[PATCH 6/9] rust: list: add iterators

[Alice Ryhl <aliceryhl@google.com>]

Rust Binder has lists containing stuff such as all contexts or all
processes, and sometimes need to iterate over them. This patch enables
Rust Binder to do that using a normal for loop.

The iterator returns the ArcBorrow type, so it is possible to grab a
refcount to values while iterating.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
 rust/kernel/list.rs | 102 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 102 insertions(+)

diff --git a/rust/kernel/list.rs b/rust/kernel/list.rs
index 7e9ed802b26b..892705dd0571 100644
--- a/rust/kernel/list.rs
+++ b/rust/kernel/list.rs
@@ -5,7 +5,9 @@
 //! A linked list implementation.
 
 use crate::init::PinInit;
+use crate::sync::ArcBorrow;
 use crate::types::Opaque;
+use core::iter::{DoubleEndedIterator, FusedIterator};
 use core::marker::PhantomData;
 use core::ptr;
 
@@ -405,6 +407,17 @@ pub fn push_all_back(&mut self, other: &mut List<T, ID>) {
         // INVARIANT: The other list is now empty, so update its pointer.
         other.first = ptr::null_mut();
     }
+
+    /// Creates an iterator over the list.
+    pub fn iter(&self) -> Iter<'_, T, ID> {
+        // INVARIANT: If the list is empty, both pointers are null. Otherwise, both pointers point
+        // at the first element of the same list.
+        Iter {
+            current: self.first,
+            stop: self.first,
+            _ty: PhantomData,
+        }
+    }
 }
 
 impl<T: ?Sized + ListItem<ID>, const ID: u64> Drop for List<T, ID> {
@@ -414,3 +427,92 @@ fn drop(&mut self) {
         }
     }
 }
+
+/// An iterator into a [`List`].
+///
+/// # Invariants
+///
+/// The `current` pointer points at a value in a list, or it is null if the iterator has reached
+/// the end of the list. The `stop` pointer points at the first value in the same list, or it is
+/// null if the list is empty.
+#[derive(Clone)]
+pub struct Iter<'a, T: ?Sized + ListItem<ID>, const ID: u64 = 0> {
+    current: *mut ListLinksFields,
+    stop: *mut ListLinksFields,
+    _ty: PhantomData<&'a ListArc<T, ID>>,
+}
+
+impl<'a, T: ?Sized + ListItem<ID>, const ID: u64> Iterator for Iter<'a, T, ID> {
+    type Item = ArcBorrow<'a, T>;
+
+    fn next(&mut self) -> Option<ArcBorrow<'a, T>> {
+        if self.current.is_null() {
+            return None;
+        }
+
+        let current = self.current;
+
+        // SAFETY: We just checked that `current` is not null, so it is in a list, and hence not
+        // dangling. There's no race because the iterator holds an immutable borrow to the list.
+        let next = unsafe { (*current).next };
+        // INVARIANT: If `current` was the last element of the list, then this updates it to null.
+        // Otherwise, we update it to the next element.
+        self.current = if next != self.stop {
+            next
+        } else {
+            ptr::null_mut()
+        };
+
+        // SAFETY: The `current` pointer points a value in the list.
+        let item = unsafe { T::view_value(ListLinks::from_fields(current)) };
+        // SAFETY:
+        // * All values in a list are stored in an `Arc`.
+        // * The value cannot be removed from the list for the duration of the lifetime annotated
+        //   on the returned `ArcBorrow`, because removing it from the list would require mutable
+        //   access to the list. However, the `ArcBorrow` is annotated with the iterator's
+        //   lifetime, and the list is immutably borrowed for that lifetime.
+        // * Values in a list never have a `UniqueArc` reference.
+        Some(unsafe { ArcBorrow::from_raw(item) })
+    }
+}
+
+impl<'a, T: ?Sized + ListItem<ID>, const ID: u64> FusedIterator for Iter<'a, T, ID> {}
+
+impl<'a, T: ?Sized + ListItem<ID>, const ID: u64> IntoIterator for &'a List<T, ID> {
+    type IntoIter = Iter<'a, T, ID>;
+    type Item = ArcBorrow<'a, T>;
+
+    fn into_iter(self) -> Iter<'a, T, ID> {
+        self.iter()
+    }
+}
+
+/// An owning iterator into a [`List`].
+pub struct IntoIter<T: ?Sized + ListItem<ID>, const ID: u64 = 0> {
+    list: List<T, ID>,
+}
+
+impl<T: ?Sized + ListItem<ID>, const ID: u64> Iterator for IntoIter<T, ID> {
+    type Item = ListArc<T, ID>;
+
+    fn next(&mut self) -> Option<ListArc<T, ID>> {
+        self.list.pop_front()
+    }
+}
+
+impl<T: ?Sized + ListItem<ID>, const ID: u64> FusedIterator for IntoIter<T, ID> {}
+
+impl<T: ?Sized + ListItem<ID>, const ID: u64> DoubleEndedIterator for IntoIter<T, ID> {
+    fn next_back(&mut self) -> Option<ListArc<T, ID>> {
+        self.list.pop_back()
+    }
+}
+
+impl<T: ?Sized + ListItem<ID>, const ID: u64> IntoIterator for List<T, ID> {
+    type IntoIter = IntoIter<T, ID>;
+    type Item = ListArc<T, ID>;
+
+    fn into_iter(self) -> IntoIter<T, ID> {
+        IntoIter { list: self }
+    }
+}

-- 
2.44.0.478.gd926399ef9-goog