Fix EngineStage.Run for warmupCount=0 (fixes #2185) (#2186)

Author: AndreyAkinshin

src/BenchmarkDotNet/Engines/EngineStage.cs

@@ -23,6 +23,12 @@ protected Measurement RunIteration(IterationMode mode, IterationStage stage, int
         internal List<Measurement> Run(IStoppingCriteria criteria, long invokeCount, IterationMode mode, IterationStage stage, int unrollFactor)
         {
             var measurements = new List<Measurement>(criteria.MaxIterationCount);
+            if (criteria.Evaluate(measurements).IsFinished)
+            {
+                WriteLine();
+                return measurements;
+            }
+
             int iterationCounter = 0;
             while (true)
             {

tests/BenchmarkDotNet.IntegrationTests/JobTests.cs

@@ -0,0 +1,40 @@
+using System.Linq;
+using System.Threading;
+using BenchmarkDotNet.Attributes;
+using BenchmarkDotNet.Configs;
+using BenchmarkDotNet.Engines;
+using BenchmarkDotNet.Jobs;
+using BenchmarkDotNet.Reports;
+using Xunit;
+using Xunit.Abstractions;
+
+namespace BenchmarkDotNet.IntegrationTests
+{
+    public class JobTests : BenchmarkTestExecutor
+    {
+        public JobTests(ITestOutputHelper output) : base(output) { }
+
+        [Fact]
+        public void ZeroWarmupCountIsApplied()
+        {
+            var job = Job.Default
+                .WithEvaluateOverhead(false)
+                .WithWarmupCount(0)
+                .WithIterationCount(1)
+                .WithInvocationCount(1)
+                .WithUnrollFactor(1);
+            var config = DefaultConfig.Instance.AddJob(job).WithOptions(ConfigOptions.DisableOptimizationsValidator);
+            var summary = CanExecute<ZeroWarmupBench>(config);
+            var report = summary.Reports.Single();
+            int workloadWarmupCount = report.AllMeasurements
+                .Count(m => m.Is(IterationMode.Workload, IterationStage.Warmup));
+            Assert.Equal(0, workloadWarmupCount);
+        }
+
+        public class ZeroWarmupBench
+        {
+            [Benchmark]
+            public void Foo() => Thread.Sleep(10);
+        }
+    }
+}