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Prevent Vec::drain_filter from double dropping on panic #61224
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Original file line number | Diff line number | Diff line change |
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@@ -2120,6 +2120,7 @@ impl<T> Vec<T> { | |
del: 0, | ||
old_len, | ||
pred: filter, | ||
panic_flag: false, | ||
} | ||
} | ||
} | ||
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@@ -2747,10 +2748,20 @@ 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, | ||
} | ||
There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. This has grown enough fields that they should really be documented. Either individually here, or with a high-level description of what we're trying to do in the implementation of There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. I added docs to each field in the latest commit. |
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#[unstable(feature = "drain_filter", reason = "recently added", issue = "43244")] | ||
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@@ -2761,20 +2772,23 @@ impl<T, F> Iterator for DrainFilter<'_, T, F> | |
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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; | ||
There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. I have a terrible compulsion to try to encode this state in some magic combination of old_len/idx/del, but this is probably clearest, and easiest for llvm to evaporate when it sees pred can't unwind. There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. I considered that route initially but came to the conclusion that even if it's possible, the simple flag would be much easier to understand and maintain. |
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// 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. | ||
There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. Is this comment no longer true? Why is it safe instead then? There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. I believe the intent was to leak instead of double drop, but it didn't quite work. I've done some minor refactoring and added additional comments. It's safe now because there are additional checks in |
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ptr::copy_nonoverlapping(src, dst, 1); | ||
} | ||
} | ||
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@@ -2792,9 +2806,46 @@ 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>, | ||
} | ||
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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); | ||
} | ||
There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. I don't understand why this while loop exists. Surely this should just be: if self.drain.idx < self.drain.old_len {
// It looks like `pred` panicked, so we didn't process all the elements.
// This is a pretty messed up state, and there isn't really an obviously right
// thing to do (and 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, hoping that doesn't mess up anyone further along in the panic.
let idx = self.drain.idx;
let num_deleted = self.drain.del;
let tail_len = self.drain.old_len - idx;
let ptr = self.drain.vec.as_mut_ptr();
if num_deleted > 0 {
ptr.add(idx).copy_to(ptr.add(idx - num_deleted), tail_len);
}
}
self.drain.vec.set_len(self.drain.old_len - self.drain.del); (Here I modernized the code a bit to use the newer raw pointer APIs, and some clearer names. It would be a nice cleanup to this code if you also did that to the Iterator's fields and its next impl as well, although not a blocker.) There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. I incorporated this in the latest commit. I also consolidated the the |
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self.drain.vec.set_len(self.drain.old_len - self.drain.del); | ||
} | ||
} | ||
} | ||
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let backshift = BackshiftOnDrop { | ||
drain: self | ||
}; | ||
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// 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); | ||
} | ||
} | ||
} |
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