[CORDA-2265]: Nodes hangs when using AppServiceHub to start a flow in a blocking way (fix) (#4376)

docs/source/changelog.rst

@@ -28,6 +28,8 @@ Unreleased
 
 * Improved exception thrown by `AttachmentsClassLoader` when an attachment cannot be used because its uploader is not trusted.
 
+* Fixed deadlocks generated by starting flow from within CordaServices.
+
 * Marked the ``Attachment`` interface as ``@DoNotImplement`` because it is not meant to be extended by CorDapp developers. If you have already
   done so, please get in contact on the usual communication channels.
 

node/src/integration-test/kotlin/net/corda/node/services/CordaServiceFlowTests.kt

@@ -0,0 +1,55 @@
+package net.corda.node.services
+
+import co.paralleluniverse.fibers.Suspendable
+import net.corda.core.flows.FlowLogic
+import net.corda.core.flows.StartableByRPC
+import net.corda.core.flows.StartableByService
+import net.corda.core.messaging.startFlow
+import net.corda.core.node.AppServiceHub
+import net.corda.core.node.services.CordaService
+import net.corda.core.serialization.SingletonSerializeAsToken
+import net.corda.core.utilities.getOrThrow
+import net.corda.testing.driver.DriverParameters
+import net.corda.testing.driver.driver
+import org.assertj.core.api.Assertions.assertThat
+import org.junit.Test
+
+class CordaServiceFlowTests {
+    @Test
+    fun `corda service can start a flow and wait for it`() {
+        driver(DriverParameters(startNodesInProcess = true)) {
+            val node = startNode().getOrThrow()
+            val text = "191ejodaimadc8i"
+
+            val length = node.rpc.startFlow(::ComputeTextLengthThroughCordaService, text).returnValue.getOrThrow()
+
+            assertThat(length).isEqualTo(text.length)
+        }
+    }
+
+    @StartableByRPC
+    class ComputeTextLengthThroughCordaService(private val text: String) : FlowLogic<Int>() {
+        @Suspendable
+        override fun call(): Int {
+            val service = serviceHub.cordaService(TextLengthComputingService::class.java)
+            return service.computeLength(text)
+        }
+    }
+
+    @StartableByService
+    class ActuallyComputeTextLength(private val text: String) : FlowLogic<Int>() {
+        @Suspendable
+        override fun call(): Int {
+            return text.length
+        }
+    }
+
+    @CordaService
+    class TextLengthComputingService(private val services: AppServiceHub) : SingletonSerializeAsToken() {
+        fun computeLength(text: String): Int {
+            // Just to check this works with Quasar.
+            require(text.isNotEmpty()) { "Length must be at least 1." }
+            return services.startFlow(ActuallyComputeTextLength(text)).returnValue.toCompletableFuture().getOrThrow()
+        }
+    }
+}

node/src/main/kotlin/net/corda/node/internal/AbstractNode.kt

@@ -17,6 +17,7 @@ import net.corda.core.identity.CordaX500Name
 import net.corda.core.identity.Party
 import net.corda.core.identity.PartyAndCertificate
 import net.corda.core.internal.*
+import net.corda.core.internal.concurrent.doneFuture
 import net.corda.core.internal.concurrent.map
 import net.corda.core.internal.concurrent.openFuture
 import net.corda.core.internal.notary.NotaryService
@@ -601,8 +602,16 @@ abstract class AbstractNode<S>(val configuration: NodeConfiguration,
         }
 
         override fun <T> startFlow(flow: FlowLogic<T>): FlowHandle<T> {
-            val stateMachine = startFlowChecked(flow)
+            val parentFlow = FlowLogic.currentTopLevel
-            return FlowHandleImpl(id = stateMachine.id, returnValue = stateMachine.resultFuture)
+            return if (parentFlow != null) {
+                val result = parentFlow.subFlow(flow)
+                // Accessing the flow id must happen after the flow has started.
+                val flowId = flow.runId
+                FlowHandleImpl(flowId, doneFuture(result))
+            } else {
+                val stateMachine = startFlowChecked(flow)
+                FlowHandleImpl(id = stateMachine.id, returnValue = stateMachine.resultFuture)
+            }
         }
 
         private fun <T> startFlowChecked(flow: FlowLogic<T>): FlowStateMachine<T> {