codership/wsrep-lib#117 Fixed empty vector access.

Access to empty vector by using operator[] may cause stdlib++
assertions to fail. Replaced the vector data access to use data()
method which is valid operation even if the vector is empty.

Added unit test to reproduce assertion with empty mutable_buffer access.

Added -D_GLIBCXX_ASSERTIONS preprocessor option to debug builds
to catch standard library misuse.

Added gcc 8 and gcc9 into travis build matrix.

.travis.yml

@@ -147,6 +147,62 @@ matrix:
             - libboost-filesystem-dev
             - libboost-thread-dev
       env: MATRIX_EVAL="CC=gcc-7 CXX=g++-7 TYPE=RelWithDebInfo STRICT=ON UNIT_TESTS=ON ASAN=OFF DBSIM=ON"
+    - os: linux
+      name: "GCC 8 Debug"
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+          packages:
+            - g++-8
+            - cmake
+            - libboost-test-dev
+            - libboost-program-options-dev
+            - libboost-filesystem-dev
+            - libboost-thread-dev
+      env: MATRIX_EVAL="CC=gcc-8 CXX=g++-8 TYPE=Debug STRICT=ON UNIT_TESTS=ON ASAN=OFF DBSIM=ON"
+    - os: linux
+      name: "GCC 8 RelWithDebInfo"
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+          packages:
+            - g++-8
+            - cmake
+            - libboost-test-dev
+            - libboost-program-options-dev
+            - libboost-filesystem-dev
+            - libboost-thread-dev
+      env: MATRIX_EVAL="CC=gcc-8 CXX=g++-8 TYPE=RelWithDebInfo STRICT=ON UNIT_TESTS=ON ASAN=OFF DBSIM=ON"
+    - os: linux
+      name: "GCC 9 Debug"
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+          packages:
+            - g++-9
+            - cmake
+            - libboost-test-dev
+            - libboost-program-options-dev
+            - libboost-filesystem-dev
+            - libboost-thread-dev
+      env: MATRIX_EVAL="CC=gcc-9 CXX=g++-9 TYPE=Debug STRICT=ON UNIT_TESTS=ON ASAN=OFF DBSIM=ON"
+    - os: linux
+      name: "GCC 9 RelWithDebInfo"
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+          packages:
+            - g++-9
+            - cmake
+            - libboost-test-dev
+            - libboost-program-options-dev
+            - libboost-filesystem-dev
+            - libboost-thread-dev
+      env: MATRIX_EVAL="CC=gcc-9 CXX=g++-9 TYPE=RelWithDebInfo STRICT=ON UNIT_TESTS=ON ASAN=OFF DBSIM=ON"
     - os: linux
       dist: trusty
       name: "Clang 3.6 Debug"

CMakeLists.txt

@@ -80,6 +80,11 @@ if (WSREP_LIB_MAINTAINER_MODE)
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Werror")
 endif()
 
+# Enable extra libstdc++ assertions with debug build.
+if (CMAKE_BUILD_TYPE STREQUAL "Debug")
+  add_definitions("-D_GLIBCXX_ASSERTIONS")
+endif()
+
 # Set up include directories
 include_directories("${CMAKE_CURRENT_SOURCE_DIR}/include")
 include_directories("${CMAKE_CURRENT_SOURCE_DIR}/wsrep-API")

include/wsrep/buffer.hpp

@@ -85,8 +85,26 @@ namespace wsrep
         }
 
         size_t size() const { return buffer_.size(); }
-        char* data() { return &buffer_[0]; }
-        const char* data() const { return &buffer_[0]; }
+
+        /**
+         * Return pointer to underlying data array. The returned pointer
+         * may or may not be null in case of empty buffer, it is up to
+         * user to check the size of the array before dereferencing the
+         * pointer.
+         *
+         * @return Pointer to underlying data array.
+         */
+        char* data() { return buffer_.data(); }
+
+        /**
+         * Return const pointer to underlying data array. The returned pointer
+         * may or may not be null in case of empty buffer, it is up to
+         * user to check the size of the array before dereferencing the
+         * pointer.
+         *
+         * @return Const pointer to underlying data array.
+         */
+        const char* data() const { return buffer_.data(); }
 
         mutable_buffer& operator= (const mutable_buffer& other)
         {

src/wsrep_provider_v26.cpp

@@ -884,7 +884,7 @@ wsrep::wsrep_provider_v26::enter_toi(
     return map_return_value(wsrep_->to_execute_start(
                                 wsrep_,
                                 client_id.get(),
-                                &wsrep_keys[0],
+                                wsrep_keys.data(),
                                 wsrep_keys.size(),
                                 &wsrep_buf,
                                 1,

test/CMakeLists.txt

@@ -7,6 +7,7 @@ add_executable(wsrep-lib_test
   mock_high_priority_service.cpp
   mock_storage_service.cpp
   test_utils.cpp
+  buffer_test.cpp
   gtid_test.cpp
   id_test.cpp
   server_context_test.cpp

test/buffer_test.cpp

@@ -0,0 +1,28 @@
+/*
+ * Copyright (C) 2019 Codership Oy <info@codership.com>
+ *
+ * This file is part of wsrep-lib.
+ *
+ * Wsrep-lib is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * Wsrep-lib is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with wsrep-lib.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+#include "wsrep/buffer.hpp"
+#include <boost/test/unit_test.hpp>
+
+BOOST_AUTO_TEST_CASE(buffer_test_empty_access)
+{
+    wsrep::mutable_buffer buf;
+    BOOST_REQUIRE(buf.size() == 0);
+    (void)buf.data();
+}