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.

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
+	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 +6,44 @@ 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"}
+)
+
+// isOS indicates whether we're running on a real operating system with
+// filesystem support.
+const isOS = false
+
+// 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 +6,105 @@ 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"}
+)
+
+// isOS indicates whether we're running on a real operating system with
+// filesystem support.
+const isOS = true
+
+// 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
+	}
 }

src/os/filesystem.go

@@ -0,0 +1,85 @@
+package os
+
+import (
+	"strings"
+)
+
+// mounts lists the mount points currently mounted in the filesystem provided by
+// the os package. To resolve a path to a mount point, it is scanned from top to
+// bottom looking for the first prefix match.
+var mounts []mountPoint
+
+type mountPoint struct {
+	// prefix is a filesystem prefix, that always starts and ends with a forward
+	// slash. To denote the root filesystem, use a single slash: "/".
+	// This allows fast checking whether a path lies within a mount point.
+	prefix string
+
+	// filesystem is the Filesystem implementation that is mounted at this mount
+	// point.
+	filesystem Filesystem
+}
+
+// Filesystem provides an interface for generic filesystem drivers mounted in
+// the os package. The errors returned must be one of the os.Err* errors, or a
+// custom error if one doesn't exist. It should not be a *PathError because
+// errors will be wrapped with a *PathError by the filesystem abstraction.
+//
+// WARNING: this interface is not finalized and may change in a future version.
+type Filesystem interface {
+	// OpenFile opens the named file.
+	OpenFile(name string, flag int, perm FileMode) (FileHandle, error)
+
+	// Mkdir creates a new directoy with the specified permission (before
+	// umask). Some filesystems may not support directories or permissions.
+	Mkdir(name string, perm FileMode) error
+
+	// Remove removes the named file or (empty) directory.
+	Remove(name string) error
+}
+
+// FileHandle is an interface that should be implemented by filesystems
+// implementing the Filesystem interface.
+//
+// WARNING: this interface is not finalized and may change in a future version.
+type FileHandle interface {
+	// Read reads up to len(b) bytes from the file.
+	Read(b []byte) (n int, err error)
+
+	// Write writes up to len(b) bytes to the file.
+	Write(b []byte) (n int, err error)
+
+	// Close closes the file, making it unusable for further writes.
+	Close() (err error)
+}
+
+// findMount returns the appropriate (mounted) filesystem to use for a given
+// filename plus the path relative to that filesystem.
+func findMount(path string) (Filesystem, string) {
+	for i := len(mounts) - 1; i >= 0; i-- {
+		mount := mounts[i]
+		if strings.HasPrefix(path, mount.prefix) {
+			return mount.filesystem, path[len(mount.prefix)-1:]
+		}
+	}
+	if isOS {
+		// Assume that the first entry in the mounts slice is the OS filesystem
+		// at the root of the directory tree. Use it as-is, to support relative
+		// paths.
+		return mounts[0].filesystem, path
+	}
+	return nil, path
+}
+
+// Mount mounts the given filesystem in the filesystem abstraction layer of the
+// os package. It is not possible to unmount filesystems. Filesystems added
+// later will override earlier filesystems.
+//
+// The provided prefix must start and end with a forward slash. This is true for
+// the root directory ("/") for example.
+func Mount(prefix string, filesystem Filesystem) {
+	if prefix[0] != '/' || prefix[len(prefix)-1] != '/' {
+		panic("os.Mount: invalid prefix")
+	}
+	mounts = append(mounts, mountPoint{prefix, filesystem})
+}

src/syscall/syscall_darwin.go

@@ -1,8 +1,7 @@
 package syscall
 
 // This file defines errno and constants to match the darwin libsystem ABI.
-// Values have been determined experimentally by compiling some C code on macOS
-// with Clang and looking at the resulting LLVM IR.
+// Values have been copied from src/syscall/zerrors_darwin_amd64.go.
 
 // This function returns the error location in the darwin ABI.
 // Discovered by compiling the following code using Clang:
@@ -24,28 +23,34 @@ func getErrno() Errno {
 }
 
 const (
-	ENOENT      Errno = 2
-	EINTR       Errno = 4
-	EMFILE      Errno = 24
-	EAGAIN      Errno = 35
-	ETIMEDOUT   Errno = 60
-	ENOSYS      Errno = 78
+	ENOENT      Errno = 0x2
+	EEXIST      Errno = 0x11
+	EINTR       Errno = 0x4
+	EMFILE      Errno = 0x18
+	EAGAIN      Errno = 0x23
+	ETIMEDOUT   Errno = 0x3c
+	ENOSYS      Errno = 0x4e
 	EWOULDBLOCK Errno = EAGAIN
 )
 
 type Signal int
 
 const (
-	SIGCHLD Signal = 20
-	SIGINT  Signal = 2
-	SIGKILL Signal = 9
-	SIGTRAP Signal = 5
-	SIGQUIT Signal = 3
-	SIGTERM Signal = 15
+	SIGCHLD Signal = 0x14
+	SIGINT  Signal = 0x2
+	SIGKILL Signal = 0x9
+	SIGTRAP Signal = 0x5
+	SIGQUIT Signal = 0x3
+	SIGTERM Signal = 0xf
 )
 
 const (
-	O_RDONLY = 0
-	O_WRONLY = 1
-	O_RDWR   = 2
+	O_RDONLY = 0x0
+	O_WRONLY = 0x1
+	O_RDWR   = 0x2
+	O_APPEND = 0x8
+	O_SYNC   = 0x80
+	O_CREAT  = 0x200
+	O_TRUNC  = 0x400
+	O_EXCL   = 0x800
 )

src/syscall/syscall_libc.go

@@ -31,6 +31,14 @@ func Open(path string, mode int, perm uint32) (fd int, err error) {
 	return 0, ENOSYS // TODO
 }
 
+func Mkdir(path string, mode uint32) (err error) {
+	return ENOSYS // TODO
+}
+
+func Unlink(path string) (err error) {
+	return ENOSYS // TODO
+}
+
 func Kill(pid int, sig Signal) (err error) {
 	return ENOSYS // TODO
 }

testdata/stdlib.go

@@ -9,9 +9,9 @@ import (
 
 func main() {
 	// package os, fmt
-	fmt.Println("stdin: ", os.Stdin.Fd())
-	fmt.Println("stdout:", os.Stdout.Fd())
-	fmt.Println("stderr:", os.Stderr.Fd())
+	fmt.Println("stdin: ", os.Stdin.Name())
+	fmt.Println("stdout:", os.Stdout.Name())
+	fmt.Println("stderr:", os.Stderr.Name())
 
 	// package math/rand
 	fmt.Println("pseudorandom number:", rand.Int31())

testdata/stdlib.txt

@@ -1,6 +1,6 @@
-stdin:  0
-stdout: 1
-stderr: 2
+stdin:  /dev/stdin
+stdout: /dev/stdout
+stderr: /dev/stderr
 pseudorandom number: 1298498081
 strings.IndexByte: 2
 strings.Replace: An-example-string