rust: pin-init: make `[pin_]chain` unwind safe

Add a drop guard before the call to the chained closure so that the value
initialized by the first stage is dropped if the closure errors or panics;
`mem::forget` the guard on success.

The previous code only ran cleanup on the explicit error path, leaking the
first-stage value if the chained closure panicked.

Reported-by: Gary Guo <gary@garyguo.net>
Closes: https://github.com/Rust-for-Linux/pin-init/issues/136
Suggested-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Mirko Adzic <adzicmirko97@gmail.com>
Link: https://patch.msgid.link/20260710-pin-init-sync-v1-7-8fa16cde87ae@garyguo.net
[ Fix Clippy missing safety comment false positive when `slot` and `guard`
  creation are merged in a single line. - Gary ]
Signed-off-by: Gary Guo <gary@garyguo.net>

1 files changed, 10 insertions, 12 deletions

diff --git a/rust/pin-init/src/lib.rs b/rust/pin-init/src/lib.rs
index 3fc4a674a487..ef9f20b11034 100644
--- a/rust/pin-init/src/lib.rs
+++ b/rust/pin-init/src/lib.rs
@@ -959,13 +959,11 @@ where
 {
     unsafe fn __pinned_init(self, slot: *mut T) -> Result<(), E> {
         // SAFETY: All requirements fulfilled since this function is `__pinned_init`.
-        unsafe { self.0.__pinned_init(slot)? };
-        // SAFETY: The above call initialized `slot` and we still have unique access.
-        let val = unsafe { &mut *slot };
-        // SAFETY: `slot` is considered pinned.
-        let val = unsafe { Pin::new_unchecked(val) };
-        // SAFETY: `slot` was initialized above.
-        (self.1)(val).inspect_err(|_| unsafe { core::ptr::drop_in_place(slot) })
+        let slot = unsafe { __internal::Slot::<__internal::Pinned, _>::new(slot) };
+        let mut guard = slot.init(self.0)?;
+        (self.1)(guard.let_binding())?;
+        core::mem::forget(guard);
+        Ok(())
     }
 }
 
@@ -1065,11 +1063,11 @@ where
 {
     unsafe fn __init(self, slot: *mut T) -> Result<(), E> {
         // SAFETY: All requirements fulfilled since this function is `__init`.
-        unsafe { self.0.__pinned_init(slot)? };
-        // SAFETY: The above call initialized `slot` and we still have unique access.
-        (self.1)(unsafe { &mut *slot }).inspect_err(|_|
-            // SAFETY: `slot` was initialized above.
-            unsafe { core::ptr::drop_in_place(slot) })
+        let slot = unsafe { __internal::Slot::<__internal::Unpinned, _>::new(slot) };
+        let mut guard = slot.init(self.0)?;
+        (self.1)(guard.let_binding())?;
+        core::mem::forget(guard);
+        Ok(())
     }
 }