Prevent Vec::drain_filter from double dropping on panic

src/liballoc/tests/vec.rs

@@ -945,6 +945,115 @@ fn drain_filter_complex() {
     }
 }
 
+#[test]
+#[cfg(not(miri))] // Miri does not support catching panics
+fn drain_filter_consumed_panic() {
+    use std::rc::Rc;
+    use std::sync::Mutex;
+
+    struct Check {
+        index: usize,
+        drop_counts: Rc<Mutex<Vec<usize>>>,
+    };
+
+    impl Drop for Check {
+        fn drop(&mut self) {
+            self.drop_counts.lock().unwrap()[self.index] += 1;
+            println!("drop: {}", self.index);
+        }
+    }
+
+    let check_count = 10;
+    let drop_counts = Rc::new(Mutex::new(vec![0_usize; check_count]));
+    let mut data: Vec<Check> = (0..check_count)
+        .map(|index| Check { index, drop_counts: Rc::clone(&drop_counts) })
+        .collect();
+
+    let _ = std::panic::catch_unwind(move || {
+        let filter = |c: &mut Check| {
+            if c.index == 2 {
+                panic!("panic at index: {}", c.index);
+            }
+            // Verify that if the filter could panic again on another element
+            // that it would not cause a double panic and all elements of the
+            // vec would still be dropped exactly once.
+            if c.index == 4 {
+                panic!("panic at index: {}", c.index);
+            }
+            c.index < 6
+        };
+        let drain = data.drain_filter(filter);
+
+        // NOTE: The DrainFilter is explictly consumed
+        drain.for_each(drop);
+    });
+
+    let drop_counts = drop_counts.lock().unwrap();
+    assert_eq!(check_count, drop_counts.len());
+
+    for (index, count) in drop_counts.iter().cloned().enumerate() {
+        assert_eq!(1, count, "unexpected drop count at index: {} (count: {})", index, count);
+    }
+}
+
+#[test]
+#[cfg(not(miri))] // Miri does not support catching panics
+fn drain_filter_unconsumed_panic() {
+    use std::rc::Rc;
+    use std::sync::Mutex;
+
+    struct Check {
+        index: usize,
+        drop_counts: Rc<Mutex<Vec<usize>>>,
+    };
+
+    impl Drop for Check {
+        fn drop(&mut self) {
+            self.drop_counts.lock().unwrap()[self.index] += 1;
+            println!("drop: {}", self.index);
+        }
+    }
+
+    let check_count = 10;
+    let drop_counts = Rc::new(Mutex::new(vec![0_usize; check_count]));
+    let mut data: Vec<Check> = (0..check_count)
+        .map(|index| Check { index, drop_counts: Rc::clone(&drop_counts) })
+        .collect();
+
+    let _ = std::panic::catch_unwind(move || {
+        let filter = |c: &mut Check| {
+            if c.index == 2 {
+                panic!("panic at index: {}", c.index);
+            }
+            // Verify that if the filter could panic again on another element
+            // that it would not cause a double panic and all elements of the
+            // vec would still be dropped exactly once.
+            if c.index == 4 {
+                panic!("panic at index: {}", c.index);
+            }
+            c.index < 6
+        };
+        let _drain = data.drain_filter(filter);
+
+        // NOTE: The DrainFilter is dropped without being consumed
+    });
+
+    let drop_counts = drop_counts.lock().unwrap();
+    assert_eq!(check_count, drop_counts.len());
+
+    for (index, count) in drop_counts.iter().cloned().enumerate() {
+        assert_eq!(1, count, "unexpected drop count at index: {} (count: {})", index, count);
+    }
+}
+
+#[test]
+fn drain_filter_unconsumed() {
+    let mut vec = vec![1, 2, 3, 4];
+    let drain = vec.drain_filter(|&mut x| x % 2 != 0);
+    drop(drain);
+    assert_eq!(vec, [2, 4]);
+}
+
 #[test]
 fn test_reserve_exact() {
     // This is all the same as test_reserve

src/liballoc/vec.rs

@@ -2120,6 +2120,7 @@ impl<T> Vec<T> {
             del: 0,
             old_len,
             pred: filter,
+            panic_flag: false,
         }
     }
 }
@@ -2751,6 +2752,7 @@ pub struct DrainFilter<'a, T, F>
     del: usize,
     old_len: usize,
     pred: F,
+    panic_flag: bool,
 }
 
 #[unstable(feature = "drain_filter", reason = "recently added", issue = "43244")]
@@ -2761,20 +2763,23 @@ 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);
                 }
             }
@@ -2792,9 +2797,53 @@ 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 {
+                    // Backshift any unprocessed elements, preventing double-drop
+                    // of any element that *should* have been previously overwritten
+                    // but was not due to a panic in the filter predicate. This is
+                    // implemented via drop so that it's guaranteed to run even in
+                    // the event of a panic while consuming the remainder of the
+                    // DrainFilter.
+                    while self.drain.idx < self.drain.old_len {
+                        let i = self.drain.idx;
+                        self.drain.idx += 1;
+                        let v = slice::from_raw_parts_mut(
+                            self.drain.vec.as_mut_ptr(),
+                            self.drain.old_len,
+                        );
+                        if self.drain.del > 0 {
+                            let del = self.drain.del;
+                            let src: *const T = &v[i];
+                            let dst: *mut T = &mut v[i - del];
+                            ptr::copy_nonoverlapping(src, dst, 1);
+                        }
+                    }
+                    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);
         }
     }
 }