fixed bug 1630: wrong CPU values when no ${top *} var was used

#1630

My per-core CPU values were wrong because Conky was ignoring half the data in /proc/stat.

In Linux, Conky supports short and long /proc/stat formats. The type is auto-detected by determine_longstat_file(). However, this function is only called if the user's config uses a ${top *} var. If the user doesn't use this widget, the CPU values are calculated wrong because 4 of the 8 stat values are ignored.

In my case, I discovered it because conky showed one of my CPU cores pegged at 100% all the time no matter what. But htop + btop showed under 5% for that core. So I did some digging, and found it was happening because that core was stuck in iowait due to a hung NFS mount.

Conky normally handles iowait correctly, but only if it detected the long stat format. I didn't have any ${top *} vars, so conky wasn't trying to detect the stat format, defaulting to short stat which ignores half the data, and this made it think the CPU was always at 100%.

This fix detects the stat format the first time it's accessed, regardless of which widgets the user config contains.

src/linux.cc

@@ -902,8 +902,12 @@ void determine_longstat_file(void) {
 #define MAX_PROCSTAT_LINELEN 255
   FILE *stat_fp;
   static int reported = 0;
+  static bool first = true;
   char buf[MAX_PROCSTAT_LINELEN + 1];
 
+  if (! first) return;
+  first = false;
+
   if (!(stat_fp = open_file("/proc/stat", &reported))) return;
   while (!feof(stat_fp) &&
          fgets(buf, MAX_PROCSTAT_LINELEN, stat_fp) != nullptr) {
@@ -1000,6 +1004,7 @@ int update_stat(void) {
   }
 
   if (!stat_template) {
+    determine_longstat_file();
     stat_template =
         KFLAG_ISSET(KFLAG_IS_LONGSTAT) ? TMPL_LONGSTAT : TMPL_SHORTSTAT;
   }