bool IsExecutableInstance()

in src/DurableTask.AzureStorage/AzureStorageOrchestrationService.cs [1052:1089]
28 lines of code
10 McCabe index (conditional complexity)

        bool IsExecutableInstance(OrchestrationRuntimeState runtimeState, IList<TaskMessage> newMessages, bool allowReplayingTerminalInstances, out string message)
        {
            if (runtimeState.ExecutionStartedEvent == null && !newMessages.Any(msg => msg.Event is ExecutionStartedEvent))
            {
                var instanceId = newMessages[0].OrchestrationInstance.InstanceId;

                if (DurableTask.Core.Common.Entities.AutoStart(instanceId, newMessages))
                {
                    message = null;
                    return true;
                }
                else
                {
                    // A non-zero event count usually happens when an instance's history is overwritten by a
                    // new instance or by a ContinueAsNew. When history is overwritten by new instances, we
                    // overwrite the old history with new history (with a new execution ID), but this is done
                    // gradually as we build up the new history over time. If we haven't yet overwritten *all*
                    // the old history and we receive a message from the old instance (this happens frequently
                    // with canceled durable timer messages) we'll end up loading just the history that hasn't
                    // been fully overwritten. We know it's invalid because it's missing the ExecutionStartedEvent.
                    message = runtimeState.Events.Count == 0 ? "No such instance" : "Invalid history (may have been overwritten by a newer instance)";
                    return false;
                }
            }

            if (runtimeState.ExecutionStartedEvent != null &&
                !allowReplayingTerminalInstances &&
                runtimeState.OrchestrationStatus != OrchestrationStatus.Running &&
                runtimeState.OrchestrationStatus != OrchestrationStatus.Pending &&
                runtimeState.OrchestrationStatus != OrchestrationStatus.Suspended)
            {
                message = $"Instance is {runtimeState.OrchestrationStatus}";
                return false;
            }

            message = null;
            return true;
        }