Improve non-buffering strategy for FileStream

Author: BCSharp

Src/IronPython.Modules/nt.cs

@@ -883,8 +883,6 @@ public static void mkdir(CodeContext context, [NotNone] Bytes path, [ParamDictio
         public static void mkdir(CodeContext context, object? path, [ParamDictionary, NotNone] IDictionary<string, object> kwargs, [NotNone] params object[] args)
             => mkdir(ConvertToFsString(context, path, nameof(path)), kwargs, args);
 
-        private const int DefaultBufferSize = 4096;
-
         [Documentation("""
             open(path, flags, mode=511, *, dir_fd=None)
 
@@ -926,6 +924,12 @@ public static object open(CodeContext/*!*/ context, [NotNone] string path, int f
             }
 
             try {
+                // FileStream buffer size must be >= 0 on .NET, and >= 1 on .NET Framework and Mono.
+                // On .NET, buffer size 0 or 1 disables buffering.
+                // On .NET Framework, buffer size 1 disables buffering.
+                // On Mono, buffer size 1 makes writes of length >= 2 bypass the buffer.
+                const int NoBuffering = 1;
+
                 FileMode fileMode = FileModeFromFlags(flags);
                 FileAccess access = FileAccessFromFlags(flags);
                 FileOptions options = FileOptionsFromFlags(flags);
@@ -940,15 +944,15 @@ public static object open(CodeContext/*!*/ context, [NotNone] string path, int f
                     // open it again w/ just read access.
                     fs = new FileStream(path, fileMode, FileAccess.Write, FileShare.None);
                     fs.Close();
-                    s = fs = new FileStream(path, FileMode.Open, FileAccess.Read, FileShare.ReadWrite, DefaultBufferSize, options);
+                    s = fs = new FileStream(path, FileMode.Open, FileAccess.Read, FileShare.ReadWrite, NoBuffering, options);
                 } else if (access == FileAccess.ReadWrite && fileMode == FileMode.Append) {
                     // .NET doesn't allow Append w/ access != Write, so open the file w/ Write
                     // and a secondary stream w/ Read, then seek to the end.
-                    s = fs = new FileStream(path, FileMode.Append, FileAccess.Write, FileShare.ReadWrite, DefaultBufferSize, options);
-                    rs = new FileStream(path, FileMode.Open, FileAccess.Read, FileShare.ReadWrite, DefaultBufferSize, options);
+                    s = fs = new FileStream(path, FileMode.Append, FileAccess.Write, FileShare.ReadWrite, NoBuffering, options);
+                    rs = new FileStream(path, FileMode.Open, FileAccess.Read, FileShare.ReadWrite, NoBuffering, options);
                     rs.Seek(0L, SeekOrigin.End);
                 } else {
-                    s = fs = new FileStream(path, fileMode, access, FileShare.ReadWrite, DefaultBufferSize, options);
+                    s = fs = new FileStream(path, fileMode, access, FileShare.ReadWrite, NoBuffering, options);
                 }
                 rs ??= s;
 

Src/IronPython/Runtime/PythonFileManager.cs

@@ -29,19 +29,10 @@ internal sealed class StreamBox {
         private int _id = -1;
         private Stream _readStream;
         private Stream _writeStream;
-        private readonly bool _writeNeedsFlush;
 
         public StreamBox(Stream readStream, Stream writeStream) {
             _readStream = readStream;
             _writeStream = writeStream;
-
-            // a hack but it does improve perfomance a lot if flushing is not needed
-            _writeNeedsFlush = writeStream switch {
-                FileStream => true,         // FileStream is buffered by default, used on Windows and mostly Mono
-                PosixFileStream => false,   // PosixFileStream is unbuffered, used on .NET/Posix
-                UnixStream => false,        // UnixStream is unbuffered, used sometimes on Mono
-                _ => true                   // if in doubt, flush
-            };
         }
 
         public StreamBox(Stream stream) : this(stream, stream) { }
@@ -162,8 +153,8 @@ public int Write(IPythonBuffer buffer) {
             count = buffer.NumBytes();
             _writeStream.Write(bytes, 0, count);
 #endif
-            if (_writeNeedsFlush) {
-                _writeStream.Flush(); // IO at this level is not supposed to buffer so we need to call Flush.
+            if (ClrModule.IsMono && count == 1) {
+                _writeStream.Flush(); // IO at this level is not supposed to buffer so we need to call Flush (only needed on Mono)
             }
             if (!IsSingleStream) {
                 _readStream.Seek(_writeStream.Position, SeekOrigin.Begin);

Tests/test_io_stdlib.py

@@ -6,7 +6,7 @@
 ## Run selected tests from test_io from StdLib
 ##
 
-from iptest import is_ironpython, is_mono, generate_suite, run_test
+from iptest import is_ironpython, is_mono, is_windows, generate_suite, run_test
 
 import test.test_io
 
@@ -100,7 +100,7 @@ def load_tests(loader, standard_tests, pattern):
             test.test_io.PyMiscIOTest('test_warn_on_dealloc_fd'), # AssertionError: ResourceWarning not triggered
         ]
 
-        if is_mono:
+        if is_mono or is_windows:
             failing_tests += [
                 test.test_io.PyTextIOWrapperTest('test_seek_append_bom'), # OSError: [Errno -2146232800] Unable seek backward to overwrite data that previously existed in a file opened in Append mode.
             ]