In debug mode, detect cases of btree comparators that violate transitivity, i.e.…

In debug mode, detect cases of btree comparators that violate transitivity, i.e. comp(A,B) && comp(B,C) -> comp(A,C).

When inserting a new element, we verify that the key is ordered correctly with respect to all the other values on the node, which can be done in constant time.

PiperOrigin-RevId: 523729309
Change-Id: Idb5a5912a9aa5411d086cb9fa76791523046778a

absl/container/BUILD.bazel

@@ -1001,6 +1001,7 @@ cc_test(
         "//absl/random",
         "//absl/strings",
         "//absl/types:compare",
+        "//absl/types:optional",
         "@com_google_googletest//:gtest_main",
     ],
 )

absl/container/CMakeLists.txt

@@ -80,6 +80,7 @@ absl_cc_test(
     absl::counting_allocator
     absl::flags
     absl::hash_testing
+    absl::optional
     absl::random_random
     absl::raw_logging_internal
     absl::strings

absl/container/btree_test.cc

@@ -46,6 +46,7 @@
 #include "absl/strings/str_split.h"
 #include "absl/strings/string_view.h"
 #include "absl/types/compare.h"
+#include "absl/types/optional.h"
 
 ABSL_FLAG(int, test_values, 10000, "The number of values to use for tests");
 
@@ -3137,6 +3138,78 @@ TEST(Btree, InvalidComparatorsCaught) {
     absl::btree_set<int, ThreeWaySumGreaterZeroCmp> set;
     EXPECT_DEATH(set.insert({0, 1, 2}), "lhs_comp_rhs < 0 -> rhs_comp_lhs > 0");
   }
+  // Verify that we detect cases of comparators that violate transitivity.
+  // When the comparators below check for the presence of an optional field,
+  // they violate transitivity because instances that have the optional field
+  // compare differently with each other from how they compare with instances
+  // that don't have the optional field.
+  struct ClockTime {
+    absl::optional<int> hour;
+    int minute;
+  };
+  // `comp(a,b) && comp(b,c) && !comp(a,c)` violates transitivity.
+  ClockTime a = {absl::nullopt, 1};
+  ClockTime b = {2, 5};
+  ClockTime c = {6, 0};
+  {
+    struct NonTransitiveTimeCmp {
+      bool operator()(ClockTime lhs, ClockTime rhs) const {
+        if (lhs.hour.has_value() && rhs.hour.has_value() &&
+            *lhs.hour != *rhs.hour) {
+          return *lhs.hour < *rhs.hour;
+        }
+        return lhs.minute < rhs.minute;
+      }
+    };
+    NonTransitiveTimeCmp cmp;
+    ASSERT_TRUE(cmp(a, b) && cmp(b, c) && !cmp(a, c));
+    absl::btree_set<ClockTime, NonTransitiveTimeCmp> set;
+    EXPECT_DEATH(set.insert({a, b, c}), "is_ordered_correctly");
+    absl::btree_multiset<ClockTime, NonTransitiveTimeCmp> mset;
+    EXPECT_DEATH(mset.insert({a, a, b, b, c, c}), "is_ordered_correctly");
+  }
+  {
+    struct ThreeWayNonTransitiveTimeCmp {
+      absl::weak_ordering operator()(ClockTime lhs, ClockTime rhs) const {
+        if (lhs.hour.has_value() && rhs.hour.has_value() &&
+            *lhs.hour != *rhs.hour) {
+          return *lhs.hour < *rhs.hour ? absl::weak_ordering::less
+                                       : absl::weak_ordering::greater;
+        }
+        return lhs.minute < rhs.minute    ? absl::weak_ordering::less
+               : lhs.minute == rhs.minute ? absl::weak_ordering::equivalent
+                                          : absl::weak_ordering::greater;
+      }
+    };
+    ThreeWayNonTransitiveTimeCmp cmp;
+    ASSERT_TRUE(cmp(a, b) < 0 && cmp(b, c) < 0 && cmp(a, c) > 0);
+    absl::btree_set<ClockTime, ThreeWayNonTransitiveTimeCmp> set;
+    EXPECT_DEATH(set.insert({a, b, c}), "is_ordered_correctly");
+    absl::btree_multiset<ClockTime, ThreeWayNonTransitiveTimeCmp> mset;
+    EXPECT_DEATH(mset.insert({a, a, b, b, c, c}), "is_ordered_correctly");
+  }
+}
+
+TEST(Btree, MutatedKeysCaught) {
+  if (!IsAssertEnabled()) GTEST_SKIP() << "Assertions not enabled.";
+
+  struct IntPtrCmp {
+    bool operator()(int *lhs, int *rhs) const { return *lhs < *rhs; }
+  };
+  {
+    absl::btree_set<int *, IntPtrCmp> set;
+    int arr[] = {0, 1, 2};
+    set.insert({&arr[0], &arr[1], &arr[2]});
+    arr[0] = 100;
+    EXPECT_DEATH(set.insert(&arr[0]), "is_ordered_correctly");
+  }
+  {
+    absl::btree_multiset<int *, IntPtrCmp> set;
+    int arr[] = {0, 1, 2};
+    set.insert({&arr[0], &arr[0], &arr[1], &arr[1], &arr[2], &arr[2]});
+    arr[0] = 100;
+    EXPECT_DEATH(set.insert(&arr[0]), "is_ordered_correctly");
+  }
 }
 
 #ifndef _MSC_VER

absl/container/internal/btree.h

@@ -892,6 +892,38 @@ class btree_node {
     }
   }
 
+  // Returns whether key i is ordered correctly with respect to the other keys
+  // in the node. The motivation here is to detect comparators that violate
+  // transitivity. Note: we only do comparisons of keys on this node rather than
+  // the whole tree so that this is constant time.
+  template <typename Compare>
+  bool is_ordered_correctly(field_type i, const Compare &comp) const {
+    if (std::is_base_of<BtreeTestOnlyCheckedCompareOptOutBase,
+                        Compare>::value ||
+        params_type::kIsKeyCompareStringAdapted) {
+      return true;
+    }
+
+    const auto compare = [&](field_type a, field_type b) {
+      const absl::weak_ordering cmp =
+          compare_internal::do_three_way_comparison(comp, key(a), key(b));
+      return cmp < 0 ? -1 : cmp > 0 ? 1 : 0;
+    };
+    int cmp = -1;
+    constexpr bool kCanHaveEquivKeys =
+        params_type::template can_have_multiple_equivalent_keys<key_type>();
+    for (field_type j = start(); j < finish(); ++j) {
+      if (j == i) {
+        if (cmp > 0) return false;
+        continue;
+      }
+      int new_cmp = compare(j, i);
+      if (new_cmp < cmp || (!kCanHaveEquivKeys && new_cmp == 0)) return false;
+      cmp = new_cmp;
+    }
+    return true;
+  }
+
   // Emplaces a value at position i, shifting all existing values and
   // children at positions >= i to the right by 1.
   template <typename... Args>
@@ -2816,6 +2848,13 @@ inline auto btree<P>::internal_emplace(iterator iter, Args &&...args)
   }
   iter.node_->emplace_value(static_cast<field_type>(iter.position_), alloc,
                             std::forward<Args>(args)...);
+  assert(
+      iter.node_->is_ordered_correctly(static_cast<field_type>(iter.position_),
+                                       original_key_compare(key_comp())) &&
+      "If this assert fails, then either (1) the comparator may violate "
+      "transitivity, i.e. comp(a,b) && comp(b,c) -> comp(a,c) (see "
+      "https://en.cppreference.com/w/cpp/named_req/Compare), or (2) a "
+      "key may have been mutated after it was inserted into the tree.");
   ++size_;
   iter.update_generation();
   return iter;