unix: add {get,set,remove,list}xattr on darwin

It's only when dest is set to NULL that the OS X implementations of
getxattr() and listxattr() return the current sizes of the named
attributes. An empty byte array is not sufficient. To maintain the same
behaviour as the linux implementation, we wrap around the system calls
and pass in NULL when dest is empty.

The parameters for the OS X implementation of setxattr() vary slightly
compared to the linux system call, specifically the 'position'
parameter:

 linux:
     int setxattr(
         const char *path,
         const char *name,
         const void *value,
         size_t size,
         int flags
     );

 darwin:
     int setxattr(
         const char *path,
         const char *name,
         void *value,
         size_t size,
         u_int32_t position,
         int options
     );

'position' specifies the offset within the extended attribute. In the
current implementation, only the resource fork extended attribute makes
use of this argument. For all others, position is reserved.  We simply
default to setting it to zero.  If that's needed by the package user, a
function with a different name needs to be implemented instead.

Similarly for removexattr(), we wrap around and explicitly zero out the
options provided. We do so for interoperability with the linux variant.
If options are needed by the package user, a function with a different
name needs to be implemented instead.

Fixes golang/go#14456

Change-Id: I2581e1fa8dc9324bced7fda7f8ada10fe2ede3f5
Reviewed-on: https://go-review.googlesource.com/113995
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

unix/zsyscall_darwin_386.go

@@ -399,6 +399,83 @@ func pipe() (r int, w int, err error) {
 
 // THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
 
+func getxattr(path string, attr string, dest *byte, size int, position uint32, options int) (sz int, err error) {
+	var _p0 *byte
+	_p0, err = BytePtrFromString(path)
+	if err != nil {
+		return
+	}
+	var _p1 *byte
+	_p1, err = BytePtrFromString(attr)
+	if err != nil {
+		return
+	}
+	r0, _, e1 := Syscall6(SYS_GETXATTR, uintptr(unsafe.Pointer(_p0)), uintptr(unsafe.Pointer(_p1)), uintptr(unsafe.Pointer(dest)), uintptr(size), uintptr(position), uintptr(options))
+	sz = int(r0)
+	if e1 != 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
+// THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
+
+func setxattr(path string, attr string, data *byte, size int, position uint32, options int) (err error) {
+	var _p0 *byte
+	_p0, err = BytePtrFromString(path)
+	if err != nil {
+		return
+	}
+	var _p1 *byte
+	_p1, err = BytePtrFromString(attr)
+	if err != nil {
+		return
+	}
+	_, _, e1 := Syscall6(SYS_SETXATTR, uintptr(unsafe.Pointer(_p0)), uintptr(unsafe.Pointer(_p1)), uintptr(unsafe.Pointer(data)), uintptr(size), uintptr(position), uintptr(options))
+	if e1 != 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
+// THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
+
+func removexattr(path string, attr string, options int) (err error) {
+	var _p0 *byte
+	_p0, err = BytePtrFromString(path)
+	if err != nil {
+		return
+	}
+	var _p1 *byte
+	_p1, err = BytePtrFromString(attr)
+	if err != nil {
+		return
+	}
+	_, _, e1 := Syscall(SYS_REMOVEXATTR, uintptr(unsafe.Pointer(_p0)), uintptr(unsafe.Pointer(_p1)), uintptr(options))
+	if e1 != 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
+// THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
+
+func listxattr(path string, dest *byte, size int, options int) (sz int, err error) {
+	var _p0 *byte
+	_p0, err = BytePtrFromString(path)
+	if err != nil {
+		return
+	}
+	r0, _, e1 := Syscall6(SYS_LISTXATTR, uintptr(unsafe.Pointer(_p0)), uintptr(unsafe.Pointer(dest)), uintptr(size), uintptr(options), 0, 0)
+	sz = int(r0)
+	if e1 != 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
+// THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
+
 func kill(pid int, signum int, posix int) (err error) {
 	_, _, e1 := Syscall(SYS_KILL, uintptr(pid), uintptr(signum), uintptr(posix))
 	if e1 != 0 {