fs/zfs: fall back to VFS when zfs stats are unavailable

lib/fs/zfs/plugin.go

@@ -24,6 +24,12 @@ import (
 
 type zfsPlugin struct{}
 
+// Overridable for tests.
+var (
+	zfsDevicePath = "/dev/zfs"
+	getZfsStats   = GetZfsStats
+)
+
 // NewPlugin creates a new ZFS filesystem plugin.
 func NewPlugin() fs.FsPlugin {
 	return &zfsPlugin{}
@@ -44,15 +50,19 @@ func (p *zfsPlugin) Priority() int {
 }
 
 // GetStats returns filesystem statistics for ZFS.
+//
+// When /dev/zfs is missing, or when zfs tooling cannot collect stats (common
+// in unprivileged containers that still see the device node but lack
+// permission to use it), fall back to VFS so mountpoint capacity remains
+// available via statfs.
 func (p *zfsPlugin) GetStats(device string, partition fs.PartitionInfo) (*fs.FsStats, error) {
-	// Check if ZFS is available - if /dev/zfs doesn't exist, fall back to VFS
-	if _, err := os.Stat("/dev/zfs"); os.IsNotExist(err) {
+	if _, err := os.Stat(zfsDevicePath); err != nil {
 		return nil, fs.ErrFallbackToVFS
 	}
 
-	capacity, free, avail, err := GetZfsStats(device)
+	capacity, free, avail, err := getZfsStats(device)
 	if err != nil {
-		return nil, err
+		return nil, fs.ErrFallbackToVFS
 	}
 
 	return &fs.FsStats{

lib/fs/zfs/plugin_test.go

@@ -0,0 +1,90 @@
+// Copyright 2024 Google Inc. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package zfs
+
+import (
+	"errors"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/google/cadvisor/lib/fs"
+)
+
+func TestGetStatsFallsBackWhenDevMissing(t *testing.T) {
+	oldPath := zfsDevicePath
+	zfsDevicePath = filepath.Join(t.TempDir(), "missing-zfs")
+	defer func() { zfsDevicePath = oldPath }()
+
+	_, err := NewPlugin().GetStats("tank/root", fs.PartitionInfo{Mountpoint: "/"})
+	if !errors.Is(err, fs.ErrFallbackToVFS) {
+		t.Fatalf("GetStats() error = %v, want ErrFallbackToVFS", err)
+	}
+}
+
+func TestGetStatsFallsBackOnZfsError(t *testing.T) {
+	// Simulate /dev/zfs present but unusable (e.g. EPERM in an unprivileged
+	// container): device node exists, yet zfs list fails.
+	dev := filepath.Join(t.TempDir(), "zfs")
+	if err := os.WriteFile(dev, nil, 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	oldPath, oldFn := zfsDevicePath, getZfsStats
+	zfsDevicePath = dev
+	getZfsStats = func(string) (uint64, uint64, uint64, error) {
+		return 0, 0, 0, os.ErrPermission
+	}
+	defer func() {
+		zfsDevicePath = oldPath
+		getZfsStats = oldFn
+	}()
+
+	_, err := NewPlugin().GetStats("tank/root", fs.PartitionInfo{Mountpoint: "/"})
+	if !errors.Is(err, fs.ErrFallbackToVFS) {
+		t.Fatalf("GetStats() error = %v, want ErrFallbackToVFS", err)
+	}
+}
+
+func TestGetStatsSuccess(t *testing.T) {
+	dev := filepath.Join(t.TempDir(), "zfs")
+	if err := os.WriteFile(dev, nil, 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	oldPath, oldFn := zfsDevicePath, getZfsStats
+	zfsDevicePath = dev
+	getZfsStats = func(pool string) (uint64, uint64, uint64, error) {
+		if pool != "tank/root" {
+			t.Errorf("pool = %q, want tank/root", pool)
+		}
+		return 300, 100, 100, nil
+	}
+	defer func() {
+		zfsDevicePath = oldPath
+		getZfsStats = oldFn
+	}()
+
+	stats, err := NewPlugin().GetStats("tank/root", fs.PartitionInfo{Mountpoint: "/"})
+	if err != nil {
+		t.Fatalf("GetStats() unexpected error: %v", err)
+	}
+	if stats.Capacity != 300 || stats.Free != 100 || stats.Available != 100 {
+		t.Errorf("stats = %+v, want capacity=300 free=100 available=100", stats)
+	}
+	if stats.Type != fs.ZFS {
+		t.Errorf("Type = %q, want %q", stats.Type, fs.ZFS)
+	}
+}