[PATCH 3/5] rust: alloc: add Vec::push_within_capacity

[Alice Ryhl <aliceryhl@google.com>]

This introduces a new method called `push_within_capacity` for appending
to a vector without attempting to allocate if the capacity is full. Rust
Binder will use this in various places to safely push to a vector while
holding a spinlock.

The existing Vec::push method is reimplemented in terms of the new
method.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
 rust/kernel/alloc/kvec.rs | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs
index 9943358c70aa63f5ad7ed9782cb8879d7a80a8fb..df930ff0d0b85b8b03c9b7932a2b31dfb62612ed 100644
--- a/rust/kernel/alloc/kvec.rs
+++ b/rust/kernel/alloc/kvec.rs
@@ -284,6 +284,31 @@ pub fn spare_capacity_mut(&mut self) -> &mut [MaybeUninit<T>] {
     /// ```
     pub fn push(&mut self, v: T, flags: Flags) -> Result<(), AllocError> {
         self.reserve(1, flags)?;
+        let err = self.push_within_capacity(v);
+        // SAFETY: The call to `reserve` was successful, so `push_within_capacity` cannot fail.
+        unsafe { err.unwrap_unchecked() };
+        Ok(())
+    }
+
+    /// Appends an element to the back of the [`Vec`] instance.
+    ///
+    /// Fails if the vector does not have capacity for the new element.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let mut v = KVec::with_capacity(10, GFP_KERNEL);
+    /// for i in 0..10 {
+    ///     v.push_within_capacity(i).unwrap();
+    /// }
+    ///
+    /// assert!(v.push_within_capacity(11).is_err());
+    /// # Ok::<(), Error>(())
+    /// ```
+    pub fn push_within_capacity(&mut self, v: T) -> Result<(), T> {
+        if self.len() >= self.capacity() {
+            return Err(v);
+        }
 
         // SAFETY:
         // - `self.len` is smaller than `self.capacity` and hence, the resulting pointer is

-- 
2.49.0.rc1.451.g8f38331e32-goog