Introduce ApplyOptions and use in allocators

src/ImageSharp/Memory/AllocationTrackingState.cs

@@ -21,6 +21,12 @@ internal struct AllocationTrackingState
     /// </summary>
     /// <param name="allocator">The allocator that owns the reservation.</param>
     /// <param name="lengthInBytes">The reserved allocation size, in bytes.</param>
+    /// <remarks>
+    /// Must complete-before the owning object's reference is observable to any other thread.
+    /// <see cref="MemoryAllocator"/> guarantees this by attaching synchronously on the allocating
+    /// thread before returning the owner; reference publication then provides the release fence
+    /// that makes these field writes visible to a subsequent <see cref="Release"/> on another thread.
+    /// </remarks>
     internal void Attach(MemoryAllocator allocator, long lengthInBytes)
     {
         this.allocator = allocator;

src/ImageSharp/Memory/Allocators/MemoryAllocator.cs

@@ -83,18 +83,26 @@ public abstract class MemoryAllocator
     public static MemoryAllocator Create(MemoryAllocatorOptions options)
     {
         UniformUnmanagedMemoryPoolMemoryAllocator allocator = new(options.MaximumPoolSizeMegabytes);
+        allocator.ApplyOptions(options);
+        return allocator;
+    }
+
+    /// <summary>
+    /// Applies the supplied <see cref="MemoryAllocatorOptions"/> to this instance.
+    /// </summary>
+    /// <param name="options">The options to apply. Properties left as <see langword="null"/> are ignored.</param>
+    private protected void ApplyOptions(MemoryAllocatorOptions options)
+    {
         if (options.AllocationLimitMegabytes.HasValue)
         {
-            allocator.MemoryGroupAllocationLimitBytes = options.AllocationLimitMegabytes.Value * 1024L * 1024L;
-            allocator.SingleBufferAllocationLimitBytes = (int)Math.Min(allocator.SingleBufferAllocationLimitBytes, allocator.MemoryGroupAllocationLimitBytes);
+            this.MemoryGroupAllocationLimitBytes = options.AllocationLimitMegabytes.Value * 1024L * 1024L;
+            this.SingleBufferAllocationLimitBytes = (int)Math.Min(this.SingleBufferAllocationLimitBytes, this.MemoryGroupAllocationLimitBytes);
         }
 
         if (options.AccumulativeAllocationLimitMegabytes.HasValue)
         {
-            allocator.AccumulativeAllocationLimitBytes = options.AccumulativeAllocationLimitMegabytes.Value * 1024L * 1024L;
+            this.AccumulativeAllocationLimitBytes = options.AccumulativeAllocationLimitMegabytes.Value * 1024L * 1024L;
         }
-
-        return allocator;
     }
 
     /// <summary>

src/ImageSharp/Memory/Allocators/SimpleGcMemoryAllocator.cs

@@ -22,19 +22,7 @@ public sealed class SimpleGcMemoryAllocator : MemoryAllocator
     /// Initializes a new instance of the <see cref="SimpleGcMemoryAllocator"/> class with custom limits.
     /// </summary>
     /// <param name="options">The <see cref="MemoryAllocatorOptions"/> to apply.</param>
-    public SimpleGcMemoryAllocator(MemoryAllocatorOptions options)
-    {
-        if (options.AllocationLimitMegabytes.HasValue)
-        {
-            this.MemoryGroupAllocationLimitBytes = options.AllocationLimitMegabytes.Value * 1024L * 1024L;
-            this.SingleBufferAllocationLimitBytes = (int)Math.Min(this.SingleBufferAllocationLimitBytes, this.MemoryGroupAllocationLimitBytes);
-        }
-
-        if (options.AccumulativeAllocationLimitMegabytes.HasValue)
-        {
-            this.AccumulativeAllocationLimitBytes = options.AccumulativeAllocationLimitMegabytes.Value * 1024L * 1024L;
-        }
-    }
+    public SimpleGcMemoryAllocator(MemoryAllocatorOptions options) => this.ApplyOptions(options);
 
     /// <inheritdoc />
     protected internal override int GetBufferCapacityInBytes() => int.MaxValue;

src/ImageSharp/Memory/Allocators/UniformUnmanagedMemoryPoolMemoryAllocator.cs

@@ -70,6 +70,15 @@ internal sealed class UniformUnmanagedMemoryPoolMemoryAllocator : MemoryAllocato
         this.nonPoolAllocator = new UnmanagedMemoryAllocator(unmanagedBufferSizeInBytes);
     }
 
+    internal UniformUnmanagedMemoryPoolMemoryAllocator(
+        int sharedArrayPoolThresholdInBytes,
+        int poolBufferSizeInBytes,
+        long maxPoolSizeInBytes,
+        int unmanagedBufferSizeInBytes,
+        MemoryAllocatorOptions options)
+        : this(sharedArrayPoolThresholdInBytes, poolBufferSizeInBytes, maxPoolSizeInBytes, unmanagedBufferSizeInBytes)
+        => this.ApplyOptions(options);
+
     /// <inheritdoc />
     protected internal override int GetBufferCapacityInBytes() => this.poolBufferSizeInBytes;
 

src/ImageSharp/Memory/DiscontiguousBuffers/MemoryGroup{T}.cs

@@ -102,8 +102,8 @@ internal abstract partial class MemoryGroup<T> : IMemoryGroup<T>, IDisposable
         int bufferAlignmentInElements,
         AllocationOptions options = AllocationOptions.None)
     {
-        int bufferCapacityInBytes = allocator.GetBufferCapacityInBytes();
         Guard.NotNull(allocator, nameof(allocator));
+        int bufferCapacityInBytes = allocator.GetBufferCapacityInBytes();
 
         if (totalLengthInElements < 0)
         {

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

@@ -477,6 +477,37 @@ public class UniformUnmanagedPoolMemoryAllocatorTests
         allocator.AllocateGroup<byte>(oneMb, 1024).Dispose();
     }
 
+    [Fact]
+    public void AllocateGroup_AccumulativeLimit_NonPoolFallback_TracksOncePerGroup()
+    {
+        // Configure the pool with zero capacity so multi-segment requests bypass both the
+        // single-buffer-from-pool path and MemoryGroup<T>.TryAllocate(pool, ...) and fall
+        // through to MemoryGroup<T>.Allocate(nonPoolAllocator, ...). The unmanaged segment
+        // size is small enough that the request must span multiple segments, which is the
+        // path where per-segment double-counting could regress.
+        UniformUnmanagedMemoryPoolMemoryAllocator allocator = new(
+            sharedArrayPoolThresholdInBytes: 64 * 1024,
+            poolBufferSizeInBytes: 128 * 1024,
+            maxPoolSizeInBytes: 0,
+            unmanagedBufferSizeInBytes: 256 * 1024,
+            new MemoryAllocatorOptions { AccumulativeAllocationLimitMegabytes = 1 });
+
+        // 768 KB exceeds the pool buffer size, so the request takes the multi-segment
+        // non-pool fallback (three 256 KB segments). If tracking double-counted (group
+        // plus each segment), reservation would be 768 KB + 768 KB = 1.5 MB and exceed
+        // the 1 MB limit on allocation itself.
+        MemoryGroup<byte> g = allocator.AllocateGroup<byte>(768 * 1024, 1024);
+        Assert.True(g.Count > 1, "Test setup must exercise the multi-segment fallback path.");
+
+        // Reservation should be exactly 768 KB; another 512 KB would push to 1.25 MB and throw.
+        Assert.Throws<InvalidMemoryOperationException>(() => allocator.Allocate<byte>(512 * 1024));
+
+        g.Dispose();
+
+        // After disposal the reservation is fully released; a second equivalent group succeeds.
+        allocator.AllocateGroup<byte>(768 * 1024, 1024).Dispose();
+    }
+
     [ConditionalFact(typeof(Environment), nameof(Environment.Is64BitProcess))]
     public void MemoryAllocator_Create_SetHighLimit()
     {