[PATCH 6/7] rust: pin-init: make `[pin_]init_array_from_fn` unwind safe

[Gary Guo <gary@garyguo.net>]

From: Mirko Adzic <adzicmirko97@gmail.com>

The previous code only ran cleanup on the explicit error path. If the per-
element initializer panicked partway through, the elements already written
into the array would be leaked: their `Drop` impls would never run. This
violates the pinning requirement.

Fix the unwind safety issue by adding a guard type that drops element on
both error and panic path.

To avoid having to duplicate code between `pin_init_array_from_fn` and the
non-pin variant, extract the code to a shared `ArrayInit` type; this type
is internal and not visible via API.

Reported-by: Gary Guo <gary@garyguo.net>
Closes: https://github.com/Rust-for-Linux/pin-init/issues/136
Signed-off-by: Mirko Adzic <adzicmirko97@gmail.com>
[ Split guard type and the initializer type, move the guard type to be
  within __pinned_init. - Gary ]
Co-developed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Gary Guo <gary@garyguo.net>
---
 rust/pin-init/src/lib.rs | 122 +++++++++++++++++++++++++++++++----------------
 1 file changed, 80 insertions(+), 42 deletions(-)

diff --git a/rust/pin-init/src/lib.rs b/rust/pin-init/src/lib.rs
index 90e9d501d44a..3fc4a674a487 100644
--- a/rust/pin-init/src/lib.rs
+++ b/rust/pin-init/src/lib.rs
@@ -1186,6 +1186,82 @@ pub fn uninit<T, E>() -> impl Init<MaybeUninit<T>, E> {
     unsafe { init_from_closure(|_| Ok(())) }
 }
 
+/// Array initializer from element initializer.
+struct ArrayInit<T: ?Sized, F>(F, __internal::PhantomInvariant<T>);
+
+// SAFETY: On success, all `N` elements of the array have been initialized. On error or panic, the
+// elements that have been initialized so far are dropped, thus leaving the array uninitialized and
+// ready to deallocate.
+unsafe impl<T, F, I, E, const N: usize> PinInit<[T; N], E> for ArrayInit<T, F>
+where
+    F: FnMut(usize) -> I,
+    I: PinInit<T, E>,
+{
+    unsafe fn __pinned_init(mut self, slot: *mut [T; N]) -> Result<(), E> {
+        /// # Invariants
+        ///
+        /// - `ptr[..num_init]` contains initialized elements of type `T`
+        /// - `ptr[num_init..N]` (where N is the size of the array) contains uninitialized memory
+        struct ArrayInitGuard<T> {
+            /// A pointer to the first element of the array.
+            ptr: *mut T,
+            /// The number of initialized elements in the array.
+            num_init: usize,
+        }
+
+        impl<T> Drop for ArrayInitGuard<T> {
+            #[inline]
+            fn drop(&mut self) {
+                // SAFETY: Per type invariant, `self.ptr[..self.num_init]` are initialized.
+                unsafe {
+                    core::ptr::drop_in_place(core::ptr::slice_from_raw_parts_mut(
+                        self.ptr,
+                        self.num_init,
+                    ))
+                };
+            }
+        }
+
+        // INVARIANT: nothing is initialized yet.
+        let mut guard = ArrayInitGuard {
+            ptr: slot.cast::<T>(),
+            num_init: 0,
+        };
+
+        for i in 0..N {
+            // INVARIANT: Elements `self.ptr[..self.num_init]` have been initialized
+            // thus far. This holds true for every `self.num_init = i`.
+            guard.num_init = i;
+
+            let init = (self.0)(i);
+            // SAFETY:
+            // - The subslot is derived from `slot` with a valid offset.
+            // - If `Err` is touched, the subslot is not touched further, the guard will drop
+            //   previously initialized elements only.
+            // - `slot` is pinned so is the subslot.
+            unsafe { init.__pinned_init(&raw mut (*slot)[i]) }?;
+        }
+
+        // Dismiss the drop guard now that all elements are initialized.
+        core::mem::forget(guard);
+        Ok(())
+    }
+}
+
+// SAFETY: Follows the `PinInit` impl. `__init` executes the same code as `__pinned_init`.
+unsafe impl<T, F, I, E, const N: usize> Init<[T; N], E> for ArrayInit<T, F>
+where
+    F: FnMut(usize) -> I,
+    I: Init<T, E>,
+{
+    #[inline(always)]
+    unsafe fn __init(self, slot: *mut [T; N]) -> Result<(), E> {
+        // SAFETY: `I: Init` cancels out the pinning requirement on subslots. The other safety
+        // requirements follow that of `__init`.
+        unsafe { self.__pinned_init(slot) }
+    }
+}
+
 /// Initializes an array by initializing each element via the provided initializer.
 ///
 /// # Examples
@@ -1197,31 +1273,12 @@ pub fn uninit<T, E>() -> impl Init<MaybeUninit<T>, E> {
 /// assert_eq!(array.len(), 1_000);
 /// ```
 pub fn init_array_from_fn<I, const N: usize, T, E>(
-    mut make_init: impl FnMut(usize) -> I,
+    make_init: impl FnMut(usize) -> I,
 ) -> impl Init<[T; N], E>
 where
     I: Init<T, E>,
 {
-    let init = move |slot: *mut [T; N]| {
-        let slot = slot.cast::<T>();
-        for i in 0..N {
-            let init = make_init(i);
-            // SAFETY: Since 0 <= `i` < N, it is still in bounds of `[T; N]`.
-            let ptr = unsafe { slot.add(i) };
-            // SAFETY: The pointer is derived from `slot` and thus satisfies the `__init`
-            // requirements.
-            if let Err(e) = unsafe { init.__init(ptr) } {
-                // SAFETY: The loop has initialized the elements `slot[0..i]` and since we return
-                // `Err` below, `slot` will be considered uninitialized memory.
-                unsafe { ptr::drop_in_place(ptr::slice_from_raw_parts_mut(slot, i)) };
-                return Err(e);
-            }
-        }
-        Ok(())
-    };
-    // SAFETY: The initializer above initializes every element of the array. On failure it drops
-    // any initialized elements and returns `Err`.
-    unsafe { init_from_closure(init) }
+    ArrayInit(make_init, __internal::PhantomInvariant::new())
 }
 
 /// Initializes an array by initializing each element via the provided initializer.
@@ -1240,31 +1297,12 @@ pub fn init_array_from_fn<I, const N: usize, T, E>(
 /// assert_eq!(array.len(), 1_000);
 /// ```
 pub fn pin_init_array_from_fn<I, const N: usize, T, E>(
-    mut make_init: impl FnMut(usize) -> I,
+    make_init: impl FnMut(usize) -> I,
 ) -> impl PinInit<[T; N], E>
 where
     I: PinInit<T, E>,
 {
-    let init = move |slot: *mut [T; N]| {
-        let slot = slot.cast::<T>();
-        for i in 0..N {
-            let init = make_init(i);
-            // SAFETY: Since 0 <= `i` < N, it is still in bounds of `[T; N]`.
-            let ptr = unsafe { slot.add(i) };
-            // SAFETY: The pointer is derived from `slot` and thus satisfies the `__init`
-            // requirements.
-            if let Err(e) = unsafe { init.__pinned_init(ptr) } {
-                // SAFETY: The loop has initialized the elements `slot[0..i]` and since we return
-                // `Err` below, `slot` will be considered uninitialized memory.
-                unsafe { ptr::drop_in_place(ptr::slice_from_raw_parts_mut(slot, i)) };
-                return Err(e);
-            }
-        }
-        Ok(())
-    };
-    // SAFETY: The initializer above initializes every element of the array. On failure it drops
-    // any initialized elements and returns `Err`.
-    unsafe { pin_init_from_closure(init) }
+    ArrayInit(make_init, __internal::PhantomInvariant::new())
 }
 
 /// Construct an initializer in a closure and run it.

-- 
2.54.0