fix: open() uses single SYS_OPENAT call — eliminates OtherScheme EOPNOTSUPP

The two-step pattern (open full path as scheme root, then open reference
relative to root_fd) failed for namespace-internal paths like
/scheme/namespace/scheme-creation-cap. The namespace scheme resolves the
full path and returns OtherScheme pointing to SchemeList (a kernel scheme
that does not implement kopenat). The second call then hits
SchemeList.kopenat() → EOPNOTSUPP.

The namespace scheme already handles reference resolution internally via
open_scheme_resource(), so a single SYS_OPENAT call with the full path
resolves everything. This also bypasses FILETABLE.insert_upper (which
previously collided with the untracked ns_fd at UPPER index 0).

redox-rt/src/sys.rs

@@ -589,32 +589,20 @@ pub fn getns() -> Result<usize> {
 
 pub fn open<T: AsRef<str>>(path: T, flags: usize) -> Result<usize> {
     let _siglock = tmp_disable_signals();
-    let fcntl_flags = flags & syscall::O_FCNTL_MASK;
-    let redox_path = RedoxPath::from_absolute(path.as_ref()).ok_or(Error::new(EINVAL))?;
-    let (_, reference) = redox_path.as_parts().ok_or(Error::new(EINVAL))?;
 
     let ns_fd = crate::current_namespace_fd()?;
 
-    // Open the scheme root using raw SYS_OPENAT (auto-assigned POSIX fd).
-    // This bypasses FILETABLE.insert_upper, which could collide with the
-    // namespace fd (ns_fd) that is not always tracked in the UPPER table.
-    let root_fd = syscall::openat(
+    // Single SYS_OPENAT call: the kernel resolves the full path through the
+    // namespace scheme, which handles OtherScheme redirection internally.
+    // The previous two-step approach (open scheme root, then open reference)
+    // failed when the namespace returned OtherScheme, because the target
+    // scheme did not implement kopenat for the reference path.
+    let posix_fd = syscall::openat(
         ns_fd,
         path.as_ref(),
-        syscall::O_DIRECTORY | O_CLOEXEC,
+        flags,
         0,
     )?;
-    register_external_fd(root_fd)?;
-
-    let posix_fd = syscall::openat(
-        root_fd,
-        reference.as_ref(),
-        flags,
-        fcntl_flags,
-    )?;
-
-    // Close root_fd and remove from FILETABLE, then register posix_fd.
-    close(root_fd)?;
     register_external_fd(posix_fd)?;
 
     Ok(posix_fd)