feature: Add a build flavor 'Minimal'

In an effort to use NetMQ with Unity3D, one problem was trying to
satisfy the dependencies for `System.ServiceModel.Primitives`. It
required also providing `System.ServiceModel.{Http,Duplex,NetTcp}` and
perhaps more before I gave up. These dependencies were only noticed
while Unity3D built the project. When using NetMQ in a regular dotnet
project, I've had no problems with the `System.ServiceModel.Primitives`
dependencies.

I added an alternative implementation of `IBufferPool` using the
`ArrayPool` from `System.Buffers` but it is unused. I instrumented the
build system to have a "Flavor" that by default is set to "Standard",
which changes nothing from before.

For my usecase, however, I set the flavor to "Minimal".

    $ dotnet build -p:Flavor=Minimal

This excludes "System.ServiceModel.Primitives" and defines a
preprocessor flag of `FLAVOR_MINIMAL` that excludes the
BufferManagerBufferPool class and uses the ArrayBufferPool instead.

Author: shanecelis

src/NetMQ/BufferPool.cs

@@ -1,5 +1,7 @@
 using System;
+#if ! FLAVOR_MINIMAL
 using System.ServiceModel.Channels;
+#endif
 using System.Threading;
 using System.Buffers;
 
@@ -25,6 +27,10 @@ public interface IBufferPool : IDisposable
         void Return(byte[] buffer);
     }
 
+    /// <summary>
+    /// This implementation of <see cref="IBufferPool"/> uses System.Buffer's ArrayPool
+    /// class to manage a pool of buffers.
+    /// </summary>
     public class ArrayBufferPool : IBufferPool
     {
         private readonly ArrayPool<byte> m_arrayPool;
@@ -84,6 +90,8 @@ protected virtual void Dispose(bool disposing)
             // We don't have a means of clearing ArrayPool like BufferManager does.
         }
     }
+
+#if ! FLAVOR_MINIMAL
     /// <summary>
     /// This implementation of <see cref="IBufferPool"/> uses WCF's <see cref="BufferManager"/>
     /// class to manage a pool of buffers.
@@ -148,6 +156,7 @@ protected virtual void Dispose(bool disposing)
             m_bufferManager.Clear();
         }
     }
+#endif
 
     /// <summary>
     /// This simple implementation of <see cref="IBufferPool"/> does no buffer pooling. Instead, it uses regular
@@ -194,6 +203,7 @@ protected virtual void Dispose(bool disposing)
         }
     }
 
+
     /// <summary>
     /// Contains a singleton instance of <see cref="IBufferPool"/> used for allocating byte arrays
     /// for <see cref="Msg"/> instances with type <see cref="MsgType.Pool"/>.
@@ -204,7 +214,7 @@ protected virtual void Dispose(bool disposing)
     /// <para/>
     /// The default implementation is <see cref="GCBufferPool"/>.
     /// <list type="bullet">
-    /// <item>Call <see cref="SetBufferManagerBufferPool"/> to replace it with a <see cref="BufferManagerBufferPool"/>.</item>
+    /// <item>Call <see cref="SetBufferManagerBufferPool"/> to replace it with a pooling implementation.</item>
     /// <item>Call <see cref="SetGCBufferPool"/> to reinstate the default <see cref="GCBufferPool"/>.</item>
     /// <item>Call <see cref="SetCustomBufferPool"/> to substitute a custom implementation for the allocation and
     /// deallocation of message buffers.</item>
@@ -223,15 +233,19 @@ public static void SetGCBufferPool()
         }
 
         /// <summary>
-        /// Set BufferPool to use the <see cref="BufferManagerBufferPool"/> to manage the buffer-pool.
+        /// Set BufferPool to use a buffer pooling implementation to manage the buffer-pool.
         /// </summary>
         /// <param name="maxBufferPoolSize">the maximum size to allow for the buffer pool</param>
         /// <param name="maxBufferSize">the maximum size to allow for each individual buffer in the pool</param>
         /// <exception cref="InsufficientMemoryException">There was insufficient memory to create the requested buffer pool.</exception>
         /// <exception cref="ArgumentOutOfRangeException">Either maxBufferPoolSize or maxBufferSize was less than zero.</exception>
         public static void SetBufferManagerBufferPool(long maxBufferPoolSize, int maxBufferSize)
         {
+#if FLAVOR_MINIMAL
+            SetCustomBufferPool(new ArrayBufferPool(maxBufferPoolSize, maxBufferSize));
+#else
             SetCustomBufferPool(new BufferManagerBufferPool(maxBufferPoolSize, maxBufferSize));
+#endif
         }
 
         /// <summary>

src/NetMQ/NetMQ.csproj

@@ -22,6 +22,19 @@
     <IncludeSymbols>true</IncludeSymbols>
     <SymbolPackageFormat>snupkg</SymbolPackageFormat>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
+
+    <Flavor>Standard</Flavor>
+    <!-- Set flavor to minimal to exclude dependencies where possible. Found to
+         useful when building for use with Unity3D. Can change here in .csproj
+         or by using the following command argument when building:
+
+         $ dotnet build -p:Flavor=Minimal
+    -->
+    <!-- <Flavor>Minimal</Flavor> -->
+  </PropertyGroup>
+
+  <PropertyGroup Condition="'$(Flavor)'=='Minimal'">
+    <DefineConstants>$(DefineConstants);FLAVOR_MINIMAL</DefineConstants>
   </PropertyGroup>
 
   <PropertyGroup Condition=" '$(TargetFramework)' != 'netstandard2.1' ">
@@ -40,13 +53,13 @@
   </ItemGroup>
 
   <ItemGroup Condition=" '$(TargetFramework)' == 'netstandard2.0' ">
-    <PackageReference Include="System.ServiceModel.Primitives" Version="4.9.0" />
+    <PackageReference Include="System.ServiceModel.Primitives" Version="4.9.0" Condition="'$(Flavor)'!='Minimal'"/>
     <PackageReference Include="System.ValueTuple" Version="4.5.0" />
     <PackageReference Include="System.Threading.Tasks.Extensions" Version="4.5.4" />
   </ItemGroup>
 
   <ItemGroup Condition=" '$(TargetFramework)' == 'netstandard2.1' ">
-    <PackageReference Include="System.ServiceModel.Primitives" Version="4.9.0" />
+    <PackageReference Include="System.ServiceModel.Primitives" Version="4.9.0" Condition="'$(Flavor)'!='Minimal'"/>
     <PackageReference Include="System.ValueTuple" Version="4.5.0" />
   </ItemGroup>