Custom level of parallel tests

source/Octopus.Tentacle.Tests.Integration/AssemblyInfo.cs

@@ -1,3 +1,5 @@
 using NUnit.Framework;
+using Octopus.Tentacle.Tests.Integration.Support.TestAttributes;
 
-[assembly: Parallelizable(ParallelScope.All)]
+[assembly: Parallelizable(ParallelScope.All)]
+[assembly: CustomLevelOfParallelism]

source/Octopus.Tentacle.Tests.Integration/Support/HowManyParallelTests.cs

@@ -0,0 +1,18 @@
+using System;
+using NUnit.Framework;
+using Octopus.Tentacle.Tests.Integration.Support.TestAttributes;
+
+namespace Octopus.Tentacle.Tests.Integration.Support
+{
+    public class HowManyParallelTests : IntegrationTest
+    {
+        [Test]
+        public void HowManyTestsAreRunningInParallel()
+        {
+            // Only exists to make it easy to find out how many tests are running in parallel.
+            Logger.Information("The number of parallel tests are: {LevelOfParallelism} on a machine with {NumberOfCpuCores} cpu cores.", 
+                CustomLevelOfParallelismAttribute.LevelOfParallelism(),
+                Environment.ProcessorCount);
+        }
+    }
+}

source/Octopus.Tentacle.Tests.Integration/Support/TestAttributes/CustomLevelOfParallelismAttribute.cs

@@ -0,0 +1,57 @@
+using System;
+using NUnit.Framework;
+using NUnit.Framework.Api;
+using Octopus.Tentacle.Util;
+
+namespace Octopus.Tentacle.Tests.Integration.Support.TestAttributes
+{
+    /// <summary>
+    /// Halibut tests take time but don't use lots of CPU.
+    ///
+    /// For local development by default increase the level of parallelism to 2x the number of Cores.
+    ///
+    /// This can be overriden with the environment variable "CustomLevelOfParallelism" e.g.
+    /// CustomLevelOfParallelism=256
+    ///
+    /// When run in the build the default level is always used.
+    /// </summary>
+    [AttributeUsage( AttributeTargets.Assembly, AllowMultiple=false, Inherited=false )]
+    public class CustomLevelOfParallelismAttribute : PropertyAttribute
+    {
+        public CustomLevelOfParallelismAttribute() : base(LevelOfParallelismAttributePropertyName(), LevelOfParallelism())
+        {
+        }
+
+        public static int LevelOfParallelism()
+        {
+            if (TeamCityDetection.IsRunningInTeamCity())
+            {
+                return LevelOfParallelismFromEnvVar() ?? NUnitTestAssemblyRunner.DefaultLevelOfParallelism * 2;
+            }
+
+            return LevelOfParallelismFromEnvVar() ?? NUnitTestAssemblyRunner.DefaultLevelOfParallelism * 2;
+        }
+
+        static int? LevelOfParallelismFromEnvVar()
+        {
+            var nunitLevelOfParallelismSetting = Environment.GetEnvironmentVariable("CustomLevelOfParallelism");
+            if (!string.IsNullOrEmpty(nunitLevelOfParallelismSetting))
+            {
+                if (int.TryParse(nunitLevelOfParallelismSetting, out var level))
+                {
+                    return level;
+                }
+            }
+
+            return null;
+        }
+
+        static string LevelOfParallelismAttributePropertyName()
+        {
+            string propertyName = typeof(LevelOfParallelismAttribute).Name;
+            if ( propertyName.EndsWith( "Attribute" ) )
+                propertyName = propertyName.Substring( 0, propertyName.Length - 9 );
+            return propertyName;
+        }
+    }
+}