Merge bitcoin#19111: Limit scope of all global std::once_flag

fa9c675 Limit scope of all global std::once_flag (MarcoFalke)

Pull request description:

  `once_flag` is  a helper (as the name might suggest) to execute a callable only once. Thus, the scope of the flag does never need to extend beyond where the callable is called. Typically this is function scope.

  Move all the flags to function scope to
  * simplify code review
  * avoid mistakes where similarly named flags are accidentally exchanged
  * avoid polluting the global scope

ACKs for top commit:
  hebasto:
    ACK fa9c675, tested on Linux Mint 19.3 (x86_64).
  promag:
    Code review ACK fa9c675.

Tree-SHA512: 095a0c11d93d0ddcb82b3c71676090ecc7e3de3d5e7a2a63ab2583093be279242acac43523bbae2060b4dcfa8f92b54256a0e91fbbae78fa92d2d49e9db62e57

src/rpc/server.cpp

@@ -25,8 +25,6 @@
 
 static Mutex g_rpc_warmup_mutex;
 static std::atomic<bool> g_rpc_running{false};
-static std::once_flag g_rpc_interrupt_flag;
-static std::once_flag g_rpc_stop_flag;
 static bool fRPCInWarmup GUARDED_BY(g_rpc_warmup_mutex) = true;
 static std::string rpcWarmupStatus GUARDED_BY(g_rpc_warmup_mutex) = "RPC server started";
 /* Timer-creating functions */
@@ -329,6 +327,7 @@ void StartRPC()
 
 void InterruptRPC()
 {
+    static std::once_flag g_rpc_interrupt_flag;
     // This function could be called twice if the GUI has been started with -server=1.
     std::call_once(g_rpc_interrupt_flag, []() {
         LogPrint(BCLog::RPC, "Interrupting RPC\n");

src/support/lockedpool.cpp

@@ -29,7 +29,6 @@
 #endif
 
 LockedPoolManager* LockedPoolManager::_instance = nullptr;
-std::once_flag LockedPoolManager::init_flag;
 
 /*******************************************************************************/
 // Utilities

src/support/lockedpool.h

@@ -221,7 +221,8 @@ class LockedPoolManager : public LockedPool
     /** Return the current instance, or create it once */
     static LockedPoolManager& Instance()
     {
-        std::call_once(LockedPoolManager::init_flag, LockedPoolManager::CreateInstance);
+        static std::once_flag init_flag;
+        std::call_once(init_flag, LockedPoolManager::CreateInstance);
         return *LockedPoolManager::_instance;
     }
 
@@ -234,7 +235,6 @@ class LockedPoolManager : public LockedPool
     static bool LockingFailed();
 
     static LockedPoolManager* _instance;
-    static std::once_flag init_flag;
 };
 
 #endif // BITCOIN_SUPPORT_LOCKEDPOOL_H