os: implement os.(*File).WriteAt (#3697)

os: implement os.(*File).WriteAt

Signed-off-by: Achille Roussel <achille.roussel@gmail.com>

src/os/export_test.go

@@ -6,4 +6,5 @@ package os
 
 // Export for testing.
 
+var ErrWriteAtInAppendMode = errWriteAtInAppendMode
 var ErrPatternHasSeparator = errPatternHasSeparator

src/os/file.go

@@ -90,7 +90,9 @@ func OpenFile(name string, flag int, perm FileMode) (*File, error) {
 	if err != nil {
 		return nil, &PathError{Op: "open", Path: name, Err: err}
 	}
-	return NewFile(handle, name), nil
+	f := NewFile(handle, name)
+	f.appendMode = (flag & O_APPEND) != 0
+	return f, nil
 }
 
 // Open opens the file named for reading.
@@ -170,13 +172,34 @@ func (f *File) WriteString(s string) (n int, err error) {
 	return f.Write([]byte(s))
 }
 
-func (f *File) WriteAt(b []byte, off int64) (n int, err error) {
-	if f.handle == nil {
-		err = ErrClosed
-	} else {
-		err = ErrNotImplemented
+var errWriteAtInAppendMode = errors.New("os: invalid use of WriteAt on file opened with O_APPEND")
+
+// WriteAt writes len(b) bytes to the File starting at byte offset off.
+// It returns the number of bytes written and an error, if any.
+// WriteAt returns a non-nil error when n != len(b).
+//
+// If file was opened with the O_APPEND flag, WriteAt returns an error.
+func (f *File) WriteAt(b []byte, offset int64) (n int, err error) {
+	switch {
+	case offset < 0:
+		return 0, &PathError{Op: "writeat", Path: f.name, Err: errNegativeOffset}
+	case f.handle == nil:
+		return 0, &PathError{Op: "writeat", Path: f.name, Err: ErrClosed}
+	case f.appendMode:
+		// Go does not wrap this error but it would be more consistent
+		// if it did.
+		return 0, errWriteAtInAppendMode
+	}
+	for len(b) > 0 {
+		m, e := f.handle.WriteAt(b, offset)
+		if e != nil {
+			err = &PathError{Op: "writeat", Path: f.name, Err: e}
+			break
+		}
+		n += m
+		b = b[m:]
+		offset += int64(m)
 	}
-	err = &PathError{Op: "writeat", Path: f.name, Err: err}
 	return
 }
 

src/os/file_other.go

@@ -29,8 +29,9 @@ type stdioFileHandle uint8
 // can overwrite this data, which could cause the finalizer
 // to close the wrong file descriptor.
 type file struct {
-	handle FileHandle
-	name   string
+	handle     FileHandle
+	name       string
+	appendMode bool
 }
 
 func (f *file) close() error {
@@ -38,7 +39,7 @@ func (f *file) close() error {
 }
 
 func NewFile(fd uintptr, name string) *File {
-	return &File{&file{stdioFileHandle(fd), name}}
+	return &File{&file{handle: stdioFileHandle(fd), name: name}}
 }
 
 // Read reads up to len(b) bytes from machine.Serial.
@@ -67,6 +68,10 @@ func (f stdioFileHandle) ReadAt(b []byte, off int64) (n int, err error) {
 	return 0, ErrNotImplemented
 }
 
+func (f stdioFileHandle) WriteAt(b []byte, off int64) (n int, err error) {
+	return 0, ErrNotImplemented
+}
+
 // 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 stdioFileHandle) Write(b []byte) (n int, err error) {

src/os/file_unix.go

@@ -37,9 +37,10 @@ func rename(oldname, newname string) error {
 // can overwrite this data, which could cause the finalizer
 // to close the wrong file descriptor.
 type file struct {
-	handle  FileHandle
-	name    string
-	dirinfo *dirInfo // nil unless directory being read
+	handle     FileHandle
+	name       string
+	dirinfo    *dirInfo // nil unless directory being read
+	appendMode bool
 }
 
 func (f *file) close() (err error) {
@@ -51,7 +52,7 @@ func (f *file) close() (err error) {
 }
 
 func NewFile(fd uintptr, name string) *File {
-	return &File{&file{unixFileHandle(fd), name, nil}}
+	return &File{&file{handle: unixFileHandle(fd), name: name}}
 }
 
 func Pipe() (r *File, w *File, err error) {
@@ -124,6 +125,18 @@ func (f unixFileHandle) ReadAt(b []byte, offset int64) (n int, err error) {
 	return
 }
 
+// WriteAt writes len(b) bytes to the File starting at byte offset off.
+// It returns the number of bytes written and an error, if any.
+// WriteAt returns a non-nil error when n != len(b).
+//
+// If file was opened with the O_APPEND flag, WriteAt returns an error.
+//
+// TODO: move to file_anyos once WriteAt is implemented for windows.
+func (f unixFileHandle) WriteAt(b []byte, offset int64) (int, error) {
+	n, err := syscall.Pwrite(syscallFd(f), b, offset)
+	return n, handleSyscallError(err)
+}
+
 // Seek wraps syscall.Seek.
 func (f unixFileHandle) Seek(offset int64, whence int) (int64, error) {
 	newoffset, err := syscall.Seek(syscallFd(f), offset, whence)

src/os/file_windows.go

@@ -35,8 +35,9 @@ func rename(oldname, newname string) error {
 }
 
 type file struct {
-	handle FileHandle
-	name   string
+	handle     FileHandle
+	name       string
+	appendMode bool
 }
 
 func (f *file) close() error {
@@ -44,7 +45,7 @@ func (f *file) close() error {
 }
 
 func NewFile(fd uintptr, name string) *File {
-	return &File{&file{unixFileHandle(fd), name}}
+	return &File{&file{handle: unixFileHandle(fd), name: name}}
 }
 
 func Pipe() (r *File, w *File, err error) {
@@ -84,6 +85,17 @@ func (f unixFileHandle) ReadAt(b []byte, offset int64) (n int, err error) {
 	return -1, ErrNotImplemented
 }
 
+// WriteAt writes len(b) bytes to the File starting at byte offset off.
+// It returns the number of bytes written and an error, if any.
+// WriteAt returns a non-nil error when n != len(b).
+//
+// If file was opened with the O_APPEND flag, WriteAt returns an error.
+//
+// TODO: move to file_anyos once WriteAt is implemented for windows.
+func (f unixFileHandle) WriteAt(b []byte, offset int64) (n int, err error) {
+	return -1, ErrNotImplemented
+}
+
 // Seek wraps syscall.Seek.
 func (f unixFileHandle) Seek(offset int64, whence int) (int64, error) {
 	newoffset, err := syscall.Seek(syscallFd(f), offset, whence)

src/os/filesystem.go

@@ -58,6 +58,9 @@ type FileHandle interface {
 	// Write writes up to len(b) bytes to the file.
 	Write(b []byte) (n int, err error)
 
+	// WriteAt writes b to the file at the given absolute offset
+	WriteAt(b []byte, offset int64) (n int, err error)
+
 	// Close closes the file, making it unusable for further writes.
 	Close() (err error)
 }

src/os/os_test.go

@@ -5,6 +5,7 @@
 package os_test
 
 import (
+	"fmt"
 	"io"
 	"os"
 	. "os"
@@ -249,3 +250,66 @@ func TestReadAtEOF(t *testing.T) {
 		t.Fatalf("ReadAt failed: %s", err)
 	}
 }
+
+func TestWriteAt(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Log("TODO: implement Pwrite for Windows")
+		return
+	}
+	f := newFile("TestWriteAt", t)
+	defer Remove(f.Name())
+	defer f.Close()
+
+	const data = "hello, world\n"
+	io.WriteString(f, data)
+
+	n, err := f.WriteAt([]byte("WORLD"), 7)
+	if err != nil || n != 5 {
+		t.Fatalf("WriteAt 7: %d, %v", n, err)
+	}
+
+	b, err := os.ReadFile(f.Name())
+	if err != nil {
+		t.Fatalf("ReadFile %s: %v", f.Name(), err)
+	}
+	if string(b) != "hello, WORLD\n" {
+		t.Fatalf("after write: have %q want %q", string(b), "hello, WORLD\n")
+	}
+}
+
+// Verify that WriteAt doesn't allow negative offset.
+func TestWriteAtNegativeOffset(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Log("TODO: implement Pwrite for Windows")
+		return
+	}
+	f := newFile("TestWriteAtNegativeOffset", t)
+	defer Remove(f.Name())
+	defer f.Close()
+
+	n, err := f.WriteAt([]byte("WORLD"), -10)
+
+	const wantsub = "negative offset"
+	if !strings.Contains(fmt.Sprint(err), wantsub) || n != 0 {
+		t.Errorf("WriteAt(-10) = %v, %v; want 0, ...%q...", n, err, wantsub)
+	}
+}
+
+// Verify that WriteAt doesn't work in append mode.
+func TestWriteAtInAppendMode(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Log("TODO: implement Pwrite for Windows")
+		return
+	}
+	defer chtmpdir(t)()
+	f, err := OpenFile("write_at_in_append_mode.txt", O_APPEND|O_CREATE|O_WRONLY, 0666)
+	if err != nil {
+		t.Fatalf("OpenFile: %v", err)
+	}
+	defer f.Close()
+
+	_, err = f.WriteAt([]byte(""), 1)
+	if err != ErrWriteAtInAppendMode {
+		t.Fatalf("f.WriteAt returned %v, expected %v", err, ErrWriteAtInAppendMode)
+	}
+}

src/syscall/syscall_libc.go

@@ -54,6 +54,15 @@ func Pread(fd int, p []byte, offset int64) (n int, err error) {
 	return
 }
 
+func Pwrite(fd int, p []byte, offset int64) (n int, err error) {
+	buf, count := splitSlice(p)
+	n = libc_pwrite(int32(fd), buf, uint(count), offset)
+	if n < 0 {
+		err = getErrno()
+	}
+	return
+}
+
 func Seek(fd int, offset int64, whence int) (newoffset int64, err error) {
 	newoffset = libc_lseek(int32(fd), offset, whence)
 	if newoffset < 0 {
@@ -342,6 +351,11 @@ func libc_read(fd int32, buf *byte, count uint) int
 //export pread
 func libc_pread(fd int32, buf *byte, count uint, offset int64) int
 
+// ssize_t pwrite(int fd, void *buf, size_t count, off_t offset);
+//
+//export pwrite
+func libc_pwrite(fd int32, buf *byte, count uint, offset int64) int
+
 // ssize_t lseek(int fd, off_t offset, int whence);
 //
 //export lseek