fix(orm): replace data race in ENTITY_METADATA with std::call_once

[kcenon]

What

Summary

Replaces a data race in the ENTITY_METADATA() macro by swapping the unsafe static bool initialized check-then-act pattern with std::call_once + std::once_flag, guaranteeing thread-safe one-time initialization of entity metadata.

Change Type

• Bugfix (fixes an issue)
• Feature (new functionality)
• Refactor (no functional changes)

Affected Components

• database/orm/entity.h — ENTITY_METADATA() macro definition

Why

Problem Solved

The ENTITY_METADATA() macro used a plain static bool initialized flag with a manual check-then-act pattern that is not thread-safe under the C++11 memory model. When multiple threads call get_metadata() for the first time concurrently, initialize_metadata() could be called multiple times, leading to undefined behavior (data race) and potential duplicate field entries in the metadata.

Related Issues

• Closes [Bug] ENTITY_METADATA() macro has data race on static bool initialized #556

Alternative Approaches Considered

1. std::atomic<bool> with double-checked locking — More complex, same overhead as call_once
2. Eager initialization in constructor — Would change entity construction order

Where

Files Changed

File	Type of Change
database/orm/entity.h	Replace static bool with std::call_once, add #include <mutex>
docs/README.kr.md	Fix pre-existing broken markdown anchors

How

Implementation Details

// Before (data race)
static bool initialized = false;
if (!initialized) {
    initialize_metadata();
    initialized = true;
}

// After (thread-safe)
static std::once_flag init_flag;
std::call_once(init_flag, [this]() { initialize_metadata(); });

This aligns with the existing threading discipline in backend_base::initialized_ which correctly uses std::atomic<bool>.

Breaking Changes

None — get_metadata() return type and behavior are unchanged.

[github-actions]

Benchmark Results

No comparison reports available. Baseline may not be established yet.