sync.h: Add GlobalMutex type

Author: ajtowns

src/sync.h

@@ -129,10 +129,22 @@ using RecursiveMutex = AnnotatedMixin<std::recursive_mutex>;
 /** Wrapped mutex: supports waiting but not recursive locking */
 using Mutex = AnnotatedMixin<std::mutex>;
 
+/** Different type to mark Mutex at global scope
+ *
+ * Thread safety analysis can't handle negative assertions about mutexes
+ * with global scope well, so mark them with a separate type, and
+ * eventually move all the mutexes into classes so they are not globally
+ * visible.
+ *
+ * See: https://github.com/bitcoin/bitcoin/pull/20272#issuecomment-720755781
+ */
+class GlobalMutex : public Mutex { };
+
 #define AssertLockHeld(cs) AssertLockHeldInternal(#cs, __FILE__, __LINE__, &cs)
 
 inline void AssertLockNotHeldInline(const char* name, const char* file, int line, Mutex* cs) EXCLUSIVE_LOCKS_REQUIRED(!cs) { AssertLockNotHeldInternal(name, file, line, cs); }
 inline void AssertLockNotHeldInline(const char* name, const char* file, int line, RecursiveMutex* cs) LOCKS_EXCLUDED(cs) { AssertLockNotHeldInternal(name, file, line, cs); }
+inline void AssertLockNotHeldInline(const char* name, const char* file, int line, GlobalMutex* cs) LOCKS_EXCLUDED(cs) { AssertLockNotHeldInternal(name, file, line, cs); }
 #define AssertLockNotHeld(cs) AssertLockNotHeldInline(#cs, __FILE__, __LINE__, &cs)
 
 /** Wrapper around std::unique_lock style lock for Mutex. */