Auto merge of #61224 - aloucks:drain_filter, r=Gankro

Prevent Vec::drain_filter from double dropping on panic

Fixes: #60977

The changes in this PR prevent leaking and double-panicking in addition to double-drop.

Tracking issue: #43244

src/liballoc/vec.rs

@@ -2152,6 +2152,7 @@ impl<T> Vec<T> {
             del: 0,
             old_len,
             pred: filter,
+            panic_flag: false,
         }
     }
 }
@@ -2779,10 +2780,20 @@ pub struct DrainFilter<'a, T, F>
     where F: FnMut(&mut T) -> bool,
 {
     vec: &'a mut Vec<T>,
+    /// The index of the item that will be inspected by the next call to `next`.
     idx: usize,
+    /// The number of items that have been drained (removed) thus far.
     del: usize,
+    /// The original length of `vec` prior to draining.
     old_len: usize,
+    /// The filter test predicate.
     pred: F,
+    /// A flag that indicates a panic has occured in the filter test prodicate.
+    /// This is used as a hint in the drop implmentation to prevent consumption
+    /// of the remainder of the `DrainFilter`. Any unprocessed items will be
+    /// backshifted in the `vec`, but no further items will be dropped or
+    /// tested by the filter predicate.
+    panic_flag: bool,
 }
 
 #[unstable(feature = "drain_filter", reason = "recently added", issue = "43244")]
@@ -2793,20 +2804,23 @@ impl<T, F> Iterator for DrainFilter<'_, T, F>
 
     fn next(&mut self) -> Option<T> {
         unsafe {
-            while self.idx != self.old_len {
+            while self.idx < self.old_len {
                 let i = self.idx;
-                self.idx += 1;
                 let v = slice::from_raw_parts_mut(self.vec.as_mut_ptr(), self.old_len);
-                if (self.pred)(&mut v[i]) {
+                self.panic_flag = true;
+                let drained = (self.pred)(&mut v[i]);
+                self.panic_flag = false;
+                // Update the index *after* the predicate is called. If the index
+                // is updated prior and the predicate panics, the element at this
+                // index would be leaked.
+                self.idx += 1;
+                if drained {
                     self.del += 1;
                     return Some(ptr::read(&v[i]));
                 } else if self.del > 0 {
                     let del = self.del;
                     let src: *const T = &v[i];
                     let dst: *mut T = &mut v[i - del];
-                    // This is safe because self.vec has length 0
-                    // thus its elements will not have Drop::drop
-                    // called on them in the event of a panic.
                     ptr::copy_nonoverlapping(src, dst, 1);
                 }
             }
@@ -2824,9 +2838,46 @@ impl<T, F> Drop for DrainFilter<'_, T, F>
     where F: FnMut(&mut T) -> bool,
 {
     fn drop(&mut self) {
-        self.for_each(drop);
-        unsafe {
-            self.vec.set_len(self.old_len - self.del);
+        struct BackshiftOnDrop<'a, 'b, T, F>
+            where
+                F: FnMut(&mut T) -> bool,
+        {
+            drain: &'b mut DrainFilter<'a, T, F>,
+        }
+
+        impl<'a, 'b, T, F> Drop for BackshiftOnDrop<'a, 'b, T, F>
+            where
+                F: FnMut(&mut T) -> bool
+        {
+            fn drop(&mut self) {
+                unsafe {
+                    if self.drain.idx < self.drain.old_len && self.drain.del > 0 {
+                        // This is a pretty messed up state, and there isn't really an
+                        // obviously right thing to do. We don't want to keep trying
+                        // to execute `pred`, so we just backshift all the unprocessed
+                        // elements and tell the vec that they still exist. The backshift
+                        // is required to prevent a double-drop of the last successfully
+                        // drained item prior to a panic in the predicate.
+                        let ptr = self.drain.vec.as_mut_ptr();
+                        let src = ptr.add(self.drain.idx);
+                        let dst = src.sub(self.drain.del);
+                        let tail_len = self.drain.old_len - self.drain.idx;
+                        src.copy_to(dst, tail_len);
+                    }
+                    self.drain.vec.set_len(self.drain.old_len - self.drain.del);
+                }
+            }
+        }
+
+        let backshift = BackshiftOnDrop {
+            drain: self
+        };
+
+        // Attempt to consume any remaining elements if the filter predicate
+        // has not yet panicked. We'll backshift any remaining elements
+        // whether we've already panicked or if the consumption here panics.
+        if !backshift.drain.panic_flag {
+            backshift.drain.for_each(drop);
         }
     }
 }