Conntrack update: differentiate ErrNoData and other errors (#1323, #1723)

Signed-off-by: Li Ling <liiling@google.com>

collector/conntrack_linux.go

@@ -16,7 +16,12 @@
 package collector
 
 import (
+	"errors"
+	"fmt"
+	"os"
+
 	"github.com/go-kit/kit/log"
+	"github.com/go-kit/kit/log/level"
 	"github.com/prometheus/client_golang/prometheus"
 )
 
@@ -50,18 +55,25 @@ func NewConntrackCollector(logger log.Logger) (Collector, error) {
 func (c *conntrackCollector) Update(ch chan<- prometheus.Metric) error {
 	value, err := readUintFromFile(procFilePath("sys/net/netfilter/nf_conntrack_count"))
 	if err != nil {
-		// Conntrack probably not loaded into the kernel.
-		return err
+		return c.handleErr(err)
 	}
 	ch <- prometheus.MustNewConstMetric(
 		c.current, prometheus.GaugeValue, float64(value))
 
 	value, err = readUintFromFile(procFilePath("sys/net/netfilter/nf_conntrack_max"))
 	if err != nil {
-		return err
+		return c.handleErr(err)
 	}
 	ch <- prometheus.MustNewConstMetric(
 		c.limit, prometheus.GaugeValue, float64(value))
 
 	return nil
 }
+
+func (c *conntrackCollector) handleErr(err error) error {
+	if errors.Is(err, os.ErrNotExist) {
+		level.Debug(c.logger).Log("msg", "conntrack probably not loaded")
+		return ErrNoData
+	}
+	return fmt.Errorf("failed to retrieve conntrack stats: %w", err)
+}