don't deadlock in demo when cancelled

src/demo/demo.h

@@ -5,7 +5,6 @@
 #include <assert.h>
 #include <unistd.h>
 #include <limits.h>
-#include <pthread.h>
 #include <stdatomic.h>
 #include <version.h>
 #include <notcurses/notcurses.h>
@@ -148,7 +147,8 @@ int demo_render(struct notcurses* nc);
 #define DEMO_RENDER(nc) { int demo_render_err = demo_render(nc); if(demo_render_err){ return demo_render_err; }}
 
 // locked by callers to notcurses_render() and notcurses_refresh(), all internal
-extern pthread_mutex_t demo_render_lock;
+void lock_demo_render(void);
+void unlock_demo_render(void);
 
 // if you won't be doing things, and it's a long sleep, consider using
 // demo_nanosleep(). it updates the HUD, which looks better to the user.

src/demo/hud.c

@@ -1,11 +1,12 @@
 #include "demo.h"
+#include <pthread.h>
 
 // we provide a heads-up display throughout the demo, detailing the demos we're
 // about to run, running, and just runned. the user can move this HUD with
 // their mouse. it should always be on the top of the z-stack.
 struct ncplane* hud = NULL;
 
-pthread_mutex_t demo_render_lock = PTHREAD_MUTEX_INITIALIZER;
+static pthread_mutex_t demo_render_lock = PTHREAD_MUTEX_INITIALIZER;
 
 // while the HUD is grabbed by the mouse, these are set to the position where
 // the grab started. they are reset once the HUD is released.
@@ -340,6 +341,10 @@ int hud_completion_notify(const demoresult* result){
   return 0;
 }
 
+static void unlock_mutex(void* vm){
+  pthread_mutex_unlock(vm);
+}
+
 // inform the HUD of an upcoming demo
 int hud_schedule(const char* demoname){
   elem* cure;
@@ -440,9 +445,20 @@ int demo_render(struct notcurses* nc){
       return -1;
     }
   }
+  int ret = 0;
+  pthread_cleanup_push(unlock_mutex, &demo_render_lock);
   // lock against a possible notcurses_refresh() on Ctrl+L
   pthread_mutex_lock(&demo_render_lock);
-  int ret = notcurses_render(nc);
+  ret = notcurses_render(nc);
   pthread_mutex_unlock(&demo_render_lock);
+  pthread_cleanup_pop(1);
   return ret;
 }
+
+void lock_demo_render(void){
+  pthread_mutex_lock(&demo_render_lock);
+}
+
+void unlock_demo_render(void){
+  pthread_mutex_unlock(&demo_render_lock);
+}

src/demo/input.c

@@ -97,9 +97,9 @@ ultramegaok_demo(void* vnc){
       }
     }
     if(id == 'L' && ni.ctrl){
-      pthread_mutex_lock(&demo_render_lock);
+      lock_demo_render();
       notcurses_refresh(nc);
-      pthread_mutex_unlock(&demo_render_lock);
+      unlock_demo_render();
       continue;
     }
     // if this was about the menu or HUD, pass to them, and continue