steamcompmgr, rendervulkan: prevent segfault that occured when closin…

…g gamescope, due to a race condition between present thread @ present_wait_thread_func & compositor thread @ steamcompmgr_exit

src/rendervulkan.cpp

@@ -2669,7 +2669,8 @@ bool acquire_next_image( void )
 }
 
 
-static std::atomic<uint64_t> g_currentPresentWaitId = {0u};
+inline std::atomic<uint64_t> g_currentPresentWaitId = {0u};
+inline std::atomic<bool> g_presentThreadShouldExit = {false};
 static std::mutex present_wait_lock;
 
 extern void mangoapp_output_update( uint64_t vblanktime );
@@ -2693,6 +2694,10 @@ static void present_wait_thread_func( void )
 				uint64_t vblanktime = get_time_in_nanos();
 				GetVBlankTimer().MarkVBlank( vblanktime, true );
 				mangoapp_output_update( vblanktime );
+			} else if ( g_presentThreadShouldExit.load(std::memory_order_acquire)) {
+				g_presentThreadShouldExit = 0;
+				g_presentThreadShouldExit.notify_all();
+				return;
 			}
 		}
 	}

src/rendervulkan.hpp

@@ -74,6 +74,10 @@ enum EStreamColorspace : int
 #include <vulkan/vulkan.h>
 #include <drm_fourcc.h>
 
+extern std::atomic<uint64_t> g_currentPresentWaitId;
+
+extern std::atomic<bool> g_presentThreadShouldExit;
+
 struct VulkanRenderer_t
 {
 	struct wlr_renderer base;

src/steamcompmgr.cpp

@@ -5840,6 +5840,13 @@ steamcompmgr_exit(void)
 		}
 	}
 
+    //request the present_wait thread to exit
+    //needed to avoid getting a segfault at exit due to race condition:
+    g_presentThreadShouldExit.store(true, std::memory_order_release);
+    g_currentPresentWaitId = 0; //present thread will check if it should exit if this is zero
+    g_presentThreadShouldExit.wait(true); //present thread will toggle this atomic when it sees the exit request
+                                          //this allows us to wait for present thread to close before deleting the backend
+
     gamescope::IBackend::Set( nullptr );
 
     wlserver_lock();