util: unify usage of on_tty() in util.c

src/shared/pager.c

@@ -57,7 +57,7 @@ int pager_open(void) {
                 if (!*pager || streq(pager, "cat"))
                         return 0;
 
-        if (isatty(STDOUT_FILENO) <= 0)
+        if (!on_tty())
                 return 0;
 
         /* Determine and cache number of columns before we spawn the

src/shared/util.c

@@ -2175,28 +2175,25 @@ int read_one_char(FILE *f, char *ret, usec_t t, bool *need_nl) {
 }
 
 int ask(char *ret, const char *replies, const char *text, ...) {
-        bool on_tty;
 
         assert(ret);
         assert(replies);
         assert(text);
 
-        on_tty = isatty(STDOUT_FILENO);
-
         for (;;) {
                 va_list ap;
                 char c;
                 int r;
                 bool need_nl = true;
 
-                if (on_tty)
+                if (on_tty())
                         fputs(ANSI_HIGHLIGHT_ON, stdout);
 
                 va_start(ap, text);
                 vprintf(text, ap);
                 va_end(ap);
 
-                if (on_tty)
+                if (on_tty())
                         fputs(ANSI_HIGHLIGHT_OFF, stdout);
 
                 fflush(stdout);
@@ -3820,6 +3817,15 @@ void columns_cache_reset(int signum) {
         cached_columns = 0;
 }
 
+bool on_tty(void) {
+        static int cached_on_tty = -1;
+
+        if (_unlikely_(cached_on_tty < 0))
+                cached_on_tty = isatty(STDOUT_FILENO) > 0;
+
+        return cached_on_tty;
+}
+
 int fd_lines(int fd) {
         struct winsize ws;
         zero(ws);

src/shared/util.h

@@ -385,6 +385,7 @@ int status_welcome(void);
 int fd_columns(int fd);
 unsigned columns(void);
 void columns_cache_reset(int _unused_ signum);
+bool on_tty(void);
 
 int fd_lines(int fd);
 unsigned lines(void);