FileStream optimizations

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

@@ -306,15 +306,7 @@ private unsafe Task WriteAsyncInternalCore(ReadOnlyMemory<byte> source, Cancella
                     }
 
                     completionSource.ReleaseNativeResource();
-
-                    if (errorCode == ERROR_HANDLE_EOF)
-                    {
-                        ThrowHelper.ThrowEndOfFileException();
-                    }
-                    else
-                    {
-                        throw Win32Marshal.GetExceptionForWin32Error(errorCode, _path);
-                    }
+                    throw Win32Marshal.GetExceptionForWin32Error(errorCode, _path);
                 }
                 else if (cancellationToken.CanBeCanceled) // ERROR_IO_PENDING
                 {

src/libraries/System.Private.CoreLib/src/System/IO/Strategies/FileStreamHelpers.Windows.cs

@@ -389,15 +389,6 @@ internal static unsafe int WriteFileNative(SafeFileHandle handle, ReadOnlySpan<b
             if (r == 0)
             {
                 errorCode = GetLastWin32ErrorAndDisposeHandleIfInvalid(handle);
-
-                if (syncUsingOverlapped && errorCode == Interop.Errors.ERROR_HANDLE_EOF)
-                {
-                    // https://docs.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-readfile#synchronization-and-file-position :
-                    // "If lpOverlapped is not NULL, then when a synchronous read operation reaches the end of a file,
-                    // ReadFile returns FALSE and GetLastError returns ERROR_HANDLE_EOF"
-                    return numBytesWritten;
-                }
-
                 return -1;
             }
             else

src/libraries/System.Private.CoreLib/src/System/IO/Strategies/LegacyFileStreamStrategy.Windows.cs

@@ -1037,15 +1037,7 @@ private unsafe Task WriteAsyncInternalCore(ReadOnlyMemory<byte> source, Cancella
                     }
 
                     completionSource.ReleaseNativeResource();
-
-                    if (errorCode == ERROR_HANDLE_EOF)
-                    {
-                        ThrowHelper.ThrowEndOfFileException();
-                    }
-                    else
-                    {
-                        throw Win32Marshal.GetExceptionForWin32Error(errorCode, _path);
-                    }
+                    throw Win32Marshal.GetExceptionForWin32Error(errorCode, _path);
                 }
                 else if (cancellationToken.CanBeCanceled) // ERROR_IO_PENDING
                 {

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

@@ -27,7 +27,7 @@ internal abstract class WindowsFileStreamStrategy : FileStreamStrategy
 
         protected long _filePosition;
         private long _appendStart; // When appending, prevent overwriting file.
-        private long _length = -1; // When the FileStream blocks the handle (_share <= FileShare.Read) keep file length in-memory, negative means that hasn't been fetched.
+        private long _length = -1; // When the handle blocks the file (_share <= FileShare.Read) keep file length in-memory, negative means that hasn't been fetched.
 
         internal WindowsFileStreamStrategy(SafeFileHandle handle, FileAccess access, FileShare share)
         {
@@ -75,13 +75,15 @@ internal WindowsFileStreamStrategy(string path, FileMode mode, FileAccess access
 
         public sealed override bool CanWrite => !_fileHandle.IsClosed && (_access & FileAccess.Write) != 0;
 
+        // When the handle blocks the file we can keep file length in memory
+        // and avoid subsequent native calls which are expensive.
         public unsafe sealed override long Length => _share > FileShare.Read ?
             FileStreamHelpers.GetFileLength(_fileHandle, _path) :
             _length < 0 ? _length = FileStreamHelpers.GetFileLength(_fileHandle, _path) : _length;
 
         protected void UpdateLengthOnChangePosition()
         {
-            // Do not update the cached length if the file can be written somewhere else
+            // Do not update the cached length if the file is not blocked
             // or if the length hasn't been fetched.
             if (_share > FileShare.Read || _length < 0)
             {
@@ -112,8 +114,12 @@ internal sealed override SafeFileHandle SafeFileHandle
         {
             get
             {
-                // Update the file offset in the handle before exposing it.
-                FileStreamHelpers.Seek(_fileHandle, _path, _filePosition, SeekOrigin.Begin);
+                if (CanSeek)
+                {
+                    // Update the file offset before exposing it since it's possible that
+                    // in memory position is out-of-sync with the actual file position.
+                    FileStreamHelpers.Seek(_fileHandle, _path, _filePosition, SeekOrigin.Begin);
+                }
                 return _fileHandle;
             }
         }