os: implement virtual filesystem support

This allows applications to mount filesystems in the os package. This is
useful for mounting external flash filesystems, for example.

TODO: this is not finished: there is no support yet for paths relative
to the current working directory.

Author: aykevl

src/os/file.go

@@ -10,49 +10,119 @@ import (
 )
 
 // Portable analogs of some common system call errors.
+// Note that these are exported for use in the Filesystem interface.
 var (
-	errUnsupported = errors.New("operation not supported")
-	notImplemented = errors.New("os: not implemented")
+	ErrUnsupported    = errors.New("operation not supported")
+	ErrNotImplemented = errors.New("operation not implemented")
+	ErrNotExist       = errors.New("file not found")
+	ErrExist          = errors.New("file exists")
 )
 
-// Stdin, Stdout, and Stderr are open Files pointing to the standard input,
-// standard output, and standard error file descriptors.
-var (
-	Stdin  = &File{0, "/dev/stdin"}
-	Stdout = &File{1, "/dev/stdout"}
-	Stderr = &File{2, "/dev/stderr"}
-)
+// Mkdir creates a directory. If the operation fails, it will return an error of
+// type *PathError.
+func Mkdir(path string, perm FileMode) error {
+	fs, suffix := findMount(path)
+	if fs == nil {
+		return &PathError{"mkdir", path, ErrNotExist}
+	}
+	err := fs.Mkdir(suffix, perm)
+	if err != nil {
+		return &PathError{"mkdir", path, err}
+	}
+	return nil
+}
+
+// Remove removes a file or (empty) directory. If the operation fails, it will
+// return an error of type *PathError.
+func Remove(path string) error {
+	fs, suffix := findMount(path)
+	if fs == nil {
+		return &PathError{"remove", path, ErrNotExist}
+	}
+	err := fs.Remove(suffix)
+	if err != nil {
+		return &PathError{"remove", path, err}
+	}
+	return nil
+}
 
 // File represents an open file descriptor.
 type File struct {
-	fd   uintptr
-	name string
+	handle FileHandle
+	name   string
+}
+
+// Name returns the name of the file with which it was opened.
+func (f *File) Name() string {
+	return f.name
+}
+
+// OpenFile opens the named file. If the operation fails, the returned error
+// will be of type *PathError.
+func OpenFile(name string, flag int, perm FileMode) (*File, error) {
+	fs, suffix := findMount(name)
+	if fs == nil {
+		return nil, &PathError{"open", name, ErrNotExist}
+	}
+	handle, err := fs.OpenFile(suffix, flag, perm)
+	if err != nil {
+		return nil, &PathError{"open", name, err}
+	}
+	return &File{name: name, handle: handle}, nil
+}
+
+// Open opens the file named for reading.
+func Open(name string) (*File, error) {
+	return OpenFile(name, O_RDONLY, 0)
+}
+
+// Create creates the named file, overwriting it if it already exists.
+func Create(name string) (*File, error) {
+	return OpenFile(name, O_RDWR|O_CREATE|O_TRUNC, 0666)
+}
+
+// Read reads up to len(b) bytes from the File. It returns the number of bytes
+// read and any error encountered. At end of file, Read returns 0, io.EOF.
+func (f *File) Read(b []byte) (n int, err error) {
+	n, err = f.handle.Read(b)
+	if err != nil {
+		err = &PathError{"read", f.name, err}
+	}
+	return
+}
+
+// Write writes len(b) bytes to the File. It returns the number of bytes written
+// and an error, if any. Write returns a non-nil error when n != len(b).
+func (f *File) Write(b []byte) (n int, err error) {
+	n, err = f.handle.Write(b)
+	if err != nil {
+		err = &PathError{"write", f.name, err}
+	}
+	return
+}
+
+// Close closes the File, rendering it unusable for I/O.
+func (f *File) Close() (err error) {
+	err = f.handle.Close()
+	if err != nil {
+		err = &PathError{"close", f.name, err}
+	}
+	return
 }
 
 // Readdir is a stub, not yet implemented
 func (f *File) Readdir(n int) ([]FileInfo, error) {
-	return nil, notImplemented
+	return nil, &PathError{"readdir", f.name, ErrNotImplemented}
 }
 
 // Readdirnames is a stub, not yet implemented
 func (f *File) Readdirnames(n int) (names []string, err error) {
-	return nil, notImplemented
+	return nil, &PathError{"readdirnames", f.name, ErrNotImplemented}
 }
 
 // Stat is a stub, not yet implemented
 func (f *File) Stat() (FileInfo, error) {
-	return nil, notImplemented
-}
-
-// NewFile returns a new File with the given file descriptor and name.
-func NewFile(fd uintptr, name string) *File {
-	return &File{fd, name}
-}
-
-// Fd returns the integer Unix file descriptor referencing the open file. The
-// file descriptor is valid only until f.Close is called.
-func (f *File) Fd() uintptr {
-	return f.fd
+	return nil, &PathError{"stat", f.name, ErrNotImplemented}
 }
 
 const (
@@ -72,32 +142,8 @@ type PathError struct {
 	Err  error
 }
 
-func (e *PathError) Error() string { return e.Op + " " + e.Path + ": " + e.Err.Error() }
-
-// Open is a super simple stub function (for now), only capable of opening stdin, stdout, and stderr
-func Open(name string) (*File, error) {
-	fd := uintptr(999)
-	switch name {
-	case "/dev/stdin":
-		fd = 0
-	case "/dev/stdout":
-		fd = 1
-	case "/dev/stderr":
-		fd = 2
-	default:
-		return nil, &PathError{"open", name, notImplemented}
-	}
-	return &File{fd, name}, nil
-}
-
-// OpenFile is a stub, passing through to the stub Open() call
-func OpenFile(name string, flag int, perm FileMode) (*File, error) {
-	return Open(name)
-}
-
-// Create is a stub, passing through to the stub Open() call
-func Create(name string) (*File, error) {
-	return Open(name)
+func (e *PathError) Error() string {
+	return e.Op + " " + e.Path + ": " + e.Err.Error()
 }
 
 type FileMode uint32
@@ -155,12 +201,12 @@ type FileInfo interface {
 
 // Stat is a stub, not yet implemented
 func Stat(name string) (FileInfo, error) {
-	return nil, notImplemented
+	return nil, &PathError{"stat", name, ErrNotImplemented}
 }
 
 // Lstat is a stub, not yet implemented
 func Lstat(name string) (FileInfo, error) {
-	return nil, notImplemented
+	return nil, &PathError{"lstat", name, ErrNotImplemented}
 }
 
 // Getwd is a stub (for now), always returning an empty string
@@ -178,11 +224,6 @@ func TempDir() string {
 	return "/tmp"
 }
 
-// Mkdir is a stub, not yet implemented
-func Mkdir(name string, perm FileMode) error {
-	return notImplemented
-}
-
 // IsExist is a stub (for now), always returning false
 func IsExist(err error) bool {
 	return false

src/os/file_other.go

@@ -6,28 +6,40 @@ import (
 	_ "unsafe"
 )
 
+// Stdin, Stdout, and Stderr are open Files pointing to the standard input,
+// standard output, and standard error file descriptors.
+var (
+	Stdin  = &File{stdioFileHandle(0), "/dev/stdin"}
+	Stdout = &File{stdioFileHandle(1), "/dev/stdout"}
+	Stderr = &File{stdioFileHandle(2), "/dev/stderr"}
+)
+
+// stdioFileHandle represents one of stdin, stdout, or stderr depending on the
+// number. It implements the FileHandle interface.
+type stdioFileHandle uint8
+
 // Read is unsupported on this system.
-func (f *File) Read(b []byte) (n int, err error) {
-	return 0, errUnsupported
+func (f stdioFileHandle) Read(b []byte) (n int, err error) {
+	return 0, ErrUnsupported
 }
 
 // Write writes len(b) bytes to the output. It returns the number of bytes
 // written or an error if this file is not stdout or stderr.
-func (f *File) Write(b []byte) (n int, err error) {
-	switch f.fd {
-	case Stdout.fd, Stderr.fd:
+func (f stdioFileHandle) Write(b []byte) (n int, err error) {
+	switch f {
+	case 1, 2: // stdout, stderr
 		for _, c := range b {
 			putchar(c)
 		}
 		return len(b), nil
 	default:
-		return 0, errUnsupported
+		return 0, ErrUnsupported
 	}
 }
 
 // Close is unsupported on this system.
-func (f *File) Close() error {
-	return errUnsupported
+func (f stdioFileHandle) Close() error {
+	return ErrUnsupported
 }
 
 //go:linkname putchar runtime.putchar

src/os/file_unix.go

@@ -6,19 +6,101 @@ import (
 	"syscall"
 )
 
+func init() {
+	// Mount the host filesystem at the root directory. This is what most
+	// programs will be expecting.
+	Mount("/", unixFilesystem{})
+}
+
+// Stdin, Stdout, and Stderr are open Files pointing to the standard input,
+// standard output, and standard error file descriptors.
+var (
+	Stdin  = &File{unixFileHandle(0), "/dev/stdin"}
+	Stdout = &File{unixFileHandle(1), "/dev/stdout"}
+	Stderr = &File{unixFileHandle(2), "/dev/stderr"}
+)
+
+// unixFilesystem is an empty handle for a Unix/Linux filesystem. All operations
+// are relative to the current working directory.
+type unixFilesystem struct {
+}
+
+func (fs unixFilesystem) Mkdir(path string, perm FileMode) error {
+	return handleSyscallError(syscall.Mkdir(path, uint32(perm)))
+}
+
+func (fs unixFilesystem) Remove(path string) error {
+	return handleSyscallError(syscall.Unlink(path))
+}
+
+func (fs unixFilesystem) OpenFile(path string, flag int, perm FileMode) (FileHandle, error) {
+	// Map os package flags to syscall flags.
+	syscallFlag := 0
+	if flag&O_RDONLY != 0 {
+		syscallFlag |= syscall.O_RDONLY
+	}
+	if flag&O_WRONLY != 0 {
+		syscallFlag |= syscall.O_WRONLY
+	}
+	if flag&O_RDWR != 0 {
+		syscallFlag |= syscall.O_RDWR
+	}
+	if flag&O_APPEND != 0 {
+		syscallFlag |= syscall.O_APPEND
+	}
+	if flag&O_CREATE != 0 {
+		syscallFlag |= syscall.O_CREAT
+	}
+	if flag&O_EXCL != 0 {
+		syscallFlag |= syscall.O_EXCL
+	}
+	if flag&O_SYNC != 0 {
+		syscallFlag |= syscall.O_SYNC
+	}
+	if flag&O_TRUNC != 0 {
+		syscallFlag |= syscall.O_TRUNC
+	}
+	fp, err := syscall.Open(path, syscallFlag, uint32(perm))
+	return unixFileHandle(fp), handleSyscallError(err)
+}
+
+// unixFileHandle is a Unix file pointer with associated methods that implement
+// the FileHandle interface.
+type unixFileHandle uintptr
+
 // Read reads up to len(b) bytes from the File. It returns the number of bytes
 // read and any error encountered. At end of file, Read returns 0, io.EOF.
-func (f *File) Read(b []byte) (n int, err error) {
-	return syscall.Read(int(f.fd), b)
+func (f unixFileHandle) Read(b []byte) (n int, err error) {
+	n, err = syscall.Read(int(f), b)
+	err = handleSyscallError(err)
+	return
 }
 
 // Write writes len(b) bytes to the File. It returns the number of bytes written
 // and an error, if any. Write returns a non-nil error when n != len(b).
-func (f *File) Write(b []byte) (n int, err error) {
-	return syscall.Write(int(f.fd), b)
+func (f unixFileHandle) Write(b []byte) (n int, err error) {
+	n, err = syscall.Write(int(f), b)
+	err = handleSyscallError(err)
+	return
 }
 
 // Close closes the File, rendering it unusable for I/O.
-func (f *File) Close() error {
-	return syscall.Close(int(f.fd))
+func (f unixFileHandle) Close() error {
+	return handleSyscallError(syscall.Close(int(f)))
+}
+
+// handleSyscallError converts syscall errors into regular os package errors.
+// The err parameter must be either nil or of type syscall.Errno.
+func handleSyscallError(err error) error {
+	if err == nil {
+		return nil
+	}
+	switch err.(syscall.Errno) {
+	case syscall.EEXIST:
+		return ErrExist
+	case syscall.ENOENT:
+		return ErrNotExist
+	default:
+		return err
+	}
 }