<deque>: Add missed masking of _First_used_block_idx in `deque::s…

…hrink_to_fit` to correctly end the loop (#4955)

stl/inc/deque

@@ -997,7 +997,8 @@ public:
 
         const auto _Mask = static_cast<size_type>(_Mapsize() - 1);
 
-        const auto _First_used_block_idx = static_cast<size_type>(_Myoff() / _Block_size);
+        const auto _Unmasked_first_used_block_idx = static_cast<size_type>(_Myoff() / _Block_size);
+        const auto _First_used_block_idx          = static_cast<size_type>(_Unmasked_first_used_block_idx & _Mask);
 
         // (_Myoff() + _Mysize() - 1) is for the last element, i.e. the back() of the deque.
         // Divide by _Block_size to get the unmasked index of the last used block.
@@ -1017,7 +1018,8 @@ public:
             }
         }
 
-        const auto _Used_block_count = static_cast<size_type>(_Unmasked_first_unused_block_idx - _First_used_block_idx);
+        const auto _Used_block_count =
+            static_cast<size_type>(_Unmasked_first_unused_block_idx - _Unmasked_first_used_block_idx);
 
         size_type _New_block_count = _Minimum_map_size; // should be power of 2
 

tests/std/tests/GH_002769_handle_deque_block_pointers/test.cpp

@@ -7,6 +7,7 @@
 #include <deque>
 #include <iterator>
 #include <memory>
+#include <random>
 #include <type_traits>
 
 using namespace std;
@@ -328,8 +329,31 @@ void test_inconsistent_difference_types() {
     assert(counter == 0);
 }
 
+// Also test GH-4954: Endless loop in deque::shrink_to_fit()
+void test_gh_4954() {
+    deque<int> qu;
+    mt19937_64 mteng;
+
+    for (int i = 0; i < 256; ++i) {
+        const auto push_count = static_cast<size_t>((mteng() & 32767U) + 1);
+        for (size_t j = 0; j < push_count; ++j) {
+            qu.push_back(0);
+        }
+
+        auto pop_count = static_cast<size_t>((mteng() & 32767U) + 1);
+        if (i % 100 == 0 || pop_count > qu.size()) {
+            pop_count = qu.size();
+        }
+        for (size_t j = 0; j < pop_count; ++j) {
+            qu.pop_front();
+        }
+        qu.shrink_to_fit();
+    }
+}
+
 int main() {
     test_gh_2769();
     test_gh_3717();
+    test_gh_4954();
     test_inconsistent_difference_types();
 }