Emit sensitive task event for MPP (#585)

When the first part of a multi-part payment is received, we emit a
`SensitiveTaskEvents.TaskStarted` hoping that iOS will let the app
running long enough for subsequent parts to arrive.

src/commonMain/kotlin/fr/acinq/lightning/NodeEvents.kt

@@ -49,6 +49,7 @@ sealed interface SensitiveTaskEvents : NodeEvents {
         data class InteractiveTx(val channelId: ByteVector32, val fundingTxIndex: Long) : TaskIdentifier() {
             constructor(fundingParams: InteractiveTxParams) : this(fundingParams.channelId, (fundingParams.sharedInput as? SharedFundingInput.Multisig2of2)?.fundingTxIndex?.let { it + 1 } ?: 0)
         }
+        data class IncomingMultiPartPayment(val paymentHash: ByteVector32) : TaskIdentifier()
     }
     data class TaskStarted(val id: TaskIdentifier) : SensitiveTaskEvents
     data class TaskEnded(val id: TaskIdentifier) : SensitiveTaskEvents

src/commonMain/kotlin/fr/acinq/lightning/io/Peer.kt

@@ -795,7 +795,17 @@ class Peer(
             is Either.Left -> incomingPaymentHandler.process(item.value, currentBlockHeight)
         }
         when (result) {
-            is IncomingPaymentHandler.ProcessAddResult.Accepted -> _eventsFlow.emit(PaymentReceived(result.incomingPayment, result.received))
+            is IncomingPaymentHandler.ProcessAddResult.Accepted -> {
+                if ((result.incomingPayment.received?.receivedWith?.size ?: 0) > 1) {
+                    // this was a multi-part payment, we signal that the task is finished
+                    nodeParams._nodeEvents.tryEmit(SensitiveTaskEvents.TaskEnded(SensitiveTaskEvents.TaskIdentifier.IncomingMultiPartPayment(result.incomingPayment.paymentHash)))
+                }
+                _eventsFlow.emit(PaymentReceived(result.incomingPayment, result.received))
+            }
+            is IncomingPaymentHandler.ProcessAddResult.Pending -> if (result.pendingPayment.parts.size == 1) {
+                // this is the first part of a multi-part payment, we request to keep the app alive to receive subsequent parts
+                nodeParams._nodeEvents.tryEmit(SensitiveTaskEvents.TaskStarted(SensitiveTaskEvents.TaskIdentifier.IncomingMultiPartPayment(result.incomingPayment.paymentHash)))
+            }
             else -> Unit
         }
         result.actions.forEach { input.send(it) }

src/commonMain/kotlin/fr/acinq/lightning/payment/IncomingPaymentHandler.kt

@@ -49,7 +49,7 @@ class IncomingPaymentHandler(val nodeParams: NodeParams, val db: IncomingPayment
 
         data class Accepted(override val actions: List<PeerCommand>, val incomingPayment: IncomingPayment, val received: IncomingPayment.Received) : ProcessAddResult()
         data class Rejected(override val actions: List<PeerCommand>, val incomingPayment: IncomingPayment?) : ProcessAddResult()
-        data class Pending(val incomingPayment: IncomingPayment) : ProcessAddResult() {
+        data class Pending(val incomingPayment: IncomingPayment, val pendingPayment: PendingPayment) : ProcessAddResult() {
             override val actions: List<PeerCommand> = listOf()
         }
     }
@@ -63,7 +63,7 @@ class IncomingPaymentHandler(val nodeParams: NodeParams, val db: IncomingPayment
      * @param totalAmount total amount that should be received.
      * @param startedAtSeconds time at which we received the first partial payment (in seconds).
      */
-    private data class PendingPayment(val parts: Set<PaymentPart>, val totalAmount: MilliSatoshi, val startedAtSeconds: Long) {
+    data class PendingPayment(val parts: Set<PaymentPart>, val totalAmount: MilliSatoshi, val startedAtSeconds: Long) {
         constructor(firstPart: PaymentPart) : this(setOf(firstPart), firstPart.totalAmount, currentTimestampSeconds())
 
         val amountReceived: MilliSatoshi = parts.map { it.amount }.sum()
@@ -246,7 +246,7 @@ class IncomingPaymentHandler(val nodeParams: NodeParams, val db: IncomingPayment
                         payment.amountReceived < payment.totalAmount -> {
                             // Still waiting for more payments.
                             pending[paymentPart.paymentHash] = payment
-                            return ProcessAddResult.Pending(incomingPayment)
+                            return ProcessAddResult.Pending(incomingPayment, payment)
                         }
                         else -> {
                             if (payment.parts.filterIsInstance<PayToOpenPart>().isNotEmpty()) {