Allow temporary transfer of main logic thread ownership

Dependency to fix df-ai to work with the new CoreSuspender

library/Core.cpp

@@ -108,6 +108,26 @@ struct MainThread {
 };
 }
 
+CoreSuspendReleaseMain::CoreSuspendReleaseMain()
+{
+    MainThread::suspend().unlock();
+}
+
+CoreSuspendReleaseMain::~CoreSuspendReleaseMain()
+{
+    MainThread::suspend().lock();
+}
+
+CoreSuspendClaimMain::CoreSuspendClaimMain()
+{
+    MainThread::suspend().lock();
+}
+
+CoreSuspendClaimMain::~CoreSuspendClaimMain()
+{
+    MainThread::suspend().unlock();
+}
+
 struct Core::Private
 {
     std::thread iothread;

library/include/Core.h

@@ -300,6 +300,8 @@ namespace DFHack
         friend class ServerConnection;
         friend class CoreSuspender;
         friend class CoreSuspenderBase;
+        friend struct CoreSuspendClaimMain;
+        friend struct CoreSuspendReleaseMain;
         ServerMain *server;
     };
 
@@ -412,5 +414,25 @@ namespace DFHack
         }
     };
 
+    /*!
+     * Temporary release main thread ownership to allow alternative thread
+     * implement DF logic thread loop
+     */
+    struct DFHACK_EXPORT CoreSuspendReleaseMain {
+        CoreSuspendReleaseMain();
+        ~CoreSuspendReleaseMain();
+    };
+
+    /*!
+     * Temporary claim main thread ownership. This allows caller to call
+     * Core::Update from a different thread than original DF logic thread if
+     * logic thread has released main thread ownership with
+     * CoreSuspendReleaseMain
+     */
+    struct DFHACK_EXPORT CoreSuspendClaimMain {
+        CoreSuspendClaimMain();
+        ~CoreSuspendClaimMain();
+    };
+
     using CoreSuspendClaimer = CoreSuspender;
 }