refactor(multipart): move Safety to module

actix-multipart/src/safety.rs

@@ -0,0 +1,60 @@
+use std::{cell::Cell, marker::PhantomData, rc::Rc, task};
+
+use local_waker::LocalWaker;
+
+/// Counter. It tracks of number of clones of payloads and give access to payload only to top most.
+///
+/// - When dropped, parent task is awakened. This is to support the case where `Field` is dropped in
+///   a separate task than `Multipart`.
+/// - Assumes that parent owners don't move to different tasks; only the top-most is allowed to.
+/// - If dropped and is not top most owner, is_clean flag is set to false.
+#[derive(Debug)]
+pub(crate) struct Safety {
+    task: LocalWaker,
+    level: usize,
+    payload: Rc<PhantomData<bool>>,
+    clean: Rc<Cell<bool>>,
+}
+
+impl Safety {
+    pub(crate) fn new() -> Safety {
+        let payload = Rc::new(PhantomData);
+        Safety {
+            task: LocalWaker::new(),
+            level: Rc::strong_count(&payload),
+            clean: Rc::new(Cell::new(true)),
+            payload,
+        }
+    }
+
+    pub(crate) fn current(&self) -> bool {
+        Rc::strong_count(&self.payload) == self.level && self.clean.get()
+    }
+
+    pub(crate) fn is_clean(&self) -> bool {
+        self.clean.get()
+    }
+
+    pub(crate) fn clone(&self, cx: &task::Context<'_>) -> Safety {
+        let payload = Rc::clone(&self.payload);
+        let s = Safety {
+            task: LocalWaker::new(),
+            level: Rc::strong_count(&payload),
+            clean: self.clean.clone(),
+            payload,
+        };
+        s.task.register(cx.waker());
+        s
+    }
+}
+
+impl Drop for Safety {
+    fn drop(&mut self) {
+        if Rc::strong_count(&self.payload) != self.level {
+            // Multipart dropped leaving a Field
+            self.clean.set(false);
+        }
+
+        self.task.wake();
+    }
+}