... re-enable only the assertions (not full debug mode)

cmake/cxx.cmake

@@ -1,17 +1,17 @@
 if (CMAKE_BUILD_TYPE_UC STREQUAL "DEBUG")
-    # Enable libcxx debug mode:
-    # --> https://releases.llvm.org/15.0.0/projects/libcxx/docs/DesignDocs/DebugMode.html
+    # Enable libcxx debug mode: https://releases.llvm.org/15.0.0/projects/libcxx/docs/DesignDocs/DebugMode.html
     # The docs say the debug mode violates complexity guarantees, so do this only for Debug builds.
-    set (CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG} -D_LIBCPP_ENABLE_DEBUG_MODE=1")
+    # set (CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG} -D_LIBCPP_ENABLE_DEBUG_MODE=1")
+    # ^^ Crashes the database upon startup, needs investigation.
+    #    Besides that, the implementation looks like a poor man's MSAN specific to libcxx. Since CI tests MSAN
+    #    anyways, we can keep the debug mode disabled.
 
-    # Also, enable randomization of unspecified behavior, e.g. the order of equal elements in std::sort.
-    # "Randomization" means that Debug and Release builds use different seeds.
-    set (CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG} -D_LIBCPP_DEBUG_RANDOMIZE_UNSPECIFIED_STABILITY_SEED=12345678")
-
-    # Note: Libcxx also has provides extra assertions:
+    # Libcxx also provides extra assertions:
     # --> https://releases.llvm.org/15.0.0/projects/libcxx/docs/UsingLibcxx.html#assertions-mode
-    # These look orthogonal to the debug mode but the debug mode imlicitly enables them:
+    # These look orthogonal to the debug mode but the debug mode enables them implicitly:
     # --> https://github.com/llvm/llvm-project/blob/release/15.x/libcxx/include/__assert#L29
+    # They are cheap and straightforward, so enable them in debug builds:
+    set (CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG} -D_LIBCPP_ENABLE_ASSERTIONS=1")
 
     # TODO Once we upgrade to LLVM 18+, reconsider all of the above as they introduced "hardening modes":
     # https://libcxx.llvm.org/Hardening.html

src/Common/DNSResolver.cpp

@@ -428,7 +428,7 @@ bool DNSResolver::updateCacheImpl(
     {
         updated = true;
         String deleted_elements;
-        for (auto & it : elements_to_drop)
+        for (auto it : elements_to_drop)
         {
             if (!deleted_elements.empty())
                 deleted_elements += ", ";

src/Storages/MaterializedView/RefreshSet.cpp

@@ -23,7 +23,7 @@ RefreshSet::Handle & RefreshSet::Handle::operator=(Handle && other) noexcept
     id = std::move(other.id);
     inner_table_id = std::move(other.inner_table_id);
     dependencies = std::move(other.dependencies);
-    iter = std::move(other.iter); /// NOLINT(performance-move-const-arg)
+    iter = std::move(other.iter);
     inner_table_iter = std::move(other.inner_table_iter);
     metric_increment = std::move(other.metric_increment);
     return *this;

src/Storages/MergeTree/MergeTask.cpp

@@ -604,7 +604,7 @@ MergeTask::StageRuntimeContextPtr MergeTask::ExecuteAndFinalizeHorizontalPart::g
     new_ctx->rows_sources_temporary_file = std::move(ctx->rows_sources_temporary_file);
     new_ctx->column_sizes = std::move(ctx->column_sizes);
     new_ctx->compression_codec = std::move(ctx->compression_codec);
-    new_ctx->it_name_and_type = std::move(ctx->it_name_and_type); /// NOLINT(performance-move-const-arg)
+    new_ctx->it_name_and_type = std::move(ctx->it_name_and_type);
     new_ctx->read_with_direct_io = std::move(ctx->read_with_direct_io);
     new_ctx->need_sync = std::move(ctx->need_sync);