os: be consistent about File methods with nil receivers

Some crashed, some didn't. Make a nil receiver always
return ErrInvalid rather than crash.
Fixes #5824.
The program in the bug listing is silent now, at least on my Mac.

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/13108044

src/pkg/os/doc.go

@@ -106,6 +106,9 @@ func Hostname() (name string, err error) {
 // directory, Readdir returns the FileInfo read until that point
 // and a non-nil error.
 func (f *File) Readdir(n int) (fi []FileInfo, err error) {
+	if f == nil {
+		return nil, ErrInvalid
+	}
 	return f.readdir(n)
 }
 
@@ -122,5 +125,8 @@ func (f *File) Readdir(n int) (fi []FileInfo, err error) {
 // directory, Readdirnames returns the names read until that point and
 // a non-nil error.
 func (f *File) Readdirnames(n int) (names []string, err error) {
+	if f == nil {
+		return nil, ErrInvalid
+	}
 	return f.readdirnames(n)
 }

src/pkg/os/file.go

@@ -174,6 +174,9 @@ func (f *File) WriteAt(b []byte, off int64) (n int, err error) {
 // relative to the current offset, and 2 means relative to the end.
 // It returns the new offset and an error, if any.
 func (f *File) Seek(offset int64, whence int) (ret int64, err error) {
+	if f == nil {
+		return 0, ErrInvalid
+	}
 	r, e := f.seek(offset, whence)
 	if e == nil && f.dirinfo != nil && r != 0 {
 		e = syscall.EISDIR
@@ -216,6 +219,9 @@ func Chdir(dir string) error {
 // which must be a directory.
 // If there is an error, it will be of type *PathError.
 func (f *File) Chdir() error {
+	if f == nil {
+		return ErrInvalid
+	}
 	if e := syscall.Fchdir(f.fd); e != nil {
 		return &PathError{"chdir", f.name, e}
 	}

src/pkg/os/file_plan9.go

@@ -133,6 +133,9 @@ func OpenFile(name string, flag int, perm FileMode) (file *File, err error) {
 // Close closes the File, rendering it unusable for I/O.
 // It returns an error, if any.
 func (f *File) Close() error {
+	if f == nil {
+		return ErrInvalid
+	}
 	return f.file.close()
 }
 
@@ -156,6 +159,9 @@ func (file *file) close() error {
 // Stat returns the FileInfo structure describing file.
 // If there is an error, it will be of type *PathError.
 func (f *File) Stat() (fi FileInfo, err error) {
+	if f == nil {
+		return nil, ErrInvalid
+	}
 	d, err := dirstat(f)
 	if err != nil {
 		return nil, err
@@ -167,8 +173,11 @@ func (f *File) Stat() (fi FileInfo, err error) {
 // It does not change the I/O offset.
 // If there is an error, it will be of type *PathError.
 func (f *File) Truncate(size int64) error {
-	var d syscall.Dir
+	if f == nil {
+		return ErrInvalid
+	}
 
+	var d syscall.Dir
 	d.Null()
 	d.Length = size
 
@@ -188,6 +197,9 @@ const chmodMask = uint32(syscall.DMAPPEND | syscall.DMEXCL | syscall.DMTMP | Mod
 // Chmod changes the mode of the file to mode.
 // If there is an error, it will be of type *PathError.
 func (f *File) Chmod(mode FileMode) error {
+	if f == nil {
+		return ErrInvalid
+	}
 	var d syscall.Dir
 
 	odir, e := dirstat(f)
@@ -419,6 +431,9 @@ func Lchown(name string, uid, gid int) error {
 // Chown changes the numeric uid and gid of the named file.
 // If there is an error, it will be of type *PathError.
 func (f *File) Chown(uid, gid int) error {
+	if f == nil {
+		return ErrInvalid
+	}
 	return &PathError{"chown", f.name, syscall.EPLAN9}
 }
 

src/pkg/os/file_posix.go

@@ -86,6 +86,9 @@ func Chmod(name string, mode FileMode) error {
 // Chmod changes the mode of the file to mode.
 // If there is an error, it will be of type *PathError.
 func (f *File) Chmod(mode FileMode) error {
+	if f == nil {
+		return ErrInvalid
+	}
 	if e := syscall.Fchmod(f.fd, syscallMode(mode)); e != nil {
 		return &PathError{"chmod", f.name, e}
 	}
@@ -115,6 +118,9 @@ func Lchown(name string, uid, gid int) error {
 // Chown changes the numeric uid and gid of the named file.
 // If there is an error, it will be of type *PathError.
 func (f *File) Chown(uid, gid int) error {
+	if f == nil {
+		return ErrInvalid
+	}
 	if e := syscall.Fchown(f.fd, uid, gid); e != nil {
 		return &PathError{"chown", f.name, e}
 	}
@@ -125,6 +131,9 @@ func (f *File) Chown(uid, gid int) error {
 // It does not change the I/O offset.
 // If there is an error, it will be of type *PathError.
 func (f *File) Truncate(size int64) error {
+	if f == nil {
+		return ErrInvalid
+	}
 	if e := syscall.Ftruncate(f.fd, size); e != nil {
 		return &PathError{"truncate", f.name, e}
 	}

src/pkg/os/file_unix.go

@@ -96,6 +96,9 @@ func OpenFile(name string, flag int, perm FileMode) (file *File, err error) {
 // Close closes the File, rendering it unusable for I/O.
 // It returns an error, if any.
 func (f *File) Close() error {
+	if f == nil {
+		return ErrInvalid
+	}
 	return f.file.close()
 }
 
@@ -117,6 +120,9 @@ func (file *file) close() error {
 // Stat returns the FileInfo structure describing file.
 // If there is an error, it will be of type *PathError.
 func (f *File) Stat() (fi FileInfo, err error) {
+	if f == nil {
+		return nil, ErrInvalid
+	}
 	var stat syscall.Stat_t
 	err = syscall.Fstat(f.fd, &stat)
 	if err != nil {

src/pkg/os/file_windows.go

@@ -153,6 +153,9 @@ func OpenFile(name string, flag int, perm FileMode) (file *File, err error) {
 // Close closes the File, rendering it unusable for I/O.
 // It returns an error, if any.
 func (file *File) Close() error {
+	if f == nil {
+		return ErrInvalid
+	}
 	return file.file.close()
 }
 

src/pkg/os/stat_windows.go

@@ -12,6 +12,9 @@ import (
 // Stat returns the FileInfo structure describing file.
 // If there is an error, it will be of type *PathError.
 func (file *File) Stat() (fi FileInfo, err error) {
+	if file == nil {
+		return nil, ErrInvalid
+	}
 	if file == nil || file.fd < 0 {
 		return nil, syscall.EINVAL
 	}