librb: add rb_snprintf_try_append

For when it might fit, or it might not.

librb/include/rb_tools.h

@@ -36,13 +36,8 @@ char *rb_strcasestr(const char *s, const char *find);
 size_t rb_strlcpy(char *dst, const char *src, size_t siz);
 size_t rb_strlcat(char *dst, const char *src, size_t siz);
 size_t rb_strnlen(const char *s, size_t count);
-
+int rb_snprintf_append(char *str, size_t len, const char *format, ...) AFP(3,4);
-#ifdef __GNUC__
+int rb_snprintf_try_append(char *str, size_t len, const char *format, ...) AFP(3,4);
-int rb_snprintf_append(char *str, size_t len, const char *format, ...)
-	__attribute__ ((format(printf, 3, 4)));
-#else
-int rb_snprintf_append(char *str, const size_t size, const char *, ...);
-#endif
 
 char *rb_basename(const char *);
 char *rb_dirname(const char *);

librb/src/export-syms.txt

@@ -161,6 +161,7 @@ rb_setup_fd
 rb_setup_ssl_server
 rb_sleep
 rb_snprintf_append
+rb_snprintf_try_append
 rb_socket
 rb_socketpair
 rb_spawn_process

librb/src/tools.c

@@ -324,6 +324,41 @@ rb_snprintf_append(char *str, size_t len, const char *format, ...)
 	return (orig_len + append_len);
 }
 
+/*
+ * rb_snprintf_try_append()
+ * appends snprintf formatted string to the end of the buffer but not
+ * exceeding len
+ * returns -1 if there isn't enough space for the whole string to fit
+ */
+int
+rb_snprintf_try_append(char *str, size_t len, const char *format, ...)
+{
+	if(len == 0)
+		return -1;
+
+	int orig_len = strlen(str);
+
+	if((int)len < orig_len) {
+		str[len - 1] = '\0';
+		return -1;
+	}
+
+	va_list ap;
+	va_start(ap, format);
+	int append_len = vsnprintf(str + orig_len, len - orig_len, format, ap);
+	va_end(ap);
+
+	if (append_len < 0)
+		return append_len;
+
+	if (orig_len + append_len > (int)(len - 1)) {
+		str[orig_len] = '\0';
+		return -1;
+	}
+
+	return (orig_len + append_len);
+}
+
 /* rb_basename
  *
  * input        -