proposal: syscall: define Windows O_ALLOW_DELETE for use in os.OpenFile #34681
Description
The discussion on #32088 has so far been unable to reach consensus on setting FILE_SHARE_DELETE unconditionally in os.OpenFile. Right now it looks likely to be declined, possibly revisited in a few years.
In the interim, it is more difficult on Windows than on Unix to pass an extra flag to open a file with package syscall. syscall.Open takes fake O_* flags, not actual Windows permission bits. It wraps syscall.CreateFile, but it does a lot of argument preparation before making that call:
// package syscall
func Open(path string, mode int, perm uint32) (fd Handle, err error) {
if len(path) == 0 {
return InvalidHandle, ERROR_FILE_NOT_FOUND
}
pathp, err := UTF16PtrFromString(path)
if err != nil {
return InvalidHandle, err
}
var access uint32
switch mode & (O_RDONLY | O_WRONLY | O_RDWR) {
case O_RDONLY:
access = GENERIC_READ
case O_WRONLY:
access = GENERIC_WRITE
case O_RDWR:
access = GENERIC_READ | GENERIC_WRITE
}
if mode&O_CREAT != 0 {
access |= GENERIC_WRITE
}
if mode&O_APPEND != 0 {
access &^= GENERIC_WRITE
access |= FILE_APPEND_DATA
}
sharemode := uint32(FILE_SHARE_READ | FILE_SHARE_WRITE)
var sa *SecurityAttributes
if mode&O_CLOEXEC == 0 {
sa = makeInheritSa()
}
var createmode uint32
switch {
case mode&(O_CREAT|O_EXCL) == (O_CREAT | O_EXCL):
createmode = CREATE_NEW
case mode&(O_CREAT|O_TRUNC) == (O_CREAT | O_TRUNC):
createmode = CREATE_ALWAYS
case mode&O_CREAT == O_CREAT:
createmode = OPEN_ALWAYS
case mode&O_TRUNC == O_TRUNC:
createmode = TRUNCATE_EXISTING
default:
createmode = OPEN_EXISTING
}
h, e := CreateFile(pathp, access, sharemode, sa, createmode, FILE_ATTRIBUTE_NORMAL, 0)
return h, e
}
A direct call to syscall.CreateFile would have to duplicate all that code.
A direct call to syscall.Open would still lose out on the adjustments made inside os.Open, most importantly the call to fixLongPath:
// package os
func openFile(name string, flag int, perm FileMode) (file *File, err error) {
r, e := syscall.Open(fixLongPath(name), flag|syscall.O_CLOEXEC, syscallMode(perm))
if e != nil {
return nil, e
}
return newFile(r, name, "file"), nil
}
If package syscall on Windows defined a bit O_ALLOW_DELETE (maybe 0x100000), then syscall.Open could convert that bit into FILE_SHARE_DELETE. Then it would be easy for calls to either syscall.Open or os.OpenFile to cause the underlying call to use FILE_SHARE_DELETE if they really needed it.
This proposal assumes #32088 is declined. It should be considered withdrawn if #32088 is accepted.