Graph, Session: defaulting was changed to thread-wide using ThreadLocal<T>

src/TensorFlowNET.Core/Sessions/BaseSession.cs

@@ -38,9 +38,9 @@ namespace Tensorflow
 
         public BaseSession(string target = "", Graph g = null, SessionOptions opts = null)
         {
-            _graph = g is null ? ops.get_default_graph() : g;
+            _graph = g ?? ops.get_default_graph();
             _graph.as_default();
-            _target = UTF8Encoding.UTF8.GetBytes(target);
+            _target = Encoding.UTF8.GetBytes(target);
 
             SessionOptions newOpts = opts ?? new SessionOptions();
 

src/TensorFlowNET.Core/Sessions/Session.cs

@@ -42,7 +42,7 @@ namespace Tensorflow
 
         public Session as_default()
         {
-            tf.defaultSession = this;
+            tf._defaultSessionFactory.Value = this;
             return this;
         }
 

src/TensorFlowNET.Core/ops.cs

@@ -19,6 +19,7 @@ using System.Collections.Generic;
 using System.Runtime.InteropServices;
 using Google.Protobuf;
 using System.Linq;
+using System.Threading;
 using NumSharp;
 using static Tensorflow.Binding;
 
@@ -26,6 +27,10 @@ namespace Tensorflow
 {
     public partial class ops
     {
+        private static readonly ThreadLocal<DefaultGraphStack> _defaultGraphFactory = new ThreadLocal<DefaultGraphStack>(() => new DefaultGraphStack());
+
+        public static DefaultGraphStack default_graph_stack => _defaultGraphFactory.Value;
+
         public static int tensor_id(Tensor tensor)
         {
             return tensor.Id;
@@ -72,8 +77,6 @@ namespace Tensorflow
             return get_default_graph().get_collection_ref(key);
         }
 
-        public static DefaultGraphStack default_graph_stack = new DefaultGraphStack();
-
         /// <summary>
         /// Returns the default graph for the current thread.
         /// 
@@ -387,8 +390,6 @@ namespace Tensorflow
         /// <returns>The default `Session` being used in the current thread.</returns>
         public static Session get_default_session()
         {
-            if (tf.defaultSession == null)
-                tf.defaultSession = tf.Session();
             return tf.defaultSession;
         }
 

src/TensorFlowNET.Core/tensorflow.cs

@@ -14,12 +14,15 @@
    limitations under the License.
 ******************************************************************************/
 
+using System.Threading;
 using Tensorflow.Eager;
 
 namespace Tensorflow
 {
     public partial class tensorflow : IObjectLife
     {
+        protected internal readonly ThreadLocal<Session> _defaultSessionFactory;
+
         public TF_DataType @byte = TF_DataType.TF_UINT8;
         public TF_DataType @sbyte = TF_DataType.TF_INT8;
         public TF_DataType int16 = TF_DataType.TF_INT16;
@@ -34,7 +37,13 @@ namespace Tensorflow
 
         public Context context = new Context(new ContextOptions(), new Status());
 
-        public Session defaultSession;
+
+        public tensorflow()
+        {
+            _defaultSessionFactory = new ThreadLocal<Session>(Session);
+        }
+
+        public Session defaultSession => _defaultSessionFactory.Value;
 
         public RefVariable Variable<T>(T data,
             bool trainable = true,

test/TensorFlowNET.UnitTest/MultithreadingTests.cs

@@ -0,0 +1,71 @@
+using System;
+using System.Collections.Generic;
+using System.Runtime.InteropServices;
+using FluentAssertions;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using Tensorflow;
+using static Tensorflow.Binding;
+
+namespace TensorFlowNET.UnitTest
+{
+    [TestClass]
+    public class MultithreadingTests
+    {
+        [TestMethod]
+        public void SessionCreation()
+        {
+            tf.Session(); //create one to increase next id to 1.
+
+            MultiThreadedUnitTestExecuter.Run(8, Core);
+
+            //the core method
+            void Core(int tid)
+            {
+                tf.peak_default_graph().Should().BeNull();
+
+                //tf.Session created an other graph
+                using (var sess = tf.Session())
+                {
+                    var default_graph = tf.peak_default_graph();
+                    var sess_graph = sess.GetPrivate<Graph>("_graph");
+                    sess_graph.Should().NotBeNull();
+                    default_graph.Should().NotBeNull()
+                        .And.BeEquivalentTo(sess_graph);
+                }
+            }
+        }
+
+        [TestMethod]
+        public void GraphCreation()
+        {
+            tf.Graph(); //create one to increase next id to 1.
+
+            MultiThreadedUnitTestExecuter.Run(8, Core);
+
+            //the core method
+            void Core(int tid)
+            {
+                tf.peak_default_graph().Should().BeNull();
+                var beforehand = tf.get_default_graph(); //this should create default automatically.
+                beforehand.graph_key.Should().NotContain("0", "Already created a graph in an other thread.");
+                tf.peak_default_graph().Should().NotBeNull();
+
+                using (var sess = tf.Session())
+                {
+                    var default_graph = tf.peak_default_graph();
+                    var sess_graph = sess.GetPrivate<Graph>("_graph");
+                    sess_graph.Should().NotBeNull();
+                    default_graph.Should().NotBeNull()
+                        .And.BeEquivalentTo(sess_graph)
+                        .And.BeEquivalentTo(beforehand);
+
+                    Console.WriteLine($"{tid}-{default_graph.graph_key}");
+
+                    //var result = sess.run(new object[] {g, a});
+                    //var actualDeriv = result[0].GetData<float>()[0];
+                    //var actual = result[1].GetData<float>()[0];
+                }
+            }
+        }
+    }
+}

test/TensorFlowNET.UnitTest/SessionTest.cs

@@ -93,7 +93,7 @@ namespace TensorFlowNET.UnitTest
                 using (var sess = tf.Session())
                 {
                     var result = c.eval(sess);
-                    Assert.AreEqual(6, result.Data<double>()[0]);
+                    Assert.AreEqual(6, result.GetAtIndex<double>(0));
                 }
             }
         }