside MessageProcessor reverted, since messageId flag was set up
       expect(await foreignBridge.messageCallStatus(messageId)).to.be.equal(false)
     })
+    it('should allow to pass message back through the bridge if configured', async () => {
+      const user = accounts[8]
+      await foreignBridge.setAllowReentrantRequests(true, { from: user }).should.be.rejected
+      await foreignBridge.setAllowReentrantRequests(true, { from: owner }).should.be.fulfilled
+      expect(await foreignBridge.allowReentrantRequests()).to.be.equal(true)
+
+      const data = await foreignBridge.contract.methods
+        .requireToPassMessage(box.address, setValueData, 100000)
+        .encodeABI()
+      // Use these calls to simulate home bridge on home network
+      const resultPassMessageTx = await homeBridge.requireToPassMessage(foreignBridge.address, data, 821254, {
+        from: user
+      })
+
+      const { encodedData: message, messageId } = resultPassMessageTx.logs[0].args
+
+      const signature = await sign(authorities[0], message)
+      const vrs = signatureToVRS(signature)
+      const signatures = packSignatures([vrs])
+
+      await foreignBridge.executeSignatures(message, signatures, {
+        from: authorities[0],
+        gasPrice
+      }).should.be.fulfilled
+
+      expect(await foreignBridge.messageCallStatus(messageId)).to.be.equal(true)
+    })
   })
 
   describe('gasToken functionality', async () => {

test/arbitrary_message/home_bridge.test.js

@@ -666,6 +666,27 @@ contract('HomeAMB', async accounts => {
       // means that call to requireToPassMessage inside MessageProcessor reverted, since messageId flag was set up
       expect(await homeBridge.messageCallStatus(messageId)).to.be.equal(false)
     })
+    it('should allow to pass message back through the bridge if configured', async () => {
+      const user = accounts[8]
+      await homeBridge.setAllowReentrantRequests(true, { from: user }).should.be.rejected
+      await homeBridge.setAllowReentrantRequests(true, { from: owner }).should.be.fulfilled
+      expect(await homeBridge.allowReentrantRequests()).to.be.equal(true)
+
+      const data = await homeBridge.contract.methods.requireToPassMessage(box.address, setValueData, 100000).encodeABI()
+      // Use these calls to simulate home bridge on home network
+      const resultPassMessageTx = await foreignBridge.requireToPassMessage(homeBridge.address, data, 821254, {
+        from: user
+      })
+
+      const { encodedData: message, messageId } = resultPassMessageTx.logs[0].args
+
+      await homeBridge.executeAffirmation(message, {
+        from: authorities[0],
+        gasPrice
+      }).should.be.fulfilled
+
+      expect(await homeBridge.messageCallStatus(messageId)).to.be.equal(true)
+    })
   })
   describe('submitSignature', () => {
     let homeBridge