Merge bitcoin/bitcoin#34546: kernel: Avoid duplicating symbols in the kernel library

eafd530d20 kernel: avoid potential duplicate object in shared library/binary (Cory Fields)
24c3b47010 build: add kernel-specific warnings (Cory Fields)

Pull request description:

  This is a revival of https://github.com/bitcoin/bitcoin/pull/31807

  Introduces the [-Wunique-object-duplication](https://clang.llvm.org/docs/DiagnosticsReference.html#wunique-object-duplication) warning flag available in clang-21 for usage when building the kernel library. It warns of potential duplicate objects in shared libraries. REDUCE_EXPORTS needs to be ON to trigger it.

  Though we have a C API now that manages exporting symbols, I think it is prudent to also avoid any duplicate symbols on the internal c++ side in case we ever to decide to expose some of its headers. It also not clear that all linkers would handle these cases correctly even in the current internal usage.

ACKs for top commit:
  fanquake:
    ACK eafd530d20
  hebasto:
    ACK eafd530d20.

Tree-SHA512: 81961b50f0268dbe076497e130857f5b4b9151c748d107ec15158d1511dd25bce745e0beeb127b9cea51cb2edd78032735600606a75f7ff8a3fd572acced42e0

src/kernel/CMakeLists.txt

@@ -83,9 +83,25 @@ add_library(bitcoinkernel
   $<TARGET_OBJECTS:leveldb>
   $<TARGET_OBJECTS:crc32c>
 )
+
+# Compiler warnings that apply only to the kernel and its dependencies.
+# These can be more strict and/or warnings that only apply to shared
+# libs.
+add_library(kernel_warn_interface INTERFACE)
+if(NOT MSVC)
+  try_append_cxx_flags("-Wunique-object-duplication" TARGET kernel_warn_interface SKIP_LINK)
+endif()
+
+# Also manually apply the warnings to the kernel's internal dependencies
+target_link_libraries(bitcoin_clientversion PRIVATE kernel_warn_interface)
+target_link_libraries(bitcoin_crypto PRIVATE kernel_warn_interface)
+target_link_libraries(leveldb PRIVATE kernel_warn_interface)
+target_link_libraries(crc32c PRIVATE kernel_warn_interface)
+
 target_link_libraries(bitcoinkernel
   PRIVATE
     core_interface
+    kernel_warn_interface
     secp256k1_objs
     $<$<PLATFORM_ID:Windows>:bcrypt>
     $<TARGET_NAME_IF_EXISTS:USDT::headers>

src/support/lockedpool.cpp

@@ -399,3 +399,10 @@ void LockedPoolManager::CreateInstance()
     static LockedPoolManager instance(std::move(allocator));
     LockedPoolManager::_instance = &instance;
 }
+
+LockedPoolManager& LockedPoolManager::Instance()
+{
+    static std::once_flag init_flag;
+    std::call_once(init_flag, LockedPoolManager::CreateInstance);
+    return *LockedPoolManager::_instance;
+}

src/support/lockedpool.h

@@ -219,12 +219,7 @@ class LockedPoolManager : public LockedPool
 {
 public:
     /** Return the current instance, or create it once */
-    static LockedPoolManager& Instance()
-    {
-        static std::once_flag init_flag;
-        std::call_once(init_flag, LockedPoolManager::CreateInstance);
-        return *LockedPoolManager::_instance;
-    }
+    static LockedPoolManager& Instance();
 
 private:
     explicit LockedPoolManager(std::unique_ptr<LockedPageAllocator> allocator);