rust: rbtree: add iterator

- Add Iterator implementation for `RBTree`, allowing
  iteration over (key, value) pairs in key order.
- Add individual `keys()` and `values()` functions to iterate over keys
  or values alone.
- Update doctests to use iteration instead of explicitly getting items.

Iteration is needed by the binder driver to enumerate all values in a
tree for oneway spam detection [1].

Link: https://lore.kernel.org/rust-for-linux/20231101-rust-binder-v1-17-08ba9197f637@google.com/ [1]
Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Tested-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Matt Gilbride <mattgilbride@google.com>
Link: https://lore.kernel.org/r/20240822-b4-rbtree-v12-2-014561758a57@google.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/rbtree.rs

@@ -42,14 +42,30 @@ use core::{
 ///     assert_eq!(tree.get(&30).unwrap(), &300);
 /// }
 ///
+/// // Iterate over the nodes we just inserted.
+/// {
+///     let mut iter = tree.iter();
+///     assert_eq!(iter.next().unwrap(), (&10, &100));
+///     assert_eq!(iter.next().unwrap(), (&20, &200));
+///     assert_eq!(iter.next().unwrap(), (&30, &300));
+///     assert!(iter.next().is_none());
+/// }
+///
+/// // Print all elements.
+/// for (key, value) in &tree {
+///     pr_info!("{} = {}\n", key, value);
+/// }
+///
 /// // Replace one of the elements.
 /// tree.try_create_and_insert(10, 1000, flags::GFP_KERNEL)?;
 ///
 /// // Check that the tree reflects the replacement.
 /// {
-///     assert_eq!(tree.get(&10).unwrap(), &1000);
+///     let mut iter = tree.iter();
-///     assert_eq!(tree.get(&20).unwrap(), &200);
+///     assert_eq!(iter.next().unwrap(), (&10, &1000));
-///     assert_eq!(tree.get(&30).unwrap(), &300);
+///     assert_eq!(iter.next().unwrap(), (&20, &200));
+///     assert_eq!(iter.next().unwrap(), (&30, &300));
+///     assert!(iter.next().is_none());
 /// }
 ///
 /// // Change the value of one of the elements.
@@ -57,9 +73,11 @@ use core::{
 ///
 /// // Check that the tree reflects the update.
 /// {
-///     assert_eq!(tree.get(&10).unwrap(), &1000);
+///     let mut iter = tree.iter();
-///     assert_eq!(tree.get(&20).unwrap(), &200);
+///     assert_eq!(iter.next().unwrap(), (&10, &1000));
-///     assert_eq!(tree.get(&30).unwrap(), &3000);
+///     assert_eq!(iter.next().unwrap(), (&20, &200));
+///     assert_eq!(iter.next().unwrap(), (&30, &3000));
+///     assert!(iter.next().is_none());
 /// }
 ///
 /// // Remove an element.
@@ -67,9 +85,10 @@ use core::{
 ///
 /// // Check that the tree reflects the removal.
 /// {
-///     assert_eq!(tree.get(&10), None);
+///     let mut iter = tree.iter();
-///     assert_eq!(tree.get(&20).unwrap(), &200);
+///     assert_eq!(iter.next().unwrap(), (&20, &200));
-///     assert_eq!(tree.get(&30).unwrap(), &3000);
+///     assert_eq!(iter.next().unwrap(), (&30, &3000));
+///     assert!(iter.next().is_none());
 /// }
 ///
 /// # Ok::<(), Error>(())
@@ -109,9 +128,11 @@ use core::{
 ///
 /// // Check the nodes we just inserted.
 /// {
-///     assert_eq!(tree.get(&10).unwrap(), &100);
+///     let mut iter = tree.iter();
-///     assert_eq!(tree.get(&20).unwrap(), &200);
+///     assert_eq!(iter.next().unwrap(), (&10, &100));
-///     assert_eq!(tree.get(&30).unwrap(), &300);
+///     assert_eq!(iter.next().unwrap(), (&20, &200));
+///     assert_eq!(iter.next().unwrap(), (&30, &300));
+///     assert!(iter.next().is_none());
 /// }
 ///
 /// // Remove a node, getting back ownership of it.
@@ -119,9 +140,10 @@ use core::{
 ///
 /// // Check that the tree reflects the removal.
 /// {
-///     assert_eq!(tree.get(&10).unwrap(), &100);
+///     let mut iter = tree.iter();
-///     assert_eq!(tree.get(&20).unwrap(), &200);
+///     assert_eq!(iter.next().unwrap(), (&10, &100));
-///     assert_eq!(tree.get(&30), None);
+///     assert_eq!(iter.next().unwrap(), (&20, &200));
+///     assert!(iter.next().is_none());
 /// }
 ///
 /// // Create a preallocated reservation that we can re-use later.
@@ -133,9 +155,11 @@ use core::{
 ///
 /// // Check that the tree reflect the new insertion.
 /// {
-///     assert_eq!(tree.get(&10).unwrap(), &100);
+///     let mut iter = tree.iter();
-///     assert_eq!(tree.get(&15).unwrap(), &150);
+///     assert_eq!(iter.next().unwrap(), (&10, &100));
-///     assert_eq!(tree.get(&20).unwrap(), &200);
+///     assert_eq!(iter.next().unwrap(), (&15, &150));
+///     assert_eq!(iter.next().unwrap(), (&20, &200));
+///     assert!(iter.next().is_none());
 /// }
 ///
 /// # Ok::<(), Error>(())
@@ -167,6 +191,26 @@ impl<K, V> RBTree<K, V> {
             _p: PhantomData,
         }
     }
+
+    /// Returns an iterator over the tree nodes, sorted by key.
+    pub fn iter(&self) -> Iter<'_, K, V> {
+        // INVARIANT: `bindings::rb_first` returns a valid pointer to a tree node given a valid pointer to a tree root.
+        Iter {
+            _tree: PhantomData,
+            // SAFETY: `self.root` is a valid pointer to the tree root.
+            next: unsafe { bindings::rb_first(&self.root) },
+        }
+    }
+
+    /// Returns an iterator over the keys of the nodes in the tree, in sorted order.
+    pub fn keys(&self) -> impl Iterator<Item = &'_ K> {
+        self.iter().map(|(k, _)| k)
+    }
+
+    /// Returns an iterator over the values of the nodes in the tree, sorted by key.
+    pub fn values(&self) -> impl Iterator<Item = &'_ V> {
+        self.iter().map(|(_, v)| v)
+    }
 }
 
 impl<K, V> RBTree<K, V>
@@ -358,6 +402,56 @@ impl<K, V> Drop for RBTree<K, V> {
     }
 }
 
+impl<'a, K, V> IntoIterator for &'a RBTree<K, V> {
+    type Item = (&'a K, &'a V);
+    type IntoIter = Iter<'a, K, V>;
+
+    fn into_iter(self) -> Self::IntoIter {
+        self.iter()
+    }
+}
+
+/// An iterator over the nodes of a [`RBTree`].
+///
+/// Instances are created by calling [`RBTree::iter`].
+///
+/// # Invariants
+/// - `self.next` is a valid pointer.
+/// - `self.next` points to a node stored inside of a valid `RBTree`.
+pub struct Iter<'a, K, V> {
+    _tree: PhantomData<&'a RBTree<K, V>>,
+    next: *mut bindings::rb_node,
+}
+
+// SAFETY: The [`Iter`] gives out immutable references to K and V, so it has the same
+// thread safety requirements as immutable references.
+unsafe impl<'a, K: Sync, V: Sync> Send for Iter<'a, K, V> {}
+
+// SAFETY: The [`Iter`] gives out immutable references to K and V, so it has the same
+// thread safety requirements as immutable references.
+unsafe impl<'a, K: Sync, V: Sync> Sync for Iter<'a, K, V> {}
+
+impl<'a, K, V> Iterator for Iter<'a, K, V> {
+    type Item = (&'a K, &'a V);
+
+    fn next(&mut self) -> Option<Self::Item> {
+        if self.next.is_null() {
+            return None;
+        }
+
+        // SAFETY: By the type invariant of `Iter`, `self.next` is a valid node in an `RBTree`,
+        // and by the type invariant of `RBTree`, all nodes point to the links field of `Node<K, V>` objects.
+        let cur = unsafe { container_of!(self.next, Node<K, V>, links) };
+
+        // SAFETY: `self.next` is a valid tree node by the type invariants.
+        self.next = unsafe { bindings::rb_next(self.next) };
+
+        // SAFETY: By the same reasoning above, it is safe to dereference the node. Additionally,
+        // it is ok to return a reference to members because the iterator must outlive it.
+        Some(unsafe { (&(*cur).key, &(*cur).value) })
+    }
+}
+
 /// A memory reservation for a red-black tree node.
 ///
 ///