fix race condition in run once at start logic

Src/Coravel/Scheduling/Schedule/Scheduler.cs

@@ -27,6 +27,7 @@ public class Scheduler : IScheduler, ISchedulerConfiguration
         private string _currentWorkerName;
         private CancellationTokenSource _cancellationTokenSource;
         private bool _isFirstTick = true;
+        private object _isFirstTickLockObj = new object();
 
         public Scheduler(IMutex mutex, IServiceScopeFactory scopeFactory, IDispatcher dispatcher)
         {
@@ -102,8 +103,21 @@ public IScheduler OnWorker(string workerName)
         public async Task RunAtAsync(DateTime utcDate)
         {
             Interlocked.Increment(ref this._schedulerIterationsActiveCount);
-            bool isFirstTick = this._isFirstTick;
-            this._isFirstTick = false;
+
+            // Possibility for race condition here.
+            // Note: This was a bug fixed by issue 353.
+            bool isFirstTick = false;
+            if(this._isFirstTick)
+            {
+                lock(this._isFirstTickLockObj)
+                {
+                    // Set to `this._isFirstTick` not `true` because there might be multiple threads coming into
+                    // this critical section - but only one should count as "the first run".
+                    isFirstTick = this._isFirstTick;
+                    this._isFirstTick = false;
+                }
+            }
+
             await RunWorkersAt(utcDate, isFirstTick);
             Interlocked.Decrement(ref this._schedulerIterationsActiveCount);
         }

Src/UnitTests/CoravelUnitTests/Scheduling/IntervalTests/SchedulerRunAtStartTests.cs

@@ -1,3 +1,6 @@
+using System;
+using System.Collections.Generic;
+using System.Threading;
 using System.Threading.Tasks;
 using Coravel.Scheduling.Schedule;
 using Coravel.Scheduling.Schedule.Mutex;
@@ -38,8 +41,8 @@ public async Task TestRunsOnlyAtStart(int atHour, int atMinute, int day, int hou
             Assert.True(taskRan);
 
             taskRan = false;
+
             await RunScheduledTasksFromDayHourMinutes(scheduler, day, hour, minute);
-
             Assert.False(taskRan);
         }
 
@@ -67,9 +70,51 @@ public async Task TestRunsAtStartAndWhenDue(int atHour, int atMinute, int day, i
             Assert.True(taskRan);
 
             taskRan = false;
-            await RunScheduledTasksFromDayHourMinutes(scheduler, day, hour, minute);
 
+            await RunScheduledTasksFromDayHourMinutes(scheduler, day, hour, minute);
             Assert.True(taskRan);
         }
+
+        /// <summary>
+        /// This test runs for a long time - like 40 seconds on a fast codespaces VM. But, that's how many iterations of running this test takes to
+        /// catch a really rare race condition. This was causing the `RunOnceAtStart` method to have an bug where intermittently those tasks weren't running
+        /// when the application started up.
+        /// </summary>
+        [Fact]
+        public async Task TestConcurrentThreadsDoesNotSkipForcedRun()
+        {
+            for(int i = 0; i < 10000; i++)
+            {
+                var scheduler = new Scheduler(new InMemoryMutex(), new ServiceScopeFactoryStub(), new DispatcherStub());
+                var taskRan = false;
+                var taskRunCount = 0;
+
+                scheduler
+                    .Schedule(() => 
+                    { 
+                        Interlocked.Increment(ref taskRunCount); 
+                        taskRan = true; 
+                    })
+                    .Monthly()
+                    .RunOnceAtStart();
+
+
+                Func<Task> runTask = async () => 
+                { 
+                    await Task.Delay(1); 
+                    await scheduler.RunAtAsync(new DateTime(2000, 1, 15));
+                };
+
+                var tasks = new List<Task>()
+                {
+                    runTask(), runTask(), runTask()
+                };
+
+                await Task.WhenAll(tasks);
+
+                Assert.True(taskRan);
+                Assert.Equal(1, taskRunCount);
+            }
+        }
     }
 }