fix ManagedBufferBase pinning behavior

Author: antonfirsov

src/ImageSharp/Memory/Allocators/Internals/ManagedBufferBase.cs

@@ -24,7 +24,7 @@ public override unsafe MemoryHandle Pin(int elementIndex = 0)
             }
 
             void* ptr = (void*)this.pinHandle.AddrOfPinnedObject();
-            return new MemoryHandle(ptr, this.pinHandle);
+            return new MemoryHandle(ptr, this.pinHandle, this);
         }
 
         /// <inheritdoc />
@@ -42,4 +42,4 @@ public override void Unpin()
         /// <returns>The pinnable <see cref="object"/>.</returns>
         protected abstract object GetPinnableObject();
     }
-}
+}

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

@@ -114,6 +114,23 @@ public void CleaningRequests_AreControlledByAllocationParameter_Clean(Allocation
             }
         }
 
+        [Fact]
+        public unsafe void Allocate_MemoryIsPinnableMultipleTimes()
+        {
+            ArrayPoolMemoryAllocator allocator = this.LocalFixture.MemoryAllocator;
+            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);
+            }
+        }
+
         [Theory]
         [InlineData(false)]
         [InlineData(true)]

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

@@ -2,6 +2,7 @@
 // Licensed under the Apache License, Version 2.0.
 
 using System;
+using System.Buffers;
 using System.Runtime.InteropServices;
 using SixLabors.ImageSharp.Memory;
 using Xunit;
@@ -36,9 +37,26 @@ public void AllocateManagedByteBuffer_IncorrectAmount_ThrowsCorrect_ArgumentOutO
             Assert.Equal("length", ex.ParamName);
         }
 
+        [Fact]
+        public unsafe void Allocate_MemoryIsPinnableMultipleTimes()
+        {
+            SimpleGcMemoryAllocator allocator = this.MemoryAllocator;
+            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);
+            }
+        }
+
         [StructLayout(LayoutKind.Explicit, Size = 512)]
         private struct BigStruct
         {
         }
     }
-}
+}