src/csharp/Grpc.Core/Internal/ReusableSliceBuffer.cs

#region Copyright notice and license

// Copyright 2019 The gRPC Authors
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

#endregion

#if GRPC_CSHARP_SUPPORT_SYSTEM_MEMORY

using Grpc.Core.Utils;
using System;
using System.Threading;

using System.Buffers;

namespace Grpc.Core.Internal
{
    internal class ReusableSliceBuffer
    {
        public const int MaxCachedSegments = 1024;  // ~4MB payload for 4K slices

        readonly SliceSegment[] cachedSegments = new SliceSegment[MaxCachedSegments];
        int populatedSegmentCount = 0;

        public ReadOnlySequence<byte> PopulateFrom(IBufferReader bufferReader)
        {
            long offset = 0;
            int index = 0;
            SliceSegment prevSegment = null;
            while (bufferReader.TryGetNextSlice(out Slice slice))
            {
                // Initialize cached segment if still null or just allocate a new segment if we already reached MaxCachedSegments
                var current = index < cachedSegments.Length ? cachedSegments[index] : new SliceSegment();
                if (current == null)
                {
                    current = cachedSegments[index] = new SliceSegment();
                }

                current.Reset(slice, offset);
                prevSegment?.SetNext(current);

                index ++;
                offset += slice.Length;
                prevSegment = current;
            }
            populatedSegmentCount = index;

            // Not necessary for ending the ReadOnlySequence, but for making sure we
            // don't keep more than MaxCachedSegments alive.
            prevSegment?.SetNext(null);

            if (index == 0)
            {
                return ReadOnlySequence<byte>.Empty;
            }

            var firstSegment = cachedSegments[0];
            var lastSegment = prevSegment;
            return new ReadOnlySequence<byte>(firstSegment, 0, lastSegment, lastSegment.Memory.Length);
        }

        public void Invalidate()
        {
            if (populatedSegmentCount == 0)
            {
                return;
            }
            var segment = cachedSegments[0];
            while (segment != null)
            {
                segment.Reset(new Slice(IntPtr.Zero, 0), 0);
                segment.SetNext(null);
                segment = (SliceSegment) segment.Next;
            }
            populatedSegmentCount = 0;
        }

        // Represents a segment in ReadOnlySequence
        // Segment is backed by Slice and the instances are reusable.
        private class SliceSegment : ReadOnlySequenceSegment<byte>
        {
            readonly SliceMemoryManager pointerMemoryManager = new SliceMemoryManager();

            public void Reset(Slice slice, long runningIndex)
            {
                pointerMemoryManager.Reset(slice);
                Memory = pointerMemoryManager.Memory;  // maybe not always necessary
                RunningIndex = runningIndex;
            }

            public void SetNext(ReadOnlySequenceSegment<byte> next)
            {
                Next = next;
            }        
        }

        // Allow creating instances of Memory<byte> from Slice.
        // Represents a chunk of native memory, but doesn't manage its lifetime.
        // Instances of this class are reuseable - they can be reset to point to a different memory chunk.
        // That is important to make the instances cacheable (rather then creating new instances
        // the old ones will be reused to reduce GC pressure).
        private class SliceMemoryManager : MemoryManager<byte>
        {
            private Slice slice;

            public void Reset(Slice slice)
            {
                this.slice = slice;
            }

            public void Reset()
            {
                Reset(new Slice(IntPtr.Zero, 0));
            }

            public override Span<byte> GetSpan()
            {
                return slice.ToSpanUnsafe();
            }

            public override MemoryHandle Pin(int elementIndex = 0)
            {
                throw new NotSupportedException();
            }

            public override void Unpin()
            {
            }

            protected override void Dispose(bool disposing)
            {
                // NOP
            }
        }
    }
}
#endif