[PATCH v2 3/6] rust: rbtree: add `RBTreeIterator`

[Matt Gilbride]

From: Wedson Almeida Filho <wedsonaf@xxxxxxxxx>

- Add Iterator implementation (`RBTreeIterator`) 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@xxxxxxxxxx/ [1]
Signed-off-by: Wedson Almeida Filho <wedsonaf@xxxxxxxxx>
Reviewed-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
Tested-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
Signed-off-by: Matt Gilbride <mattgilbride@xxxxxxxxxx>
---
 rust/kernel/rbtree.rs | 125 ++++++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 107 insertions(+), 18 deletions(-)

diff --git a/rust/kernel/rbtree.rs b/rust/kernel/rbtree.rs
index a72e9f57e660..b1faac831cfc 100644
--- a/rust/kernel/rbtree.rs
+++ b/rust/kernel/rbtree.rs
@@ -54,14 +54,30 @@ struct Node<K, V> {
 ///     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)?;
 ///
 /// // Check that the tree reflects the replacement.
 /// {
-///     assert_eq!(tree.get(&10).unwrap(), &1000);
-///     assert_eq!(tree.get(&20).unwrap(), &200);
-///     assert_eq!(tree.get(&30).unwrap(), &300);
+///     let mut iter = tree.iter();
+///     assert_eq!(iter.next().unwrap(), (&10, &1000));
+///     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.
@@ -69,9 +85,11 @@ struct Node<K, V> {
 ///
 /// // Check that the tree reflects the update.
 /// {
-///     assert_eq!(tree.get(&10).unwrap(), &1000);
-///     assert_eq!(tree.get(&20).unwrap(), &200);
-///     assert_eq!(tree.get(&30).unwrap(), &3000);
+///     let mut iter = tree.iter();
+///     assert_eq!(iter.next().unwrap(), (&10, &1000));
+///     assert_eq!(iter.next().unwrap(), (&20, &200));
+///     assert_eq!(iter.next().unwrap(), (&30, &3000));
+///     assert!(iter.next().is_none());
 /// }
 ///
 /// // Remove an element.
@@ -79,9 +97,10 @@ struct Node<K, V> {
 ///
 /// // Check that the tree reflects the removal.
 /// {
-///     assert_eq!(tree.get(&10), None);
-///     assert_eq!(tree.get(&20).unwrap(), &200);
-///     assert_eq!(tree.get(&30).unwrap(), &3000);
+///     let mut iter = tree.iter();
+///     assert_eq!(iter.next().unwrap(), (&20, &200));
+///     assert_eq!(iter.next().unwrap(), (&30, &3000));
+///     assert!(iter.next().is_none());
 /// }
 ///
 /// # Ok::<(), Error>(())
@@ -121,9 +140,11 @@ struct Node<K, V> {
 ///
 /// // Check the nodes we just inserted.
 /// {
-///     assert_eq!(tree.get(&10).unwrap(), &100);
-///     assert_eq!(tree.get(&20).unwrap(), &200);
-///     assert_eq!(tree.get(&30).unwrap(), &300);
+///     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());
 /// }
 ///
 /// // Remove a node, getting back ownership of it.
@@ -131,9 +152,10 @@ struct Node<K, V> {
 ///
 /// // Check that the tree reflects the removal.
 /// {
-///     assert_eq!(tree.get(&10).unwrap(), &100);
-///     assert_eq!(tree.get(&20).unwrap(), &200);
-///     assert_eq!(tree.get(&30), None);
+///     let mut iter = tree.iter();
+///     assert_eq!(iter.next().unwrap(), (&10, &100));
+///     assert_eq!(iter.next().unwrap(), (&20, &200));
+///     assert!(iter.next().is_none());
 /// }
 ///
 /// // Turn the node into a reservation so that we can reuse it with a different key/value.
@@ -145,9 +167,11 @@ struct Node<K, V> {
 ///
 /// // Check that the tree reflect the new insertion.
 /// {
-///     assert_eq!(tree.get(&10).unwrap(), &100);
-///     assert_eq!(tree.get(&15).unwrap(), &150);
-///     assert_eq!(tree.get(&20).unwrap(), &200);
+///     let mut iter = tree.iter();
+///     assert_eq!(iter.next().unwrap(), (&10, &100));
+///     assert_eq!(iter.next().unwrap(), (&15, &150));
+///     assert_eq!(iter.next().unwrap(), (&20, &200));
+///     assert!(iter.next().is_none());
 /// }
 ///
 /// # Ok::<(), Error>(())
@@ -188,6 +212,25 @@ pub fn try_reserve_node() -> Result<RBTreeNodeReservation<K, V>> {
     pub fn try_allocate_node(key: K, value: V) -> Result<RBTreeNode<K, V>> {
         Ok(Self::try_reserve_node()?.into_node(key, value))
     }
+
+    /// Returns an iterator over the tree nodes, sorted by key.
+    pub fn iter(&self) -> RBTreeIterator<'_, K, V> {
+        RBTreeIterator {
+            _tree: PhantomData,
+            // SAFETY: `root` is valid as it's embedded in `self` and we have a valid `self`.
+            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>
@@ -350,6 +393,52 @@ fn drop(&mut self) {
     }
 }
 
+impl<'a, K, V> IntoIterator for &'a RBTree<K, V> {
+    type Item = (&'a K, &'a V);
+    type IntoIter = RBTreeIterator<'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`].
+pub struct RBTreeIterator<'a, K, V> {
+    _tree: PhantomData<&'a RBTree<K, V>>,
+    next: *mut bindings::rb_node,
+}
+
+// SAFETY: An [`RBTree`] allows the same kinds of access to its values that a struct allows to its
+// fields, so we use the same Send condition as would be used for a struct with K and V fields.
+unsafe impl<'a, K: Send, V: Send> Send for RBTreeIterator<'a, K, V> {}
+
+// SAFETY: An [`RBTree`] allows the same kinds of access to its values that a struct allows to its
+// fields, so we use the same Sync condition as would be used for a struct with K and V fields.
+unsafe impl<'a, K: Sync, V: Sync> Sync for RBTreeIterator<'a, K, V> {}
+
+impl<'a, K, V> Iterator for RBTreeIterator<'a, K, V> {
+    type Item = (&'a K, &'a V);
+
+    fn next(&mut self) -> Option<Self::Item> {
+        if self.next.is_null() {
+            return None;
+        }
+
+        // SAFETY: All links fields we create are in a `Node<K, V>`.
+        let cur = unsafe { crate::container_of!(self.next, Node<K, V>, links) };
+
+        // SAFETY: The reference to the tree used to create the iterator outlives the iterator, so
+        // the tree cannot change. By the tree invariant, all nodes are valid.
+        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.
 ///
 /// It contains the memory needed to hold a node that can be inserted into a red-black tree. One

-- 
2.44.0.rc0.258.g7320e95886-goog