demo: set up input eventfd #390

When we process input in the demo, write a packet to an
eventfd we set up in input_dispatcher(). This way,
demo components can poll() on that. #390

src/demo/demo.h

@@ -71,6 +71,10 @@ demo_getc_nblock(ncinput* ni){
   return demo_getc(&ts, ni);
 }
 
+// Get a fd which can be poll()ed to check for outstanding input. Do not close
+// this file descriptor.
+int demo_input_fd(void);
+
 // 'ni' may be NULL if the caller is uninterested in event details. Blocks
 // until an event is processed or a signal is received.
 static inline char32_t

src/demo/input.c

@@ -1,12 +1,16 @@
 #include <pthread.h>
+#include <sys/eventfd.h>
 #include "demo.h"
 
 typedef struct nciqueue {
   ncinput ni;
   struct nciqueue *next;
 } nciqueue;
 
-static bool spawned;
+// an eventfd or pipe on which we write upon receipt of input, so that demos
+// can multiplex against other fds.
+static int input_eventfd = -1;
+
 static pthread_t tid;
 static nciqueue* queue;
 static nciqueue** enqueue = &queue;
@@ -61,6 +65,8 @@ pass_along(const ncinput* ni){
   *enqueue = nq;
   enqueue = &nq->next;
   pthread_mutex_unlock(&lock);
+  const unsigned char eventcount[1] = "\x01";
+  write(input_eventfd, eventcount, sizeof(eventcount));
   pthread_cond_signal(&cond);
   return 0;
 }
@@ -113,23 +119,34 @@ ultramegaok_demo(void* vnc){
   return NULL;
 }
 
+int demo_input_fd(void){
+  return input_eventfd;
+}
+
 // listens for events, handling mouse events directly and making other ones
-// available to demos
+// available to demos. returns -1 if already spawned or resource failures.
 int input_dispatcher(struct notcurses* nc){
-  spawned = true;
+  if(input_eventfd >= 0){
+    return -1;
+  }
+  if((input_eventfd = eventfd(0, EFD_CLOEXEC | EFD_NONBLOCK)) < 0){
+    return -1;
+  }
   if(pthread_create(&tid, NULL, ultramegaok_demo, nc)){
-    spawned = false;
+    close(input_eventfd);
+    input_eventfd = -1;
     return -1;
   }
   return 0;
 }
 
 int stop_input(void){
   int ret = 0;
-  if(spawned){
+  if(input_eventfd >= 0){
     ret |= pthread_cancel(tid);
     ret |= pthread_join(tid, NULL);
-    spawned = false;
+    ret |= close(input_eventfd);
+    input_eventfd = -1;
   }
   return ret;
 }