[PATCH v3 04/12] rust: xarray: add `XArrayState`

[Andreas Hindborg <a.hindborg@kernel.org>]

Add `XArrayState` as internal state for XArray iteration and entry
operations. This struct wraps the C `xa_state` structure and holds a
reference to a `Guard` to ensure exclusive access to the XArray for the
lifetime of the state object.

The `XAS_RESTART` constant is also exposed through the bindings helper
to properly initialize the `xa_node` field.

The struct and its constructor are marked with `#[expect(dead_code)]` as
there are no users yet. We will remove this annotation in a later patch.

Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
---
 rust/bindings/bindings_helper.h |  1 +
 rust/kernel/xarray.rs           | 41 ++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 41 insertions(+), 1 deletion(-)

diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
index a067038b4b422..58605c32e8102 100644
--- a/rust/bindings/bindings_helper.h
+++ b/rust/bindings/bindings_helper.h
@@ -117,6 +117,7 @@ const xa_mark_t RUST_CONST_HELPER_XA_PRESENT = XA_PRESENT;
 
 const gfp_t RUST_CONST_HELPER_XA_FLAGS_ALLOC = XA_FLAGS_ALLOC;
 const gfp_t RUST_CONST_HELPER_XA_FLAGS_ALLOC1 = XA_FLAGS_ALLOC1;
+const size_t RUST_CONST_HELPER_XAS_RESTART = (size_t)XAS_RESTART;
 
 const vm_flags_t RUST_CONST_HELPER_VM_MERGEABLE = VM_MERGEABLE;
 const vm_flags_t RUST_CONST_HELPER_VM_READ = VM_READ;
diff --git a/rust/kernel/xarray.rs b/rust/kernel/xarray.rs
index ede48b5e1dba3..d1246ec114898 100644
--- a/rust/kernel/xarray.rs
+++ b/rust/kernel/xarray.rs
@@ -8,7 +8,10 @@
     iter,
     marker::PhantomData,
     pin::Pin,
-    ptr::NonNull, //
+    ptr::{
+        null_mut,
+        NonNull, //
+    },
 };
 use kernel::{
     alloc,
@@ -319,6 +322,42 @@ pub fn store(
     }
 }
 
+/// Internal state for XArray iteration and entry operations.
+///
+/// # Invariants
+///
+/// - `state` is always a valid `bindings::xa_state`.
+#[expect(dead_code)]
+pub(crate) struct XArrayState<'a, 'b, T: ForeignOwnable> {
+    /// Holds a reference to the lock guard to ensure the lock is not dropped
+    /// while `Self` is live.
+    _access: PhantomData<&'b Guard<'a, T>>,
+    state: bindings::xa_state,
+}
+
+impl<'a, 'b, T: ForeignOwnable> XArrayState<'a, 'b, T> {
+    #[expect(dead_code)]
+    fn new(access: &'b Guard<'a, T>, index: usize) -> Self {
+        let ptr = access.xa.xa.get();
+        // INVARIANT: We initialize `self.state` to a valid value below.
+        Self {
+            _access: PhantomData,
+            state: bindings::xa_state {
+                xa: ptr,
+                xa_index: index,
+                xa_shift: 0,
+                xa_sibs: 0,
+                xa_offset: 0,
+                xa_pad: 0,
+                xa_node: bindings::XAS_RESTART as *mut bindings::xa_node,
+                xa_alloc: null_mut(),
+                xa_update: None,
+                xa_lru: null_mut(),
+            },
+        }
+    }
+}
+
 // SAFETY: `XArray<T>` has no shared mutable state so it is `Send` iff `T` is `Send`.
 unsafe impl<T: ForeignOwnable + Send> Send for XArray<T> {}
 

-- 
2.51.2