Ensure correctness while typing a query fast

Typing a query fast enough to trigger the poll check in filter_choices()
could yield a wrong set of choices. Since filter_choices() is
interrupted, all choices have yet not been examined and potential
matches could still be left unexamined. Calling print_choices() in this
state causes the unexamined choices to never be reconsidered as
potential matches since they will have a score = 0. The solution is to
never call print_choices() if filter_choices() was interrupted.

Problem reported and partial solution provided by Jenz Guenther in
GitHub issue #268.

pick.c

@@ -71,7 +71,7 @@ struct choice {
 static int			 choicecmp(const void *, const void *);
 static void			 delete_between(char *, size_t, size_t, size_t);
 static char			*eager_strpbrk(const char *, const char *);
-static void			 filter_choices(size_t);
+static int			 filter_choices(size_t, int);
 static char			*get_choices(void);
 static enum key			 get_key(const char **);
 static void			 handle_sigwinch(int);
@@ -325,8 +325,11 @@ selected_choice(void)
 			choices_count = choices.length;
 		query_grew = 0;
 		if (dofilter) {
-			filter_choices(choices_count);
-			dofilter = selection = yscroll = 0;
+			dofilter = 0;
+			if (filter_choices(choices_count, query_length > 0))
+				selection = yscroll = 0;
+			else
+				dochoices = 0;
 		}
 
 		tty_putp(cursor_invisible, 0);
@@ -530,12 +533,14 @@ selected_choice(void)
 }
 
 /*
- * Filter choices using the current query while there is no new user input
- * available.
- * The first nchoices number of choices will be filtered.
+ * Filter the first nchoices number of choices using the current query.
+ * If intr is non-zero, regularly check for new user input and abort the
+ * filtering. This improves the performance when the cardinality of the choices
+ * is large.
+ * Returns non-zero if the filtering was not aborted.
  */
-void
-filter_choices(size_t nchoices)
+int
+filter_choices(size_t nchoices, int intr)
 {
 	struct choice	*c;
 	struct pollfd	 pfd;
@@ -555,23 +560,18 @@ filter_choices(size_t nchoices)
 			c->score = (double)query_length/match_length/c->length;
 		}
 
-		/*
-		 * Regularly check if there is any new user input available. If
-		 * true, abort filtering since the currently used query is
-		 * outdated. This improves the performance when the cardinality
-		 * of the choices is large.
-		 */
-		if (i > 0 && i % 50 == 0) {
+		if (intr && i > 0 && i % 50 == 0) {
 			pfd.fd = fileno(tty_in);
 			pfd.events = POLLIN;
 			if ((nready = poll(&pfd, 1, 0)) == -1)
 				err(1, "poll");
 			if (nready == 1 && pfd.revents & (POLLIN | POLLHUP))
-				break;
+				return 0;
 		}
 	}
-
 	qsort(choices.v, nchoices, sizeof(struct choice), choicecmp);
+
+	return 1;
 }
 
 int