Make BTrees work with custom allocators that recycle memory.

The btree data structure poisons regions of memory it's not using. It leaves
these regions poisoned when it frees memory. This means that a custom memory
allocator that tries to reuse freed memory will trigger an ASAN
use-after-poison error.

The fix is to unpoison each memory region right before freeing it.

PiperOrigin-RevId: 475309671
Change-Id: I29d55c298d3d89a83e1f960deb6e93118891ff83

absl/container/btree_test.cc

@@ -3249,6 +3249,77 @@ TEST(Btree, NotAssignableType) {
   }
 }
 
+struct ArenaLike {
+  void* recycled = nullptr;
+  size_t recycled_size = 0;
+};
+
+// A very simple implementation of arena allocation.
+template <typename T>
+class ArenaLikeAllocator : public std::allocator<T> {
+ public:
+  // Standard library containers require the ability to allocate objects of
+  // different types which they can do so via rebind.other.
+  template <typename U>
+  struct rebind {
+    using other = ArenaLikeAllocator<U>;
+  };
+
+  explicit ArenaLikeAllocator(ArenaLike* arena) noexcept : arena_(arena) {}
+
+  ~ArenaLikeAllocator() {
+    if (arena_->recycled != nullptr) {
+      delete [] static_cast<T*>(arena_->recycled);
+      arena_->recycled = nullptr;
+    }
+  }
+
+  template<typename U>
+  explicit ArenaLikeAllocator(const ArenaLikeAllocator<U>& other) noexcept
+      : arena_(other.arena_) {}
+
+  T* allocate(size_t num_objects, const void* = nullptr) {
+    size_t size = num_objects * sizeof(T);
+    if (arena_->recycled != nullptr && arena_->recycled_size == size) {
+      T* result = static_cast<T*>(arena_->recycled);
+      arena_->recycled = nullptr;
+      return result;
+    }
+    return new T[num_objects];
+  }
+
+  void deallocate(T* p, size_t num_objects) {
+    size_t size = num_objects * sizeof(T);
+
+    // Simulate writing to the freed memory as an actual arena allocator might
+    // do. This triggers an error report if the memory is poisoned.
+    memset(p, 0xde, size);
+
+    if (arena_->recycled == nullptr) {
+      arena_->recycled = p;
+      arena_->recycled_size = size;
+    } else {
+      delete [] p;
+    }
+  }
+
+  ArenaLike* arena_;
+};
+
+// This test verifies that an arena allocator that reuses memory will not be
+// asked to free poisoned BTree memory.
+TEST(Btree, ReusePoisonMemory) {
+  using Alloc = ArenaLikeAllocator<int64_t>;
+  using Set = absl::btree_set<int64_t, std::less<int64_t>, Alloc>;
+  ArenaLike arena;
+  Alloc alloc(&arena);
+  Set set(alloc);
+
+  set.insert(0);
+  set.erase(0);
+  set.insert(0);
+}
+
 }  // namespace
 }  // namespace container_internal
 ABSL_NAMESPACE_END

absl/container/internal/btree.h

@@ -958,6 +958,7 @@ class btree_node {
 
   static void deallocate(const size_type size, btree_node *node,
                          allocator_type *alloc) {
+    absl::container_internal::SanitizerUnpoisonMemoryRegion(node, size);
     absl::container_internal::Deallocate<Alignment()>(alloc, node, size);
   }