fix: parse multiple entries in /proc/self/cgroup

cgroup.go

@@ -125,12 +125,20 @@ func currentProcCgroup(fs afero.Fs) (string, error) {
 		return "", xerrors.Errorf("read %v: %w", procSelfCgroup, err)
 	}
 
-	parts := strings.Split(strings.TrimSpace(string(data)), ":")
-	if len(parts) != 3 {
-		return "", xerrors.Errorf("parse %v: %w", procSelfCgroup, err)
+	entries := strings.Split(strings.TrimSpace(string(data)), "\n")
+
+	for _, entry := range entries {
+		parts := strings.Split(strings.TrimSpace(entry), ":")
+		if len(parts) != 3 {
+			return "", xerrors.Errorf("parse entry %v: %w", procSelfCgroup, err)
+		}
+
+		if parts[0] == "0" {
+			return parts[2], nil
+		}
 	}
 
-	return parts[2], nil
+	return "", xerrors.Errorf("no cgroup entry for hierarchy 0 found")
 }
 
 // read an int64 value from path

cgroup_internal_test.go

@@ -0,0 +1,79 @@
+package clistat
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestCurrentProcCgroup(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name        string
+		procFile    string
+		expectError string
+		expectPath  string
+	}{
+		{
+			name:       "RootPath",
+			procFile:   `0::/`,
+			expectPath: "/",
+		},
+		{
+			name:       "SingleLevelPath",
+			procFile:   `0::/init.slice`,
+			expectPath: "/init.slice",
+		},
+		{
+			name:       "MultipleLevelSlice",
+			procFile:   `0::/wibble/wobble/init.slice`,
+			expectPath: "/wibble/wobble/init.slice",
+		},
+		{
+			name: "MixOfHierachy",
+			procFile: `1:net_cls:/
+0::/`,
+			expectPath: "/",
+		},
+		{
+			name: "MixOfHierarchyPaths",
+			procFile: `1:net_cls:/init.slice
+0::/`,
+			expectPath: "/",
+		},
+		{
+			name: "MixOfHierarchyPaths/Order",
+			procFile: `0::/
+1:net_cls:/init.slice`,
+			expectPath: "/",
+		},
+		{
+			name: "MixOfHierarchyPaths/Paths",
+			procFile: `0::/init.slice
+1:net_cls:/`,
+			expectPath: "/init.slice",
+		},
+		{
+			name:        "MissingHierarchy0",
+			procFile:    `1:net_cls:/`,
+			expectError: "no cgroup entry for hierarchy 0 found",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			fs := initFS(t, map[string]string{procSelfCgroup: tt.procFile})
+
+			path, err := currentProcCgroup(fs)
+			if tt.expectError != "" {
+				require.ErrorContains(t, err, tt.expectError)
+			} else {
+				require.NoError(t, err)
+				require.Equal(t, tt.expectPath, path)
+			}
+		})
+	}
+}