Fix bug in which "dead" subscribers would not be removed right away leading to bspwm running out of file descriptors

src/bspwm.c

@@ -215,6 +215,8 @@ int main(int argc, char *argv[])
 		if (!check_connection(dpy)) {
 			running = false;
 		}
+
+		prune_dead_subscribers();
 	}
 
 	if (restart) {

src/subscribe.c

@@ -163,3 +163,19 @@ void put_status(subscriber_mask_t mask, ...)
 		sb = next;
 	}
 }
+
+void prune_dead_subscribers() {
+	subscriber_list_t *sb = subscribe_head;
+	while (sb != NULL) {
+		subscriber_list_t *next = sb->next;
+		// To check if a subscriber's stream is still open and writable call
+		// write with an empty buffer and check the returned value. If the
+		// stream is not writable anymore (i.e. it has been closed because the
+		// process associated to this subscriber no longer exists) then write()
+		// will return -1.
+		if (write(fileno(sb->stream), 0, 0) == -1) {
+			remove_subscriber(sb);
+		}
+		sb = next;
+	}
+}

src/subscribe.h

@@ -68,4 +68,8 @@ void add_subscriber(subscriber_list_t *sb);
 int print_report(FILE *stream);
 void put_status(subscriber_mask_t mask, ...);
 
+/* Remove any subscriber for which the stream as been closed and is no longer
+ * writable. */
+void prune_dead_subscribers();
+
 #endif