feat(scheduling): Add workflow validation handler for Cron expressions

src/modules/Elsa.Scheduling/Handlers/ValidateWorkflowRequestHandler.cs

@@ -0,0 +1,63 @@
+using Cronos;
+using Elsa.Extensions;
+using Elsa.Mediator.Contracts;
+using Elsa.Scheduling.Bookmarks;
+using Elsa.Workflows.Management.Models;
+using Elsa.Workflows.Management.Notifications;
+using Elsa.Workflows.Runtime;
+using JetBrains.Annotations;
+
+namespace Elsa.Scheduling.Handlers;
+
+/// <summary>
+/// A <see cref="ValidateWorkflowRequestHandler"/> handler that validates a workflow cron epxression and return any errors.
+/// </summary>
+[UsedImplicitly]
+public class ValidateWorkflowRequestHandler : INotificationHandler<WorkflowDefinitionValidating>
+{
+    private readonly ITriggerIndexer _triggerIndexer;
+
+    /// <summary>
+    /// Constructor.
+    /// </summary>
+    public ValidateWorkflowRequestHandler(ITriggerIndexer triggerIndexer)
+    {
+        _triggerIndexer = triggerIndexer;
+    }
+
+    public async Task HandleAsync(WorkflowDefinitionValidating notification, CancellationToken cancellationToken)
+    {
+        var workflow = notification.Workflow;
+        var validationErrors = notification.ValidationErrors;
+
+        var cronTriggers = (await _triggerIndexer.GetTriggersAsync(workflow, cancellationToken))
+            .Where(x => x.Payload is CronTriggerPayload)
+            .ToList();
+
+        foreach (var cronTrigger in cronTriggers)
+        {
+            var cronPayload = cronTrigger.GetPayload<CronTriggerPayload>();
+
+            if (!CronExpression.TryParse(cronPayload.CronExpression, out _))
+            {
+                var message = $"The Cron Expression: {cronPayload.CronExpression} is not a valid Cron Expression!";
+                validationErrors.Add(new WorkflowValidationError(message, cronTrigger.ActivityId));
+            }
+        }
+
+        var timerTriggers = (await _triggerIndexer.GetTriggersAsync(workflow, cancellationToken))
+            .Where(x => x.Payload is TimerTriggerPayload)
+            .ToList();
+
+        foreach (var timerTrigger in timerTriggers)
+        {
+            var cronPayload = timerTrigger.GetPayload<CronTriggerPayload>();
+
+            if (!CronExpression.TryParse(cronPayload.CronExpression, out _))
+            {
+                var message = $"The Cron Expression: {cronPayload.CronExpression} is not a valid Cron Expression!";
+                validationErrors.Add(new WorkflowValidationError(message, timerTrigger.ActivityId));
+            }
+        }
+    }
+}