avoid ReadFile syscall if we know that there is nothing to read from …

…file (#56387)

* avoid ReadFile syscall if we know that there is nothing to read from file

* Apply suggestions from code review

Co-authored-by: Stephen Toub <stoub@microsoft.com>

src/libraries/System.Private.CoreLib/src/System/IO/Strategies/AsyncWindowsFileStreamStrategy.cs

@@ -50,6 +50,12 @@ private unsafe ValueTask<int> ReadAsyncInternal(Memory<byte> destination, Cancel
                 // touch the file pointer location at all.  We will adjust it
                 // ourselves, but only in memory. This isn't threadsafe.
                 _filePosition += destination.Length;
+
+                // We know for sure that there is nothing to read, so we just return here and avoid a sys-call.
+                if (destination.IsEmpty && LengthCachingSupported)
+                {
+                    return ValueTask.FromResult(0);
+                }
             }
 
             (SafeFileHandle.OverlappedValueTaskSource? vts, int errorCode) = RandomAccess.QueueAsyncReadFile(_fileHandle, destination, positionBefore, cancellationToken);

src/libraries/System.Private.CoreLib/src/System/IO/Strategies/OSFileStreamStrategy.cs

@@ -115,7 +115,7 @@ protected void UpdateLengthOnChangePosition()
             }
         }
 
-        private bool LengthCachingSupported => OperatingSystem.IsWindows() && _share <= FileShare.Read && !_exposedHandle;
+        protected bool LengthCachingSupported => OperatingSystem.IsWindows() && _share <= FileShare.Read && !_exposedHandle;
 
         /// <summary>Gets or sets the position within the current stream</summary>
         public sealed override long Position