core: Fix tx resolution bug by topological sorting of downloaded transactions

core/src/main/kotlin/com/r3corda/core/contracts/DummyContract.kt

@@ -54,11 +54,13 @@ class DummyContract : Contract {
             return TransactionType.General.Builder(notary = notary).withItems(state, Command(Commands.Create(), owner.party.owningKey))
         }
 
-        fun move(prior: StateAndRef<DummyContract.SingleOwnerState>, newOwner: PublicKey): TransactionBuilder {
-            val priorState = prior.state.data
+        fun move(prior: StateAndRef<DummyContract.SingleOwnerState>, newOwner: PublicKey) = move(listOf(prior), newOwner)
+        fun move(priors: List<StateAndRef<DummyContract.SingleOwnerState>>, newOwner: PublicKey): TransactionBuilder {
+            require(priors.size > 0)
+            val priorState = priors[0].state.data
             val (cmd, state) = priorState.withNewOwner(newOwner)
-            return TransactionType.General.Builder(notary = prior.state.notary).withItems(
-                    /* INPUT   */ prior,
+            return TransactionType.General.Builder(notary = priors[0].state.notary).withItems(
+                    /* INPUTS  */ *priors.toTypedArray(),
                     /* COMMAND */ Command(cmd, priorState.owner),
                     /* OUTPUT  */ state
             )

core/src/main/kotlin/com/r3corda/protocols/ResolveTransactionsProtocol.kt

@@ -32,6 +32,44 @@ class ResolveTransactionsProtocol(private val txHashes: Set<SecureHash>,
 
     companion object {
         private fun dependencyIDs(wtx: WireTransaction) = wtx.inputs.map { it.txhash }.toSet()
+
+        private fun topologicalSort(transactions: Iterable<SignedTransaction>): List<SignedTransaction> {
+
+            // Construct txhash -> dependent-txhashes map
+            val forwardGraph = HashMap<SecureHash, HashSet<SecureHash>>()
+            transactions.forEach { tx ->
+                tx.tx.inputs.forEach { input ->
+                    forwardGraph.getOrPut(input.txhash) { HashSet() }.add(tx.id)
+                }
+            }
+
+            // Construct txhash -> tx map
+            val transactionMap = HashMap<SecureHash, SignedTransaction>()
+            transactions.forEach { transactionMap[it.tx.id] = it }
+
+            val visited = HashSet<SecureHash>()
+            val result = LinkedList<SignedTransaction>()
+
+            fun visit(transactionHash: SecureHash) {
+                if (transactionHash in visited) {
+                    return
+                }
+                visited.add(transactionHash)
+                forwardGraph[transactionHash]?.forEach {
+                    visit(it)
+                }
+                result.addFirst(transactionMap[transactionHash]!!)
+            }
+
+            transactions.forEach {
+                visit(it.tx.id)
+            }
+
+            require(result.size == transactionMap.size)
+            require(visited.size == transactionMap.size)
+            return result
+        }
+
     }
 
     class ExcessivelyLargeTransactionGraph() : Exception()
@@ -60,7 +98,7 @@ class ResolveTransactionsProtocol(private val txHashes: Set<SecureHash>,
 
     @Suspendable
     override fun call(): List<LedgerTransaction> {
-        val newTxns: Iterable<SignedTransaction> = downloadDependencies(txHashes)
+        val newTxns: Iterable<SignedTransaction> = topologicalSort(downloadDependencies(txHashes))
 
         // For each transaction, verify it and insert it into the database. As we are iterating over them in a
         // depth-first order, we should not encounter any verification failures due to missing data. If we fail

core/src/test/kotlin/com/r3corda/core/protocols/ResolveTransactionsProtocolTest.kt

@@ -97,6 +97,30 @@ class ResolveTransactionsProtocolTest {
         }
     }
 
+    @Test
+    fun `triangle of transactions resolves fine`() {
+        val stx1 = makeTransactions().first
+
+        val stx2 = DummyContract.move(stx1.tx.outRef(0), MINI_CORP_PUBKEY).run {
+            signWith(MEGA_CORP_KEY)
+            signWith(DUMMY_NOTARY_KEY)
+            toSignedTransaction()
+        }
+
+        val stx3 = DummyContract.move(listOf(stx1.tx.outRef(0), stx2.tx.outRef(0)), MINI_CORP_PUBKEY).run {
+            signWith(MEGA_CORP_KEY)
+            signWith(DUMMY_NOTARY_KEY)
+            toSignedTransaction()
+        }
+
+        a.services.recordTransactions(stx2, stx3)
+
+        val p = ResolveTransactionsProtocol(setOf(stx3.id), a.info.identity)
+        val future = b.services.startProtocol("resolve", p)
+        net.runNetwork()
+        future.get()
+    }
+
     @Test
     fun attachment() {
         val id = a.services.storageService.attachments.importAttachment("Some test file".toByteArray().opaque().open())