feat: fan RPM monitoring (#2032)

Adds fan RPM monitoring as a peer to the existing temperature collection, addressing #1918.

---------

Co-authored-by: henrygd <hank@henrygd.me>

agent/fans.go

@@ -0,0 +1,101 @@
+package agent
+
+import (
+	"log/slog"
+	"os"
+	"path/filepath"
+	"strings"
+	"sync"
+
+	"github.com/henrygd/beszel/agent/utils"
+	"github.com/henrygd/beszel/internal/entities/system"
+)
+
+type fanSensor struct {
+	key, path string
+}
+
+var getFanSensors = newFanSensorCache(hwmonRoot)
+
+func newFanSensorCache(root string) func() ([]fanSensor, error) {
+	return sync.OnceValues(func() ([]fanSensor, error) {
+		return discoverHwmonFans(root)
+	})
+}
+
+// updateFans populates systemStats.Fans from the host's hwmon sysfs tree.
+// No-op on platforms where hwmon isn't available (see fans_other.go).
+func (a *Agent) updateFans(systemStats *system.Stats) {
+	if hwmonRoot == "" {
+		return
+	}
+	sensors, err := getFanSensors()
+	if err != nil {
+		slog.Debug("Error reading fans", "err", err)
+		return
+	}
+	fans := readFanSensors(sensors)
+	if len(fans) == 0 {
+		return
+	}
+	systemStats.Fans = fans
+	// Note: Commented out because we don't currently use this value in the UI.
+	// Compute the single "dashboard" value used by the FanSpeed alert.
+	// Per-sensor RPMs live in Stats.Fans and drive the multi-line FanChart
+	// in the UI; the alert path only needs one number to compare against
+	// the user's threshold, so we use the highest RPM across all fans
+	// a.systemInfo.DashboardFan = 0
+	// for _, rpm := range fans {
+	// 	if rpm > a.systemInfo.DashboardFan {
+	// 		a.systemInfo.DashboardFan = rpm
+	// 	}
+	// }
+}
+
+// readHwmonFans walks the given hwmon root (typically /sys/class/hwmon) and
+// returns a map of "<chip>_<label-or-fan-idx>" → RPM for every fan*_input
+// file it finds. Zero RPM is retained because it can represent a real fan that
+// has stopped; negative and malformed readings are ignored.
+func readHwmonFans(root string) (map[string]uint16, error) {
+	sensors, err := discoverHwmonFans(root)
+	if err != nil {
+		return nil, err
+	}
+	return readFanSensors(sensors), nil
+}
+
+func discoverHwmonFans(root string) ([]fanSensor, error) {
+	entries, err := os.ReadDir(root)
+	if err != nil {
+		return nil, err
+	}
+	var sensors []fanSensor
+	for _, entry := range entries {
+		chipDir := filepath.Join(root, entry.Name())
+		chipName := utils.ReadStringFile(filepath.Join(chipDir, "name"))
+		if chipName == "" {
+			chipName = entry.Name()
+		}
+		inputs, _ := filepath.Glob(filepath.Join(chipDir, "fan*_input"))
+		for _, inputPath := range inputs {
+			base := strings.TrimSuffix(filepath.Base(inputPath), "_input")
+			label := utils.ReadStringFile(filepath.Join(chipDir, base+"_label"))
+			key := chipName + "_" + base
+			if label != "" {
+				key = chipName + "_" + label
+			}
+			sensors = append(sensors, fanSensor{key, inputPath})
+		}
+	}
+	return sensors, nil
+}
+
+func readFanSensors(sensors []fanSensor) map[string]uint16 {
+	fans := make(map[string]uint16, len(sensors))
+	for _, sensor := range sensors {
+		if rpm, ok := utils.ReadUintFile(sensor.path); ok {
+			fans[sensor.key] = uint16(rpm)
+		}
+	}
+	return fans
+}

agent/fans_linux.go

@@ -0,0 +1,8 @@
+//go:build linux
+
+package agent
+
+// hwmonRoot is the sysfs entry point for hardware monitor chips. Each
+// subdirectory (hwmon0, hwmon1, …) is one chip; fan*_input files inside it
+// expose RPM readings.
+const hwmonRoot = "/sys/class/hwmon"

agent/fans_other.go

@@ -0,0 +1,7 @@
+//go:build !linux
+
+package agent
+
+// hwmonRoot is empty on non-Linux platforms — fan RPM reporting via sysfs
+// hwmon is Linux-specific. updateFans() short-circuits when this is empty.
+const hwmonRoot = ""

agent/fans_test.go

@@ -0,0 +1,87 @@
+//go:build testing
+
+package agent
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// writeFile creates path with parents and writes contents.
+func writeFile(t *testing.T, path, contents string) {
+	t.Helper()
+	require.NoError(t, os.MkdirAll(filepath.Dir(path), 0o755))
+	require.NoError(t, os.WriteFile(path, []byte(contents), 0o644))
+}
+
+// TestReadHwmonFans verifies the /sys/class/hwmon walker:
+//   - picks up fan*_input from every chip,
+//   - keys entries by chip name + sensor label (or fan idx if no label),
+//   - retains 0 RPM for stopped fans,
+//   - tolerates chips with no fan files at all.
+func TestReadHwmonFans(t *testing.T) {
+	root := t.TempDir()
+
+	// hwmon0: Raspberry Pi 5 active cooler — one fan, no label.
+	writeFile(t, filepath.Join(root, "hwmon0", "name"), "pwmfan\n")
+	writeFile(t, filepath.Join(root, "hwmon0", "fan1_input"), "6500\n")
+
+	// hwmon1: a thermal-only chip, no fan files. Must not error.
+	writeFile(t, filepath.Join(root, "hwmon1", "name"), "cpu_thermal\n")
+	writeFile(t, filepath.Join(root, "hwmon1", "temp1_input"), "55000\n")
+
+	// hwmon2: two fans — one stopped (0 RPM) and one labeled "chassis".
+	writeFile(t, filepath.Join(root, "hwmon2", "name"), "nct6798\n")
+	writeFile(t, filepath.Join(root, "hwmon2", "fan1_input"), "0\n")
+	writeFile(t, filepath.Join(root, "hwmon2", "fan2_input"), "1200\n")
+	writeFile(t, filepath.Join(root, "hwmon2", "fan2_label"), "chassis\n")
+
+	fans, err := readHwmonFans(root)
+	require.NoError(t, err)
+
+	assert.Equal(t, map[string]uint16{
+		"pwmfan_fan1":     6500,
+		"nct6798_fan1":    0,
+		"nct6798_chassis": 1200,
+	}, fans)
+}
+
+// TestReadHwmonFansMissingRoot returns an error rather than panicking when the
+// hwmon root doesn't exist (e.g. running on a kernel without hwmon support).
+func TestReadHwmonFansMissingRoot(t *testing.T) {
+	_, err := readHwmonFans(filepath.Join(t.TempDir(), "does-not-exist"))
+	assert.Error(t, err)
+}
+
+// TestReadHwmonFansEmpty returns an empty map (not nil error) when the root
+// exists but contains no chips at all.
+func TestReadHwmonFansEmpty(t *testing.T) {
+	root := t.TempDir()
+	fans, err := readHwmonFans(root)
+	require.NoError(t, err)
+	assert.Empty(t, fans)
+}
+
+func TestFanDiscoveryCache(t *testing.T) {
+	root := t.TempDir()
+	input := filepath.Join(root, "hwmon0", "fan1_input")
+	writeFile(t, filepath.Join(root, "hwmon0", "name"), "chip\n")
+	writeFile(t, input, "1000\n")
+
+	getSensors := newFanSensorCache(root)
+	sensors, err := getSensors()
+	require.NoError(t, err)
+	fans := readFanSensors(sensors)
+	assert.Equal(t, uint16(1000), fans["chip_fan1"])
+
+	writeFile(t, input, "1200\n")
+	writeFile(t, filepath.Join(root, "hwmon0", "fan1_label"), "case\n")
+	sensors, err = getSensors()
+	require.NoError(t, err)
+	fans = readFanSensors(sensors)
+	assert.Equal(t, map[string]uint16{"chip_fan1": 1200}, fans)
+}

agent/system.go

@@ -210,6 +210,9 @@ func (a *Agent) getSystemStats(cacheTimeMs uint16) system.Stats {
 	// TODO: maybe refactor to methods on systemStats
 	a.updateTemperatures(&systemStats)
 
+	// fan speeds (Linux-only; sysfs hwmon)
+	a.updateFans(&systemStats)
+
 	// GPU data
 	if a.gpuManager != nil {
 		// reset high gpu percent