Add check for subcommand cycle (#239)

Author: vpkopylov

src/CommandLineUtils/Conventions/SubcommandAttributeConvention.cs

@@ -1,6 +1,8 @@
 // Copyright (c) Nate McMaster.
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
+using McMaster.Extensions.CommandLineUtils.Abstractions;
+using McMaster.Extensions.CommandLineUtils.Errors;
 using System;
 using System.Linq;
 using System.Reflection;
@@ -28,6 +30,8 @@ public virtual void Apply(ConventionContext context)
                 var contextArgs = new object[] { context, attribute };
                 foreach (var type in attribute.Types)
                 {
+                    AssertSubcommandIsNotCycled(type, context.Application);
+
                     var impl = s_addSubcommandMethod.MakeGenericMethod(type);
                     try
                     {
@@ -42,6 +46,19 @@ public virtual void Apply(ConventionContext context)
             }
         }
 
+        private void AssertSubcommandIsNotCycled(Type modelType, CommandLineApplication parentCommand)
+        {
+            while (parentCommand != null)
+            {
+                if (parentCommand is IModelAccessor parentCommandAccessor
+                    && parentCommandAccessor.GetModelType() == modelType)
+                {
+                    throw new SubcommandCycleException(modelType);
+                }
+                parentCommand = parentCommand.Parent;
+            }
+        }
+
         private static readonly MethodInfo s_addSubcommandMethod
             = typeof(SubcommandAttributeConvention).GetRuntimeMethods()
                 .Single(m => m.Name == nameof(AddSubcommandImpl));

src/CommandLineUtils/Errors/SubcommandCycleException.cs

@@ -0,0 +1,28 @@
+// Copyright (c) Nate McMaster.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+
+namespace McMaster.Extensions.CommandLineUtils.Errors
+{
+    /// <summary>
+    /// The exception that is thrown when a subcommand cycle is detected
+    /// </summary>
+    public class SubcommandCycleException : Exception
+    {
+        /// <summary>
+        /// Initializes an instance of <see cref="SubcommandCycleException"/>.
+        /// </summary>
+        /// <param name="modelType">The type of the cycled command model</param>
+        public SubcommandCycleException(Type modelType)
+            : base($"Subcommand cycle detected: trying to add command of model {modelType} as its own direct or indirect subcommand")
+        {
+            ModelType = modelType;
+        }
+
+        /// <summary>
+        /// The type of the cycled command model
+        /// </summary>
+        public Type ModelType { get; }
+    }
+}

test/CommandLineUtils.Tests/SubcommandAttributeTests.cs

@@ -1,6 +1,7 @@
 // Copyright (c) Nate McMaster.
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
+using McMaster.Extensions.CommandLineUtils.Errors;
 using System;
 using System.IO;
 using System.Linq;
@@ -198,5 +199,37 @@ public void CommandNamesCannotDifferByCaseOnly()
                 () => CommandLineApplication.Execute<DuplicateSubCommands>(new TestConsole(_output)));
             Assert.Equal(Strings.DuplicateSubcommandName("level1"), ex.Message);
         }
+
+        [Command, Subcommand(typeof(CycledCommand2))]
+        private class CycledCommand1
+        {
+        }
+
+        [Command, Subcommand(typeof(CycledCommand1))]
+        private class CycledCommand2
+        {
+        }
+
+        [Fact]
+        public void ThrowsForCycledSubCommand()
+        {
+            var ex = Assert.Throws<SubcommandCycleException>(
+                () => CommandLineApplication.Execute<CycledCommand1>(new TestConsole(_output)));
+            Assert.Equal(typeof(CycledCommand1), ex.ModelType);
+        }
+
+        [Command, Subcommand(typeof(SelfCycledCommand))]
+        private class SelfCycledCommand
+        {
+
+        }
+
+        [Fact]
+        public void ThrowsForSelfCycledCommand()
+        {
+            var ex = Assert.Throws<SubcommandCycleException>(
+                () => CommandLineApplication.Execute<SelfCycledCommand>(new TestConsole(_output)));
+            Assert.Equal(typeof(SelfCycledCommand), ex.ModelType);
+        }
     }
 }