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RaiiMapOfListElement: fix invalid reference due to rehashing instability #15719

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Mar 28, 2021
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RaiiMapOfListElement: fix invalid reference due to rehashing instability #15719

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RaiiMapOfListElement: fix invalid reference due to rehashing instability
Mar 22, 2021
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Mar 26, 2021
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17 changes: 11 additions & 6 deletions source/common/common/cleanup.h
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Original file line number Diff line number Diff line change
Expand Up @@ -64,8 +64,11 @@ template <class Key, class Value> class RaiiMapOfListElement {

template <typename ConvertibleToKey>
RaiiMapOfListElement(MapOfList& map, const ConvertibleToKey& key, Value value)
: map_(map), list_(map_.try_emplace(key).first->second), key_(key), cancelled_(false) {
it_ = list_.emplace(list_.begin(), value);
: map_(map), key_(key), cancelled_(false) {
// The list reference itself cannot be saved because it is not stable in the event of a
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note: an alternative fix is to use absl::node_hash_map. I went ahead with this fix because it doesn't require usage site changes, and abseil generally recommends using flat_hash_map, over node_hash_map unless it's necessary.

// absl::flat_hash_map rehash.
std::list<Value>& list = map_.try_emplace(key).first->second;
it_ = list.emplace(list.begin(), value);
}

virtual ~RaiiMapOfListElement() {
Expand All @@ -79,16 +82,18 @@ template <class Key, class Value> class RaiiMapOfListElement {
private:
void erase() {
ASSERT(!cancelled_);
list_.erase(it_);
if (list_.empty()) {
auto list_it = map_.find(key_);
ASSERT(list_it != map_.end());

list_it->second.erase(it_);
if (list_it->second.empty()) {
map_.erase(key_);
}
cancelled_ = true;
}

MapOfList& map_;
std::list<Value>& list_;
// Because of absl::flat_hash_map iterator instability we have to keep a copy of the key
// Because of absl::flat_hash_map iterator instability we have to keep a copy of the key.
const Key key_;
typename MapOfList::mapped_type::iterator it_;
bool cancelled_;
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20 changes: 20 additions & 0 deletions test/common/common/cleanup_test.cc
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Expand Up @@ -114,4 +114,24 @@ TEST(RaiiMapOfListElement, MultipleEntriesSameKey) {
EXPECT_EQ(map.size(), 0);
}

TEST(RaiiMapOfListElement, DeleteAfterMapRehash) {
absl::flat_hash_map<int, std::list<int>> map;
std::list<RaiiMapOfListElement<int, int>> list;
// According to https://abseil.io/docs/cpp/guides/container the max load factor on
// absl::flat_hash_map is 87.5%. Using bucket_count and multiplying by 2 should give us enough
// head room to cause rehashing.
int rehash_limit = (map.bucket_count() == 0 ? 1 : map.bucket_count()) * 2;

for (int i = 0; i <= rehash_limit; i++) {
list.emplace_back(map, i, i);
EXPECT_EQ(map.size(), i + 1);
auto it = map.find(i);
ASSERT_NE(map.end(), it);
EXPECT_EQ(it->second.size(), 1);
}

list.clear();
EXPECT_EQ(map.size(), 0);
}

} // namespace Envoy