utils/chunked_vector: Fix sigsegv during reserve()

Fixes the case of make_room() invoked with last_chunk_capacity_deficit
but _size not in the last reserved chunk.

Found during code review, no known user impact.

Fixes scylladb#10363.

Message-Id: <20220411222605.641614-1-tgrabiec@scylladb.com>

test/boost/chunked_vector_test.cc

@@ -178,3 +178,32 @@ BOOST_AUTO_TEST_CASE(tests_reserve_partial) {
         BOOST_REQUIRE_EQUAL(v.capacity(), orig_size);
     }
 }
+
+// Tests the case of make_room() invoked with last_chunk_capacity_deficit but _size not in
+// the last reserved chunk.
+BOOST_AUTO_TEST_CASE(test_shrinking_and_expansion_involving_chunk_boundary) {
+    using vector_type = utils::chunked_vector<std::unique_ptr<uint64_t>>;
+    vector_type v;
+
+    // Fill two chunks
+    v.reserve(vector_type::max_chunk_capacity() * 3 / 2);
+    for (uint64_t i = 0; i < vector_type::max_chunk_capacity() * 3 / 2; ++i) {
+        v.emplace_back(std::make_unique<uint64_t>(i));
+    }
+
+    // Make the last chunk smaller than max size to trigger the last_chunk_capacity_deficit path in make_room()
+    v.shrink_to_fit();
+
+    // Leave the last chunk reserved but empty
+    for (uint64_t i = 0; i < vector_type::max_chunk_capacity(); ++i) {
+        v.pop_back();
+    }
+
+    // Try to reserve more than the currently reserved capacity and trigger last_chunk_capacity_deficit path
+    // with _size not in the last chunk. Should not sigsegv.
+    v.reserve(vector_type::max_chunk_capacity() * 4);
+
+    for (uint64_t i = 0; i < vector_type::max_chunk_capacity() * 2; ++i) {
+        v.emplace_back(std::make_unique<uint64_t>(i));
+    }
+}

utils/chunked_vector.hh

@@ -376,7 +376,9 @@ chunked_vector<T, max_contiguous_allocation>::make_room(size_t n, bool stop_afte
         auto new_last_chunk_capacity = last_chunk_capacity + capacity_increase;
         // FIXME: realloc? maybe not worth the complication; only works for PODs
         auto new_last_chunk = new_chunk(new_last_chunk_capacity);
-        migrate(addr(_capacity - last_chunk_capacity), addr(_size), new_last_chunk.get());
+        if (_size > _capacity - last_chunk_capacity) {
+            migrate(addr(_capacity - last_chunk_capacity), addr(_size), new_last_chunk.get());
+        }
         _chunks.back() = std::move(new_last_chunk);
         _capacity += capacity_increase;
     }