syscall: support O_SYNC flag for os.OpenFile on windows

os.OpenFile on windows did not use the O_SYNC flag. This meant
that even if the user set O_SYNC, os.OpenFile would ignore it.

This change adds a new flag FILE_FLAG_WRITE_THROUGH, which is
the equivalent of O_SYNC flag on Linux and is documented in
https://docs.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-createfilea

Fixes #35358

Change-Id: Ib338caed5bb2f215723bfe30a2551a83998d92c9
GitHub-Last-Rev: 82c6275
GitHub-Pull-Request: #64027
Reviewed-on: https://go-review.googlesource.com/c/go/+/541015
Reviewed-by: Than McIntosh <[email protected]>
Reviewed-by: Emmanuel Odeke <[email protected]>
Run-TryBot: Jes Cok <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: David Chase <[email protected]>
Reviewed-by: Quim Muntal <[email protected]>

Author: callthingsoff

src/syscall/syscall_windows.go

@@ -409,6 +409,10 @@ func Open(path string, mode int, perm uint32) (fd Handle, err error) {
 		// Necessary for opening directory handles.
 		attrs |= FILE_FLAG_BACKUP_SEMANTICS
 	}
+	if mode&O_SYNC != 0 {
+		const _FILE_FLAG_WRITE_THROUGH = 0x80000000
+		attrs |= _FILE_FLAG_WRITE_THROUGH
+	}
 	return CreateFile(pathp, access, sharemode, sa, createmode, attrs, 0)
 }