Fix potential race condition to Quartz scheduler causing duplicate scheduler error

src/activities/Elsa.Activities.Temporal.Quartz/Extensions/TimersOptionsExtensions.cs

@@ -28,6 +28,7 @@ namespace Elsa
             Action<QuartzHostedServiceOptions>? configureQuartzHostedService = default)
         {
             timersOptions.Services
+                .AddSingleton<QuartzSchedulerProvider>()
                 .AddSingleton<IWorkflowDefinitionScheduler, QuartzWorkflowDefinitionScheduler>()
                 .AddSingleton<IWorkflowInstanceScheduler, QuartzWorkflowInstanceScheduler>()
                 .AddSingleton<ICrontabParser, QuartzCrontabParser>()

src/activities/Elsa.Activities.Temporal.Quartz/Services/QuartzSchedulerProvider.cs

@@ -0,0 +1,36 @@
+using System.Threading;
+using System.Threading.Tasks;
+using Quartz;
+
+namespace Elsa.Activities.Temporal.Quartz.Services
+{
+    public class QuartzSchedulerProvider
+    {
+        private readonly ISchedulerFactory _schedulerFactory;
+        private readonly SemaphoreSlim _semaphore = new(1);
+        private IScheduler? _scheduler;
+
+        public QuartzSchedulerProvider(ISchedulerFactory schedulerFactory) => _schedulerFactory = schedulerFactory;
+
+        public async Task<IScheduler> GetSchedulerAsync(CancellationToken cancellationToken)
+        {
+            if (_scheduler != null)
+                return _scheduler;
+            
+            await _semaphore.WaitAsync(cancellationToken);
+            
+            try
+            {
+                if (_scheduler != null)
+                    return _scheduler;
+                
+                _scheduler = await _schedulerFactory.GetScheduler(cancellationToken);
+                return _scheduler!;
+            }
+            finally
+            {
+                _semaphore.Release();
+            }
+        }
+    }
+}

src/activities/Elsa.Activities.Temporal.Quartz/Services/QuartzWorkflowDefinitionScheduler.cs

@@ -12,13 +12,13 @@ namespace Elsa.Activities.Temporal.Quartz.Services
     public class QuartzWorkflowDefinitionScheduler : IWorkflowDefinitionScheduler
     {
         private static readonly string RunWorkflowJobKey = nameof(RunQuartzWorkflowDefinitionJob);
-        private readonly ISchedulerFactory _schedulerFactory;
+        private readonly QuartzSchedulerProvider _schedulerProvider;
         private readonly ILogger _logger;
         private readonly SemaphoreSlim _semaphore = new(1);
 
-        public QuartzWorkflowDefinitionScheduler(ISchedulerFactory schedulerFactory, ILogger<QuartzWorkflowDefinitionScheduler> logger)
+        public QuartzWorkflowDefinitionScheduler(QuartzSchedulerProvider schedulerProvider, ILogger<QuartzWorkflowDefinitionScheduler> logger)
         {
-            _schedulerFactory = schedulerFactory;
+            _schedulerProvider = schedulerProvider;
             _logger = logger;
         }
 
@@ -41,7 +41,7 @@ namespace Elsa.Activities.Temporal.Quartz.Services
         
         public async Task UnscheduleAsync(string workflowDefinitionId, string activityId, CancellationToken cancellationToken)
         {
-            var scheduler = await _schedulerFactory.GetScheduler(cancellationToken);
+            var scheduler = await _schedulerProvider.GetSchedulerAsync(cancellationToken);
             var trigger = CreateTriggerKey(workflowDefinitionId, activityId);
             var existingTrigger = await scheduler.GetTrigger(trigger, cancellationToken);
 
@@ -51,7 +51,7 @@ namespace Elsa.Activities.Temporal.Quartz.Services
         
         public async Task UnscheduleAsync(string workflowDefinitionId, CancellationToken cancellationToken)
         {
-            var scheduler = await _schedulerFactory.GetScheduler(cancellationToken);
+            var scheduler = await _schedulerProvider.GetSchedulerAsync(cancellationToken);
             var groupName = CreateTriggerGroupKey(workflowDefinitionId);
             var existingTriggers = await scheduler.GetTriggerKeys(GroupMatcher<TriggerKey>.GroupEquals(groupName), cancellationToken);
 
@@ -61,18 +61,18 @@ namespace Elsa.Activities.Temporal.Quartz.Services
 
         public async Task UnscheduleAllAsync(CancellationToken cancellationToken)
         {
-            var scheduler = await _schedulerFactory.GetScheduler(cancellationToken);
+            var scheduler = await _schedulerProvider.GetSchedulerAsync(cancellationToken);
             var jobKeys = await scheduler.GetJobKeys(GroupMatcher<JobKey>.GroupStartsWith("workflow-definition"), cancellationToken);
             await scheduler.DeleteJobs(jobKeys, cancellationToken);
         }
 
         private async Task ScheduleJob(ITrigger trigger, CancellationToken cancellationToken)
         {
+            var scheduler = await _schedulerProvider.GetSchedulerAsync(cancellationToken);
             await _semaphore.WaitAsync(cancellationToken);
 
             try
             {
-                var scheduler = await _schedulerFactory.GetScheduler(cancellationToken);
                 var existingTrigger = await scheduler.GetTrigger(trigger.Key, cancellationToken);
 
                 // For workflow definitions we only schedule the job if one doesn't exist already because another node may have created it beforehand.

src/activities/Elsa.Activities.Temporal.Quartz/Services/QuartzWorkflowInstanceScheduler.cs

@@ -12,13 +12,13 @@ namespace Elsa.Activities.Temporal.Quartz.Services
     public class QuartzWorkflowInstanceScheduler : IWorkflowInstanceScheduler
     {
         private static readonly string RunWorkflowJobKey = nameof(RunQuartzWorkflowInstanceJob);
-        private readonly ISchedulerFactory _schedulerFactory;
+        private readonly QuartzSchedulerProvider _schedulerProvider;
         private readonly ILogger _logger;
         private readonly SemaphoreSlim _semaphore = new(1);
 
-        public QuartzWorkflowInstanceScheduler(ISchedulerFactory schedulerFactory, ILogger<QuartzWorkflowInstanceScheduler> logger)
+        public QuartzWorkflowInstanceScheduler(QuartzSchedulerProvider schedulerProvider, ILogger<QuartzWorkflowInstanceScheduler> logger)
         {
-            _schedulerFactory = schedulerFactory;
+            _schedulerProvider = schedulerProvider;
             _logger = logger;
         }
 
@@ -41,7 +41,7 @@ namespace Elsa.Activities.Temporal.Quartz.Services
         
         public async Task UnscheduleAsync(string workflowInstanceId, string activityId, CancellationToken cancellationToken)
         {
-            var scheduler = await _schedulerFactory.GetScheduler(cancellationToken);
+            var scheduler = await _schedulerProvider. GetSchedulerAsync(cancellationToken);
             var trigger = CreateTriggerKey(workflowInstanceId, activityId);
             var existingTrigger = await scheduler.GetTrigger(trigger, cancellationToken);
 
@@ -51,7 +51,7 @@ namespace Elsa.Activities.Temporal.Quartz.Services
         
         public async Task UnscheduleAsync(string workflowInstanceId, CancellationToken cancellationToken)
         {
-            var scheduler = await _schedulerFactory.GetScheduler(cancellationToken);
+            var scheduler = await _schedulerProvider. GetSchedulerAsync(cancellationToken);
             var groupName = CreateTriggerGroupKey(workflowInstanceId);
             var existingTriggers = await scheduler.GetTriggerKeys(GroupMatcher<TriggerKey>.GroupEquals(groupName), cancellationToken);
 
@@ -61,18 +61,18 @@ namespace Elsa.Activities.Temporal.Quartz.Services
 
         public async Task UnscheduleAllAsync(CancellationToken cancellationToken)
         {
-            var scheduler = await _schedulerFactory.GetScheduler(cancellationToken);
+            var scheduler = await _schedulerProvider. GetSchedulerAsync(cancellationToken);
             var jobKeys = await scheduler.GetJobKeys(GroupMatcher<JobKey>.GroupStartsWith("workflow-instance"), cancellationToken);
             await scheduler.DeleteJobs(jobKeys, cancellationToken);
         }
 
         private async Task ScheduleJob(ITrigger trigger, CancellationToken cancellationToken)
         {
+            var scheduler = await _schedulerProvider. GetSchedulerAsync(cancellationToken);
             await _semaphore.WaitAsync(cancellationToken);
 
             try
             {
-                var scheduler = await _schedulerFactory.GetScheduler(cancellationToken);
                 var existingTrigger = await scheduler.GetTrigger(trigger.Key, cancellationToken);
 
                 if (existingTrigger != null)
@@ -102,7 +102,7 @@ namespace Elsa.Activities.Temporal.Quartz.Services
             var groupName = CreateTriggerGroupKey(workflowInstanceId);
             return new TriggerKey($"activity:{activityId}", groupName);
         }
-        
+
         private string CreateTriggerGroupKey(string workflowInstanceId) => $"workflow-instance:{workflowInstanceId}";
     }
 }