<vector>: Fix ASan annotations when clearing (microsoft#2464)

Co-authored-by: Stephan T. Lavavej <stl@nuwen.net>

stl/inc/vector

@@ -515,6 +515,12 @@ private:
 
     _CONSTEXPR20 void _Modify_annotation(const difference_type _Count) const noexcept {
         // Extends/shrinks the annotated range by _Count
+        if (_Count == 0) { // nothing to do
+            // This also avoids calling _Apply_annotation() with null pointers
+            // when the vector has zero capacity, see GH-2464.
+            return;
+        }
+
         auto& _My_data = _Mypair._Myval2;
         _Apply_annotation(_My_data._Myfirst, _My_data._Myend, _My_data._Mylast, _My_data._Mylast + _Count);
     }
@@ -1243,7 +1249,7 @@ private:
 
         // Trim.
         _Destroy_range(_Next, _Mylast, _Getal());
-        _ASAN_VECTOR_MODIFY(static_cast<difference_type>(_Next - _Mylast));
+        _ASAN_VECTOR_MODIFY(static_cast<difference_type>(_Next - _Mylast)); // negative when destroying elements
         _Mylast = _Next;
 
         // Append.
@@ -1586,7 +1592,7 @@ public:
 
             const pointer _Newlast = _Move_unchecked(_Lastptr, _Mylast, _Firstptr);
             _Destroy_range(_Newlast, _Mylast, _Getal());
-            _ASAN_VECTOR_MODIFY(static_cast<difference_type>(_Newlast - _Mylast));
+            _ASAN_VECTOR_MODIFY(static_cast<difference_type>(_Newlast - _Mylast)); // negative when destroying elements
             _Mylast = _Newlast;
         }
 
@@ -1598,9 +1604,16 @@ public:
         pointer& _Myfirst = _My_data._Myfirst;
         pointer& _Mylast  = _My_data._Mylast;
 
+        if (_Myfirst == _Mylast) { // already empty, nothing to do
+            // This is an optimization for debug mode: we can avoid taking the debug lock to invalidate iterators.
+            // Note that when clearing an empty vector, this will preserve past-the-end iterators, which is allowed by
+            // N4901 [tab:container.seq.req] "a.clear() [...] may invalidate the past-the-end iterator".
+            return;
+        }
+
         _My_data._Orphan_all();
         _Destroy_range(_Myfirst, _Mylast, _Getal());
-        _ASAN_VECTOR_MODIFY(static_cast<difference_type>(_Mylast - _Myfirst));
+        _ASAN_VECTOR_MODIFY(static_cast<difference_type>(_Myfirst - _Mylast)); // negative when destroying elements
         _Mylast = _Myfirst;
     }
 
@@ -1936,6 +1949,7 @@ private:
                 _ASAN_VECTOR_CREATE_GUARD;
                 _Mylast = _Uninitialized_move(_First, _Last, _Myfirst, _Al);
             }
+
             return;
         }
 

tests/std/tests/GH_002030_asan_annotate_vector/test.cpp

@@ -921,6 +921,44 @@ void test_insert_n_throw() {
     }
 }
 
+template <class Alloc>
+void test_clear() {
+    using T = typename Alloc::value_type;
+
+    vector<T, Alloc> v;
+    v.push_back(T());
+    v.assign(v.capacity() + 1, T());
+    assert(verify_vector(v));
+    v.clear();
+    assert(verify_vector(v));
+    v.clear();
+    assert(verify_vector(v));
+}
+
+template <class Alloc>
+void test_empty() {
+    using T = typename Alloc::value_type;
+
+    vector<T, Alloc> v1;
+    v1.clear();
+    v1.resize(0);
+    v1.shrink_to_fit();
+    v1.assign(0, T());
+    v1.assign({});
+    v1 = {};
+
+    vector<T, Alloc> v2;
+    v1.assign(v2.begin(), v2.end());
+
+    input_iterator_tester<T, 11> in;
+    auto e = in.end();
+    v1.assign(e, e); // empty range of input iterators
+
+    vector<T, Alloc> v3;
+    v1 = v3;
+    v3 = move(v1);
+}
+
 template <class Alloc>
 void run_tests() {
     test_push_pop<Alloc>();
@@ -933,6 +971,8 @@ void run_tests() {
     test_insert_range<Alloc>();
     test_assign<Alloc>();
     test_resize<Alloc>();
+    test_clear<Alloc>();
+    test_empty<Alloc>();
 }
 
 template <class T, template <class, class, class> class AllocT>