Throw a diagnostic exception if your FlowLogic.call method is not marked as @Suspendable.

This catches a bunch of unit tests where it's missing and also resolves an issue I saw Roger hit the other day.

node/src/main/kotlin/net/corda/node/services/events/NodeSchedulerService.kt

@@ -235,14 +235,20 @@ class NodeSchedulerService(private val services: ServiceHubInternal,
 
     private fun onTimeReached(scheduledState: ScheduledStateRef) {
         serverThread.execute {
-            services.database.transaction {
-                val scheduledFlow = getScheduledFlow(scheduledState)
-                if (scheduledFlow != null) {
-                    val future = services.startFlow(scheduledFlow, FlowInitiator.Scheduled(scheduledState)).resultFuture
-                    future.then {
-                        unfinishedSchedules.countDown()
+            var flowName: String? = "(unknown)"
+            try {
+                services.database.transaction {
+                    val scheduledFlow = getScheduledFlow(scheduledState)
+                    if (scheduledFlow != null) {
+                        flowName = scheduledFlow.javaClass.name
+                        val future = services.startFlow(scheduledFlow, FlowInitiator.Scheduled(scheduledState)).resultFuture
+                        future.then {
+                            unfinishedSchedules.countDown()
+                        }
                     }
                 }
+            } catch (e: Exception) {
+                log.error("Failed to start scheduled flow $flowName for $scheduledState due to an internal error", e)
             }
         }
     }

node/src/main/kotlin/net/corda/node/services/statemachine/StateMachineManager.kt

@@ -2,6 +2,7 @@ package net.corda.node.services.statemachine
 
 import co.paralleluniverse.fibers.Fiber
 import co.paralleluniverse.fibers.FiberExecutorScheduler
+import co.paralleluniverse.fibers.Suspendable
 import co.paralleluniverse.fibers.instrument.SuspendableHelper
 import co.paralleluniverse.strands.Strand
 import com.codahale.metrics.Gauge
@@ -427,6 +428,7 @@ class StateMachineManager(val serviceHub: ServiceHubInternal,
     }
 
     private fun initFiber(fiber: FlowStateMachineImpl<*>) {
+        verifyFlowLogicIsSuspendable(fiber.logic)
         fiber.database = database
         fiber.serviceHub = serviceHub
         fiber.actionOnSuspend = { ioRequest ->
@@ -458,6 +460,19 @@ class StateMachineManager(val serviceHub: ServiceHubInternal,
         }
     }
 
+    private fun verifyFlowLogicIsSuspendable(logic: FlowLogic<Any?>) {
+        // Quasar requires (in Java 8) that at least the call method be annotated suspendable. Unfortunately, it's
+        // easy to forget to add this when creating a new flow, so we check here to give the user a better error.
+        //
+        // The Kotlin compiler can sometimes generate a synthetic bridge method from a single call declaration, which
+        // forwards to the void method and then returns Unit. However annotations do not get copied across to this
+        // bridge, so we have to do a more complex scan here.
+        val call = logic.javaClass.methods.first { !it.isSynthetic && it.name == "call" && it.parameterCount == 0 }
+        if (call.getAnnotation(Suspendable::class.java) == null) {
+            throw FlowException("${logic.javaClass.name}.call() is not annotated as @Suspendable. Please fix this.")
+        }
+    }
+
     private fun endAllFiberSessions(fiber: FlowStateMachineImpl<*>, result: Try<*>, propagated: Boolean) {
         openSessions.values.removeIf { session ->
             if (session.fiber == fiber) {

node/src/test/kotlin/net/corda/node/services/events/NodeSchedulerServiceTest.kt

@@ -1,5 +1,6 @@
 package net.corda.node.services.events
 
+import co.paralleluniverse.fibers.Suspendable
 import net.corda.core.contracts.*
 import net.corda.core.flows.FlowLogic
 import net.corda.core.flows.FlowLogicRef
@@ -136,6 +137,7 @@ class NodeSchedulerServiceTest : SingletonSerializeAsToken() {
     }
 
     class TestFlowLogic(val increment: Int = 1) : FlowLogic<Unit>() {
+        @Suspendable
         override fun call() {
             (serviceHub as TestReference).testReference.calls += increment
             (serviceHub as TestReference).testReference.countDown.countDown()

node/src/test/kotlin/net/corda/node/services/statemachine/FlowFrameworkTests.kt

@@ -913,6 +913,7 @@ class FlowFrameworkTests {
         override val progressTracker: ProgressTracker = ProgressTracker(START_STEP)
         lateinit var exceptionThrown: E
 
+        @Suspendable
         override fun call(): Nothing {
             progressTracker.currentStep = START_STEP
             exceptionThrown = exception()

samples/irs-demo/src/test/kotlin/net/corda/irs/flows/UpdateBusinessDayFlow.kt

@@ -23,6 +23,7 @@ object UpdateBusinessDayFlow {
 
     @InitiatedBy(Broadcast::class)
     private class UpdateBusinessDayHandler(val otherParty: Party) : FlowLogic<Unit>() {
+        @Suspendable
         override fun call() {
             val message = receive<UpdateBusinessDayMessage>(otherParty).unwrap { it }
             (serviceHub.clock as TestClock).updateDate(message.date)