First somewhat working solution

.gitignore

@@ -0,0 +1,2 @@
+*.out
+*.test

README.md

@@ -0,0 +1,12 @@
+Features:
+- performance in non mock path (no interfaces)
+- profound testing (we use property based tsting to ensure simularity with real implementation
+
+Non features:
+- make a varaity of different backends (like NetFS, s2, etc.). I try to keep package as clean from dependencies as possible
+- simulating of concurrent effect of filesystem (e.g. concurrent ReadDir with file removing)
+
+TODO:
+- [ ] Make count in test to see how much function envocation we have
+- [ ] Document what fileMode are supported
+- [ ] O_APPEND

go.mod

@@ -1,3 +1,14 @@
 module github.com/myxo/gofs
 
 go 1.22.0
+
+require (
+	github.com/stretchr/testify v1.9.0
+	pgregory.net/rapid v1.1.0
+)
+
+require (
+	github.com/davecgh/go-spew v1.1.1 // indirect
+	github.com/pmezard/go-difflib v1.0.0 // indirect
+	gopkg.in/yaml.v3 v3.0.1 // indirect
+)

go.sum

@@ -0,0 +1,13 @@
+github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
+github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
+github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
+github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
+github.com/stretchr/objx v0.5.2/go.mod h1:FRsXN1f5AsAjCGJKqEizvkpNtU+EGNCLh3NxZ/8L+MA=
+github.com/stretchr/testify v1.9.0 h1:HtqpIVDClZ4nwg75+f6Lvsy/wHu+3BoSGCbBAcpTsTg=
+github.com/stretchr/testify v1.9.0/go.mod h1:r2ic/lqez/lEtzL7wO/rwa5dbSLXVDPFyf8C91i36aY=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
+gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
+gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
+pgregory.net/rapid v1.1.0 h1:CMa0sjHSru3puNx+J0MIAuiiEV4N0qj8/cMWGBBCsjw=
+pgregory.net/rapid v1.1.0/go.mod h1:PY5XlDGj0+V1FCq0o192FdRhpKHGTRIWBgqjDBTrq04=

gofs.go

@@ -1,7 +1,250 @@
 package gofs
 
+import (
+	"io"
+	"os"
+)
+
 type FS interface {
-
+	Create(name string) (*File, error)
+	CreateTemp(dir, pattern string) (*File, error)
+	// NewFile(fd uintptr, name string) *File // TODO: ???
+	Open(name string) (*File, error)
+	OpenFile(name string, flag int, perm os.FileMode) (*File, error)
+	Chdir(dir string) error
+	Chmod(name string, mode os.FileMode) error
+	Chown(name string, uid, gid int) error
+	Mkdir(name string, perm os.FileMode) error
+	MkdirAll(path string, perm os.FileMode) error
+	MkdirTemp(dir, pattern string) (string, error)
+	ReadFile(name string) ([]byte, error)
+	Readlink(name string) (string, error)
+	ReadDir(name string) ([]os.DirEntry, error)
+	Remove(name string) error
+	RemoveAll(path string) error
+	Rename(oldpath, newpath string) error
+	Truncate(name string, size int64) error
+	WriteFile(name string, data []byte, perm os.FileMode) error
+}
+
+type File struct {
+	mockFile *FakeFile
+	osFile   *os.File
+}
+
+func NewFromOs(fp *os.File) *File {
+	return &File{osFile: fp}
+}
+
+func (f *File) Fd() uintptr {
+	if f.osFile != nil {
+		return f.osFile.Fd()
+	}
+	return 0
+}
+
+func (f *File) Chdir() error {
+	if f.osFile != nil {
+		return f.osFile.Chdir()
+	}
+	return f.mockFile.Chdir()
+}
+
+func (f *File) Chmod(mode os.FileMode) error {
+	if f.osFile != nil {
+		return f.osFile.Chmod(mode)
+	}
+	return f.mockFile.Chmod(mode)
+}
+
+func (f *File) Chown(uid, gid int) error { panic("todo") }
+
+func (f *File) Close() error {
+	if f.osFile != nil {
+		return f.osFile.Close()
+	}
+	return f.mockFile.Close()
+}
+
+func (f *File) Name() string {
+	if f.osFile != nil {
+		return f.osFile.Name()
+	}
+	return f.mockFile.Name()
+}
+
+func (f *File) Read(b []byte) (n int, err error) {
+	if f.osFile != nil {
+		return f.osFile.Read(b)
+	}
+	return f.mockFile.Read(b)
+}
+
+func (f *File) ReadAt(b []byte, off int64) (n int, err error) {
+	if f.osFile != nil {
+		return f.osFile.ReadAt(b, off)
+	}
+	return f.mockFile.ReadAt(b, off)
 }
 
+func (f *File) ReadDir(n int) ([]os.DirEntry, error) {
+	if f.osFile != nil {
+		return f.osFile.ReadDir(n)
+	}
+	return f.mockFile.ReadDir(n)
+}
+
+func (f *File) ReadFrom(r io.Reader) (n int64, err error) {
+	if f.osFile != nil {
+		return f.osFile.ReadFrom(r)
+	}
+	return f.mockFile.ReadFrom(r)
+}
+
+func (f *File) Readdir(n int) ([]os.FileInfo, error) {
+	if f.osFile != nil {
+		return f.osFile.Readdir(n)
+	}
+	return f.mockFile.Readdir(n)
+}
+
+func (f *File) Readdirnames(n int) (names []string, err error) {
+	if f.osFile != nil {
+		return f.osFile.Readdirnames(n)
+	}
+	return f.mockFile.Readdirnames(n)
+}
+
+func (f *File) Seek(offset int64, whence int) (ret int64, err error) {
+	if f.osFile != nil {
+		return f.osFile.Seek(offset, whence)
+	}
+	return f.mockFile.Seek(offset, whence)
+}
+
+func (f *File) Stat() (os.FileInfo, error) {
+	if f.osFile != nil {
+		return f.osFile.Stat()
+	}
+	return f.mockFile.Stat()
+}
+
+func (f *File) Sync() error {
+	if f.osFile != nil {
+		return f.osFile.Sync()
+	}
+	return f.mockFile.Sync()
+}
+
+func (f *File) Truncate(size int64) error {
+	if f.osFile != nil {
+		return f.osFile.Truncate(size)
+	}
+	return f.mockFile.Truncate(size)
+}
 
+func (f *File) Write(b []byte) (n int, err error) {
+	if f.osFile != nil {
+		return f.osFile.Write(b)
+	}
+	return f.mockFile.Write(b)
+}
+
+func (f *File) WriteAt(b []byte, off int64) (n int, err error) {
+	if f.osFile != nil {
+		return f.osFile.WriteAt(b, off)
+	}
+	return f.mockFile.WriteAt(b, off)
+}
+
+func (f *File) WriteString(s string) (n int, err error) {
+	if f.osFile != nil {
+		return f.osFile.WriteString(s)
+	}
+	return f.mockFile.WriteString(s)
+}
+func (f *File) WriteTo(w io.Writer) (n int64, err error) { panic("todo") }
+
+//func (f *File) SetDeadline(t time.Time) error{ panic("todo") }
+//func (f *File) SetReadDeadline(t time.Time) error{ panic("todo") }
+//func (f *File) SetWriteDeadline(t time.Time) error{ panic("todo") }
+
+type OsFs struct{}
+
+var _ FS = &OsFs{}
+
+func (OsFs) Create(name string) (*File, error) {
+	fp, err := os.Create(name)
+	return NewFromOs(fp), err
+}
+
+func (OsFs) CreateTemp(dir, pattern string) (*File, error) {
+	fp, err := os.CreateTemp(dir, pattern)
+	return NewFromOs(fp), err
+}
+
+func (OsFs) Open(name string) (*File, error) {
+	fp, err := os.Open(name)
+	return NewFromOs(fp), err
+}
+
+func (OsFs) OpenFile(name string, flag int, perm os.FileMode) (*File, error) {
+	fp, err := os.OpenFile(name, flag, perm)
+	return NewFromOs(fp), err
+}
+
+func (OsFs) Chdir(dir string) error {
+	return os.Chdir(dir)
+}
+
+func (OsFs) Chmod(name string, mode os.FileMode) error {
+	return os.Chmod(name, mode)
+}
+
+func (OsFs) Chown(name string, uid, gid int) error {
+	return os.Chown(name, uid, gid)
+}
+
+func (OsFs) Mkdir(name string, perm os.FileMode) error {
+	return os.Mkdir(name, perm)
+}
+
+func (OsFs) MkdirAll(path string, perm os.FileMode) error {
+	return os.MkdirAll(path, perm)
+}
+
+func (OsFs) MkdirTemp(dir, pattern string) (string, error) {
+	return os.MkdirTemp(dir, pattern)
+}
+
+func (OsFs) ReadFile(name string) ([]byte, error) {
+	return os.ReadFile(name)
+}
+
+func (OsFs) Readlink(name string) (string, error) {
+	return os.Readlink(name)
+}
+
+func (OsFs) Remove(name string) error {
+	return os.Remove(name)
+}
+
+func (OsFs) RemoveAll(path string) error {
+	return os.RemoveAll(path)
+}
+
+func (OsFs) Rename(oldpath, newpath string) error {
+	return os.Rename(oldpath, newpath)
+}
+
+func (OsFs) Truncate(name string, size int64) error {
+	return os.Truncate(name, size)
+}
+
+func (OsFs) WriteFile(name string, data []byte, perm os.FileMode) error {
+	return os.WriteFile(name, data, perm)
+}
+
+func (OsFs) ReadDir(name string) ([]os.DirEntry, error) {
+	return os.ReadDir(name)
+}