Fix mount points being collected multiple times in filesystem_linux

Signed-off-by: Markus Sütter <markus.suetter@secunet.com>

Author: 0xf09f95b4

collector/filesystem_linux.go

@@ -23,6 +23,7 @@ import (
 	"io"
 	"log/slog"
 	"os"
+	"slices"
 	"strings"
 	"sync"
 	"time"
@@ -213,14 +214,25 @@ func parseFilesystemLabels(r io.Reader) ([]filesystemLabels, error) {
 			m++
 		}
 
+		mountPoint := rootfsStripPrefix(parts[4])
+		device := parts[m+3]
+
+		// Ensure that we don't collect the same mountPoint/device combination twice, as that leads
+		// to duplicate metrics which can lead to "Metric collected before" errors.
+		if slices.ContainsFunc(filesystems, func(f filesystemLabels) bool {
+			return f.mountPoint == mountPoint && f.device == device
+		}) {
+			continue
+		}
+
 		// Ensure we handle the translation of \040 and \011
 		// as per fstab(5).
 		parts[4] = strings.ReplaceAll(parts[4], "\\040", " ")
 		parts[4] = strings.ReplaceAll(parts[4], "\\011", "\t")
 
 		filesystems = append(filesystems, filesystemLabels{
-			device:      parts[m+3],
-			mountPoint:  rootfsStripPrefix(parts[4]),
+			device:      device,
+			mountPoint:  mountPoint,
 			fsType:      parts[m+2],
 			options:     parts[5],
 			major:       fmt.Sprint(major),

collector/filesystem_linux_test.go

@@ -45,6 +45,31 @@ func Test_parseFilesystemLabelsError(t *testing.T) {
 	}
 }
 
+func Test_parseFilesystemLabelsDoubleEntry(t *testing.T) {
+	// This testcase /proc/self/mountinfo contains a bindmount of the /nix/store
+	// mountpoint on itself. This creates two entries of the same mountpoint which
+	// previously lead to "Metric collected before" errors in node_exporter.
+	proc_mounts := `79 64 254:0 /nix/store /nix/store ro,relatime shared:8 - erofs /dev/mapper/nix-store ro,user_xattr,acl,cache_strategy=readaround
+41 79 254:0 /nix/store /nix/store ro,nosuid,nodev,relatime shared:9 - erofs /dev/mapper/nix-store ro,user_xattr,acl,cache_strategy=readaround`
+
+	file := strings.NewReader(proc_mounts)
+
+	filesystems, err := parseFilesystemLabels(file)
+	if err != nil {
+		t.Errorf("unexpected error: %v", err)
+	}
+
+	found := 0
+	for _, fs := range filesystems {
+		if fs.mountPoint == "/nix/store" && fs.device == "/dev/mapper/nix-store" {
+			found++
+		}
+	}
+	if found > 1 {
+		t.Errorf("Mount point collected more than once!")
+	}
+}
+
 func TestMountPointDetails(t *testing.T) {
 	if _, err := kingpin.CommandLine.Parse([]string{"--path.procfs", "./fixtures/proc"}); err != nil {
 		t.Fatal(err)