pass pinnable correctly

src/ImageSharp/Memory/Allocators/Internals/UniformUnmanagedMemoryPool.Buffer{T}.cs

@@ -36,7 +36,7 @@ namespace SixLabors.ImageSharp.Memory.Internals
                 this.BufferHandle.DangerousAddRef(ref unused);
 
                 void* pbData = Unsafe.Add<T>(this.Pointer, elementIndex);
-                return new MemoryHandle(pbData);
+                return new MemoryHandle(pbData, pinnable: this);
             }
 
             /// <inheritdoc />

src/ImageSharp/Memory/Allocators/Internals/UnmanagedBuffer{T}.cs

@@ -42,7 +42,7 @@ namespace SixLabors.ImageSharp.Memory.Internals
             this.bufferHandle.DangerousAddRef(ref unused);
 
             void* pbData = Unsafe.Add<T>(this.Pointer, elementIndex);
-            return new MemoryHandle(pbData);
+            return new MemoryHandle(pbData, pinnable: this);
         }
 
         /// <inheritdoc />

src/ImageSharp/Memory/UnmanagedMemoryManager{T}.cs

@@ -49,7 +49,7 @@ namespace SixLabors.ImageSharp.Memory
         /// <inheritdoc/>
         public override MemoryHandle Pin(int elementIndex = 0)
         {
-            return new MemoryHandle(((T*)this.pointer) + elementIndex);
+            return new MemoryHandle(((T*)this.pointer) + elementIndex, pinnable: this);
         }
 
         /// <inheritdoc/>

tests/ImageSharp.Tests/Image/ImageTests.WrapMemory.cs

@@ -71,7 +71,7 @@ namespace SixLabors.ImageSharp.Tests
                 public override unsafe MemoryHandle Pin(int elementIndex = 0)
                 {
                     void* ptr = (void*)this.bmpData.Scan0;
-                    return new MemoryHandle(ptr);
+                    return new MemoryHandle(ptr, pinnable: this);
                 }
 
                 public override void Unpin()

tests/ImageSharp.Tests/Memory/Allocators/UniformUnmanagedPoolMemoryAllocatorTests.cs

@@ -130,6 +130,23 @@ namespace SixLabors.ImageSharp.Tests.Memory.Allocators
             Assert.Equal(1, g.Count);
         }
 
+        [Fact]
+        public unsafe void Allocate_MemoryIsPinnableMultipleTimes()
+        {
+            var allocator = new UniformUnmanagedMemoryPoolMemoryAllocator(null);
+            using IMemoryOwner<byte> memoryOwner = allocator.Allocate<byte>(100);
+
+            using (MemoryHandle pin = memoryOwner.Memory.Pin())
+            {
+                Assert.NotEqual(IntPtr.Zero, (IntPtr)pin.Pointer);
+            }
+
+            using (MemoryHandle pin = memoryOwner.Memory.Pin())
+            {
+                Assert.NotEqual(IntPtr.Zero, (IntPtr)pin.Pointer);
+            }
+        }
+
         [Fact]
         public void MemoryAllocator_Create_WithoutSettings_AllocatesDiscontiguousMemory()
         {

tests/ImageSharp.Tests/TestUtilities/TestMemoryAllocator.cs

@@ -146,12 +146,12 @@ namespace SixLabors.ImageSharp.Tests.Memory
                 }
 
                 void* ptr = (void*)this.pinHandle.AddrOfPinnedObject();
-                return new MemoryHandle(ptr, this.pinHandle);
+                return new MemoryHandle(ptr, pinnable: this);
             }
 
             public override void Unpin()
             {
-                throw new NotImplementedException();
+                this.pinHandle.Free();
             }
 
             /// <inheritdoc />