Update after review

Author: BCSharp

Src/IronPython.Modules/mmap.cs

@@ -116,8 +116,8 @@ file stream handle (if any), so that when the original file is disposed, the han
 (if any), plus `Dispose` of it if it owned the file stream.
 
 `SafeFileHandle` closes the underlying file descriptor on dispose and sets it to invalid. It is OK
-to close the handle several times, or even if it add-reffed and in use somewhere else. The
-descriptor will be closed ass soon as the refcount is released, and in the meantime, it will prevent
+to close the handle several times, or even if it is add-reffed and in use somewhere else. The
+descriptor will be closed as soon as the refcount is released, and in the meantime, it will prevent
 future addrefs.
 
 MmapDefault - Rules of Engagement

Src/IronPython.Modules/nt.cs

@@ -438,7 +438,7 @@ private static int UnixDup(int fd, int fd2, out Stream? stream) {
             if (ClrModule.IsMono) {
                 // Elaborate workaround on Mono to avoid UnixStream as out
                 stream = new Mono.Unix.UnixStream(res, ownsHandle: false);
-                FileAccess fileAccess = stream.CanRead ? stream.CanWrite ? FileAccess.ReadWrite : FileAccess.Read : FileAccess.Write;
+                FileAccess fileAccess = stream.CanWrite ? stream.CanRead ? FileAccess.ReadWrite : FileAccess.Write : FileAccess.Read;
                 stream.Dispose();
                 try {
                     // FileStream on Mono created with a file descriptor might not work: https://github.com/mono/mono/issues/12783
@@ -916,7 +916,6 @@ public static object open(CodeContext/*!*/ context, [NotNone] string path, int f
                 // Use PosixFileStream to operate on fd directly
                 // On Mono, we must use FileStream due to limitations in MemoryMappedFile
                 Stream s = PosixFileStream.Open(path, flags, unchecked((uint)mode), out int fd);
-                //Stream s = PythonIOModule.FileIO.OpenFilePosix(path, flags, mode, out int fd);
                 if ((flags & O_APPEND) != 0) {
                     s.Seek(0L, SeekOrigin.End);
                 }

Src/IronPython/Modules/_fileio.cs

@@ -109,7 +109,6 @@ public FileIO(CodeContext/*!*/ context, [NotNone] string name, [NotNone] string
                         // Use PosixFileStream to operate on fd directly
                         // On Mono, we must use FileStream due to limitations in MemoryMappedFile
                         var stream = PosixFileStream.Open(name, flags, 0b_110_110_110, out int fd);  // mode: rw-rw-rw-
-                        //var stream = OpenFilePosix(name, flags, 0b_110_110_110, out int fd);
                         if ((flags & O_APPEND) != 0) {
                             stream.Seek(0L, SeekOrigin.End);
                         }
@@ -152,7 +151,7 @@ public FileIO(CodeContext/*!*/ context, [NotNone] string name, [NotNone] string
                         }
                         if (RuntimeInformation.IsOSPlatform(OSPlatform.Linux) || RuntimeInformation.IsOSPlatform(OSPlatform.OSX)) {
                             // On POSIX, register the file descriptor with the file manager right after file opening
-                            // Because the .NET case is already handled above before `switch`, this branch runs only on Mono
+                            // This branch is needed for Mono, the .NET case is already handled above before `switch`
                             _context.FileManager.GetOrAssignId(_streams);
                             // according to [documentation](https://learn.microsoft.com/en-us/dotnet/api/system.io.filestream.safefilehandle?view=net-9.0#remarks)
                             // accessing SafeFileHandle sets the current stream position to 0