Trading: move session ID into the initial args to avoid possible confusion at the start.

src/main/kotlin/contracts/protocols/TwoPartyTradeProtocol.kt

@@ -45,14 +45,16 @@ abstract class TwoPartyTradeProtocol {
     class SellerInitialArgs(
             val assetToSell: StateAndRef<OwnableState>,
             val price: Amount,
-            val myKeyPair: KeyPair
+            val myKeyPair: KeyPair,
+            val buyerSessionID: Long
     )
 
     abstract fun runSeller(otherSide: SingleMessageRecipient, args: SellerInitialArgs): Seller
 
     class BuyerInitialArgs(
             val acceptablePrice: Amount,
-            val typeToBuy: Class<out OwnableState>
+            val typeToBuy: Class<out OwnableState>,
+            val sessionID: Long
     )
 
     abstract fun runBuyer(otherSide: SingleMessageRecipient, args: BuyerInitialArgs): Buyer
@@ -95,8 +97,7 @@ private class TwoPartyTradeProtocolImpl(private val smm: StateMachineManager) :
             // Make the first message we'll send to kick off the protocol.
             val hello = SellerTradeInfo(args.assetToSell, args.price, args.myKeyPair.public, sessionID)
 
-            // Zero is a special session ID that is being listened to by the buyer (i.e. before a session is started).
+            val partialTX = sendAndReceive<SignedWireTransaction>(TRADE_TOPIC, args.buyerSessionID, sessionID, hello)
-            val partialTX = sendAndReceive<SignedWireTransaction>(TRADE_TOPIC, 0, sessionID, hello)
             logger().trace { "Received partially signed transaction" }
 
             partialTX.verifySignatures()
@@ -124,7 +125,7 @@ private class TwoPartyTradeProtocolImpl(private val smm: StateMachineManager) :
             val timestamped: TimestampedWireTransaction = fullySigned.toTimestampedTransaction(serviceHub.timestampingService)
             logger().trace { "Built finished transaction, sending back to secondary!" }
 
-            send(TRADE_TOPIC, sessionID, timestamped)
+            send(TRADE_TOPIC, args.buyerSessionID, timestamped)
 
             return Pair(timestamped, timestamped.verifyToLedgerTransaction(serviceHub.timestampingService, serviceHub.identityService))
         }
@@ -138,8 +139,8 @@ private class TwoPartyTradeProtocolImpl(private val smm: StateMachineManager) :
     // The buyer's side of the protocol. See note above Seller to learn about the caveats here.
     class BuyerImpl : Buyer() {
         override fun call(args: BuyerInitialArgs): Pair<TimestampedWireTransaction, LedgerTransaction> {
-            // Wait for a trade request to come in on special session ID zero.
+            // Wait for a trade request to come in on our pre-provided session ID.
-            val tradeRequest = receive<SellerTradeInfo>(TRADE_TOPIC, 0)
+            val tradeRequest = receive<SellerTradeInfo>(TRADE_TOPIC, args.sessionID)
 
             // What is the seller trying to sell us?
             val assetTypeName = tradeRequest.assetForSale.state.javaClass.name
@@ -183,10 +184,10 @@ private class TwoPartyTradeProtocolImpl(private val smm: StateMachineManager) :
 
             logger().trace { "Sending partially signed transaction to seller" }
 
-            // We'll just reuse the session ID the seller selected here for convenience.
             // TODO: Protect against the buyer terminating here and leaving us in the lurch without the final tx.
+            // TODO: Protect against a malicious buyer sending us back a different transaction to the one we built.
             val fullySigned = sendAndReceive<TimestampedWireTransaction>(TRADE_TOPIC,
-                    tradeRequest.sessionID, tradeRequest.sessionID, stx)
+                    tradeRequest.sessionID, args.sessionID, stx)
 
             logger().trace { "Got fully signed transaction, verifying ... "}
 

src/test/kotlin/core/messaging/TwoPartyTradeProtocolTests.kt

@@ -20,6 +20,7 @@ import core.testutils.*
 import org.junit.After
 import org.junit.Before
 import org.junit.Test
+import java.security.SecureRandom
 import java.util.concurrent.Executors
 import java.util.logging.Formatter
 import java.util.logging.Level
@@ -76,19 +77,23 @@ class TwoPartyTradeProtocolTests : TestWithInMemoryNetwork() {
             val tpSeller = TwoPartyTradeProtocol.create(StateMachineManager(alicesServices, backgroundThread))
             val tpBuyer = TwoPartyTradeProtocol.create(StateMachineManager(bobsServices, backgroundThread))
 
+            val buyerSessionID = SecureRandom.getInstanceStrong().nextLong()
+
             val aliceResult = tpSeller.runSeller(
                     bobsAddress,
                     TwoPartyTradeProtocol.SellerInitialArgs(
                             lookup("alice's paper"),
                             1000.DOLLARS,
-                            ALICE_KEY
+                            ALICE_KEY,
+                            buyerSessionID
                     )
             )
             val bobResult = tpBuyer.runBuyer(
                     alicesAddress,
                     TwoPartyTradeProtocol.BuyerInitialArgs(
                         1000.DOLLARS,
-                        CommercialPaper.State::class.java
+                        CommercialPaper.State::class.java,
+                        buyerSessionID
                     )
             )
 
@@ -129,19 +134,23 @@ class TwoPartyTradeProtocolTests : TestWithInMemoryNetwork() {
             val smmBuyer = StateMachineManager(bobsServices, MoreExecutors.directExecutor())
             val tpBuyer = TwoPartyTradeProtocol.create(smmBuyer)
 
+            val buyerSessionID = SecureRandom.getInstanceStrong().nextLong()
+
             tpSeller.runSeller(
                     bobsAddress,
                     TwoPartyTradeProtocol.SellerInitialArgs(
                             lookup("alice's paper"),
                             1000.DOLLARS,
-                            ALICE_KEY
+                            ALICE_KEY,
+                            buyerSessionID
                     )
             )
             tpBuyer.runBuyer(
                     alicesAddress,
                     TwoPartyTradeProtocol.BuyerInitialArgs(
                             1000.DOLLARS,
-                            CommercialPaper.State::class.java
+                            CommercialPaper.State::class.java,
+                            buyerSessionID
                     )
             )