Use Stream's BeginReadInternal from FileStream (dotnet/coreclr#27737)

FileStream has two modes, decided upon at construction time.  When it's created in non-async mode, the Read/WriteAsync methods end up queueing work items to invoke the synchronous Read/Write methods.  To do this, the base methods on Stream delegate to Begin/EndRead/Write (since they were around first) and then the resulting IAsyncResult is wrapped.  However, Stream has an optimization that checks to see whether the derived stream actually overrides Begin/EndXx, and if it doesn't, then it skips using those and goes straight to queueing a work item to Read/Write.  However, FileStream does override those, but when it delegates to the base implementation because it's in non-async mode (rather than because it's a type derived from FileStream), going through Begin/EndXx is just unnecessary overhead.  So, in the right circumstances, we can call to Stream's special helper instead.

Signed-off-by: dotnet-bot <dotnet-bot@microsoft.com>

src/System.Private.CoreLib/shared/System/IO/FileStream.cs

@@ -2,11 +2,12 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System.Diagnostics;
+using System.Runtime.InteropServices;
 using System.Runtime.Serialization;
 using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.Win32.SafeHandles;
-using System.Diagnostics;
 
 namespace System.IO
 {
@@ -339,30 +340,38 @@ public override Task<int> ReadAsync(byte[] buffer, int offset, int count, Cancel
             if (buffer.Length - offset < count)
                 throw new ArgumentException(SR.Argument_InvalidOffLen /*, no good single parameter name to pass*/);
 
-            // If we have been inherited into a subclass, the following implementation could be incorrect
-            // since it does not call through to Read() which a subclass might have overridden.
-            // To be safe we will only use this implementation in cases where we know it is safe to do so,
-            // and delegate to our base class (which will call into Read/ReadAsync) when we are not sure.
-            // Similarly, if we weren't opened for asynchronous I/O, call to the base implementation so that
-            // Read is invoked asynchronously.
-            if (GetType() != typeof(FileStream) || !_useAsyncIO)
+            if (GetType() != typeof(FileStream))
+            {
+                // If we have been inherited into a subclass, the following implementation could be incorrect
+                // since it does not call through to Read() which a subclass might have overridden.
+                // To be safe we will only use this implementation in cases where we know it is safe to do so,
+                // and delegate to our base class (which will call into Read/ReadAsync) when we are not sure.
                 return base.ReadAsync(buffer, offset, count, cancellationToken);
+            }
 
             if (cancellationToken.IsCancellationRequested)
                 return Task.FromCanceled<int>(cancellationToken);
 
             if (IsClosed)
                 throw Error.GetFileNotOpen();
 
+            if (!_useAsyncIO)
+            {
+                // If we weren't opened for asynchronous I/O, we still call to the base implementation so that
+                // Read is invoked asynchronously.  But we can do so using the base Stream's internal helper
+                // that bypasses delegating to BeginRead, since we already know this is FileStream rather
+                // than something derived from it and what our BeginRead implementation is going to do.
+                return (Task<int>)base.BeginReadInternal(buffer, offset, count, null, null, serializeAsynchronously: true, apm: false);
+            }
+
             return ReadAsyncTask(buffer, offset, count, cancellationToken);
         }
 
         public override ValueTask<int> ReadAsync(Memory<byte> buffer, CancellationToken cancellationToken = default)
         {
-            if (!_useAsyncIO || GetType() != typeof(FileStream))
+            if (GetType() != typeof(FileStream))
             {
-                // If we're not using async I/O, delegate to the base, which will queue a call to Read.
-                // Or if this isn't a concrete FileStream, a derived type may have overridden ReadAsync(byte[],...),
+                // If this isn't a concrete FileStream, a derived type may have overridden ReadAsync(byte[],...),
                 // which was introduced first, so delegate to the base which will delegate to that.
                 return base.ReadAsync(buffer, cancellationToken);
             }
@@ -377,6 +386,17 @@ public override ValueTask<int> ReadAsync(Memory<byte> buffer, CancellationToken
                 throw Error.GetFileNotOpen();
             }
 
+            if (!_useAsyncIO)
+            {
+                // If we weren't opened for asynchronous I/O, we still call to the base implementation so that
+                // Read is invoked asynchronously.  But if we have a byte[], we can do so using the base Stream's
+                // internal helper that bypasses delegating to BeginRead, since we already know this is FileStream
+                // rather than something derived from it and what our BeginRead implementation is going to do.
+                return MemoryMarshal.TryGetArray(buffer, out ArraySegment<byte> segment) ?
+                    new ValueTask<int>((Task<int>)base.BeginReadInternal(segment.Array!, segment.Offset, segment.Count, null, null, serializeAsynchronously: true, apm: false)) :
+                    base.ReadAsync(buffer, cancellationToken);
+            }
+
             Task<int>? t = ReadAsyncInternal(buffer, cancellationToken, out int synchronousResult);
             return t != null ?
                 new ValueTask<int>(t) :
@@ -447,28 +467,38 @@ public override Task WriteAsync(byte[] buffer, int offset, int count, Cancellati
             if (buffer.Length - offset < count)
                 throw new ArgumentException(SR.Argument_InvalidOffLen /*, no good single parameter name to pass*/);
 
-            // If we have been inherited into a subclass, the following implementation could be incorrect
-            // since it does not call through to Write() or WriteAsync() which a subclass might have overridden.
-            // To be safe we will only use this implementation in cases where we know it is safe to do so,
-            // and delegate to our base class (which will call into Write/WriteAsync) when we are not sure.
-            if (!_useAsyncIO || GetType() != typeof(FileStream))
+            if (GetType() != typeof(FileStream))
+            {
+                // If we have been inherited into a subclass, the following implementation could be incorrect
+                // since it does not call through to Write() or WriteAsync() which a subclass might have overridden.
+                // To be safe we will only use this implementation in cases where we know it is safe to do so,
+                // and delegate to our base class (which will call into Write/WriteAsync) when we are not sure.
                 return base.WriteAsync(buffer, offset, count, cancellationToken);
+            }
 
             if (cancellationToken.IsCancellationRequested)
                 return Task.FromCanceled(cancellationToken);
 
             if (IsClosed)
                 throw Error.GetFileNotOpen();
 
+            if (!_useAsyncIO)
+            {
+                // If we weren't opened for asynchronous I/O, we still call to the base implementation so that
+                // Write is invoked asynchronously.  But we can do so using the base Stream's internal helper
+                // that bypasses delegating to BeginWrite, since we already know this is FileStream rather
+                // than something derived from it and what our BeginWrite implementation is going to do.
+                return (Task)base.BeginWriteInternal(buffer, offset, count, null, null, serializeAsynchronously: true, apm: false);
+            }
+
             return WriteAsyncInternal(new ReadOnlyMemory<byte>(buffer, offset, count), cancellationToken).AsTask();
         }
 
         public override ValueTask WriteAsync(ReadOnlyMemory<byte> buffer, CancellationToken cancellationToken = default)
         {
-            if (!_useAsyncIO || GetType() != typeof(FileStream))
+            if (GetType() != typeof(FileStream))
             {
-                // If we're not using async I/O, delegate to the base, which will queue a call to Write.
-                // Or if this isn't a concrete FileStream, a derived type may have overridden WriteAsync(byte[],...),
+                // If this isn't a concrete FileStream, a derived type may have overridden WriteAsync(byte[],...),
                 // which was introduced first, so delegate to the base which will delegate to that.
                 return base.WriteAsync(buffer, cancellationToken);
             }
@@ -483,6 +513,17 @@ public override ValueTask WriteAsync(ReadOnlyMemory<byte> buffer, CancellationTo
                 throw Error.GetFileNotOpen();
             }
 
+            if (!_useAsyncIO)
+            {
+                // If we weren't opened for asynchronous I/O, we still call to the base implementation so that
+                // Write is invoked asynchronously.  But if we have a byte[], we can do so using the base Stream's
+                // internal helper that bypasses delegating to BeginWrite, since we already know this is FileStream
+                // rather than something derived from it and what our BeginWrite implementation is going to do.
+                return MemoryMarshal.TryGetArray(buffer, out ArraySegment<byte> segment) ?
+                    new ValueTask((Task)BeginWriteInternal(segment.Array!, segment.Offset, segment.Count, null, null, serializeAsynchronously: true, apm: false)) :
+                    base.WriteAsync(buffer, cancellationToken);
+            }
+
             return WriteAsyncInternal(buffer, cancellationToken);
         }