feat: full NFSv4.2 server and client in pure Go

Assisted-by: GLM 5.3 Flash

internal/nfsfs/ro.go

@@ -0,0 +1,89 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: MIT
+
+package nfsfs
+
+import (
+	"errors"
+)
+
+// ErrReadOnly marks an operation a read only export refuses, NFS4ERR_ROFS
+// on the wire.
+var ErrReadOnly = errors.New("nfsfs: the export is read only")
+
+// A roFS wraps an export and hides its mutating halves. The Writer half
+// is gone, so the dispatcher answers NFS4ERR_ROFS for every operation
+// that would change anything through it. The optional halves keep their
+// reads and refuse their writes with ErrReadOnly, so an export served
+// read only still reports its extended attributes, its holes and its
+// named attributes, and refuses to touch them.
+type roFS struct{ FS }
+
+// ReadOnly serves fs read only: the reads of every half pass through, the
+// writes of the mutating halves answer ErrReadOnly, and the mutating
+// Writer half disappears from the type.
+func ReadOnly(fs FS) FS {
+	return roFS{fs}
+}
+
+// GetXattr reads one named attribute of the object.
+func (ro roFS) GetXattr(h Handle, name string, max int) ([]byte, error) {
+	x, ok := ro.FS.(XattrFS)
+	if !ok {
+		return nil, ErrXattrNotSupp
+	}
+	return x.GetXattr(h, name, max)
+}
+
+// ListXattr names the attributes of the object.
+func (ro roFS) ListXattr(h Handle, max int) ([]string, error) {
+	x, ok := ro.FS.(XattrFS)
+	if !ok {
+		return nil, ErrXattrNotSupp
+	}
+	return x.ListXattr(h, max)
+}
+
+// SetXattr refuses a write on a read only export.
+func (ro roFS) SetXattr(h Handle, name string, value []byte, mode uint32) error {
+	return ErrReadOnly
+}
+
+// RemoveXattr refuses a write on a read only export.
+func (ro roFS) RemoveXattr(h Handle, name string) error {
+	return ErrReadOnly
+}
+
+// SeekHole reports the first hole at or after the offset.
+func (ro roFS) SeekHole(h Handle, offset int64) (int64, bool, error) {
+	s, ok := ro.FS.(interface {
+		SeekHole(Handle, int64) (int64, bool, error)
+		SeekData(Handle, int64) (int64, bool, error)
+	})
+	if !ok {
+		return 0, false, ErrNoSparse
+	}
+	return s.SeekHole(h, offset)
+}
+
+// SeekData reports the first data byte at or after the offset.
+func (ro roFS) SeekData(h Handle, offset int64) (int64, bool, error) {
+	s, ok := ro.FS.(interface {
+		SeekHole(Handle, int64) (int64, bool, error)
+		SeekData(Handle, int64) (int64, bool, error)
+	})
+	if !ok {
+		return 0, false, ErrNoSparse
+	}
+	return s.SeekData(h, offset)
+}
+
+// Allocate refuses a space reservation on a read only export.
+func (ro roFS) Allocate(h Handle, offset, length int64) error {
+	return ErrReadOnly
+}
+
+// Deallocate refuses a hole punch on a read only export.
+func (ro roFS) Deallocate(h Handle, offset, length int64) error {
+	return ErrReadOnly
+}