Skip device-scoped batteries and stop masking missing sysfs attributes

internal/battery/sysfs.go

@@ -8,7 +8,7 @@ import (
 	"strings"
 )
 
-const PowerSupplyBase = "/sys/class/power_supply"
+var PowerSupplyBase = "/sys/class/power_supply"
 
 func SysfsReadString(path string) (string, error) {
 	data, err := os.ReadFile(path)
@@ -35,7 +35,15 @@ func SysfsWriteInt(path string, value int) error {
 }
 
 func SysfsWriteString(path string, value string) error {
-	if err := os.WriteFile(path, []byte(value), 0644); err != nil {
+	f, err := os.OpenFile(path, os.O_WRONLY|os.O_TRUNC, 0)
+	if err != nil {
+		return fmt.Errorf("writing %q to %s: %w", value, path, err)
+	}
+	if _, err := f.WriteString(value); err != nil {
+		f.Close()
+		return fmt.Errorf("writing %q to %s: %w", value, path, err)
+	}
+	if err := f.Close(); err != nil {
 		return fmt.Errorf("writing %q to %s: %w", value, path, err)
 	}
 	return nil
@@ -62,14 +70,25 @@ func ListBatteries() []string {
 	}
 	var bats []string
 	for _, e := range entries {
-		typePath := filepath.Join(PowerSupplyBase, e.Name(), "type")
-		t, err := SysfsReadString(typePath)
+		t, err := SysfsReadString(filepath.Join(PowerSupplyBase, e.Name(), "type"))
 		if err != nil {
 			continue
 		}
-		if strings.EqualFold(t, "battery") {
-			bats = append(bats, e.Name())
+		if !strings.EqualFold(t, "battery") {
+			continue
+		}
+		if IsDeviceScoped(e.Name()) {
+			continue
 		}
+		bats = append(bats, e.Name())
 	}
 	return bats
 }
+
+func IsDeviceScoped(name string) bool {
+	scope, err := SysfsReadString(filepath.Join(PowerSupplyBase, name, "scope"))
+	if err != nil {
+		return false
+	}
+	return strings.EqualFold(scope, "device")
+}

internal/battery/sysfs_test.go

@@ -1,6 +1,8 @@
 package battery
 
 import (
+	"errors"
+	"io/fs"
 	"os"
 	"path/filepath"
 	"testing"
@@ -107,6 +109,7 @@ func TestSysfsReadInt(t *testing.T) {
 func TestSysfsWriteInt(t *testing.T) {
 	dir := t.TempDir()
 	path := filepath.Join(dir, "write_int")
+	os.WriteFile(path, []byte("0"), 0644)
 
 	if err := SysfsWriteInt(path, 80); err != nil {
 		t.Fatalf("unexpected error: %v", err)
@@ -124,6 +127,7 @@ func TestSysfsWriteInt(t *testing.T) {
 func TestSysfsWriteString(t *testing.T) {
 	dir := t.TempDir()
 	path := filepath.Join(dir, "write_str")
+	os.WriteFile(path, []byte("inhibit-charge"), 0644)
 
 	if err := SysfsWriteString(path, "auto"); err != nil {
 		t.Fatalf("unexpected error: %v", err)
@@ -155,3 +159,84 @@ func TestSysfsExists(t *testing.T) {
 		}
 	})
 }
+
+func TestSysfsWriteStringMissingAttribute(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "charge_control_start_threshold")
+
+	err := SysfsWriteString(path, "40")
+	if err == nil {
+		t.Fatal("expected an error writing to a missing attribute")
+	}
+	if !errors.Is(err, fs.ErrNotExist) {
+		t.Fatalf("got %v, want a not-exist error", err)
+	}
+	if errors.Is(err, fs.ErrPermission) {
+		t.Fatalf("missing attribute reported as a permission error: %v", err)
+	}
+	if SysfsExists(path) {
+		t.Fatal("SysfsWriteString must not create the attribute")
+	}
+}
+
+func writeFixture(t *testing.T, dir, name string, attrs map[string]string) {
+	t.Helper()
+	d := filepath.Join(dir, name)
+	if err := os.MkdirAll(d, 0755); err != nil {
+		t.Fatal(err)
+	}
+	for k, v := range attrs {
+		if err := os.WriteFile(filepath.Join(d, k), []byte(v+"\n"), 0644); err != nil {
+			t.Fatal(err)
+		}
+	}
+}
+
+func TestListBatteries(t *testing.T) {
+	dir := t.TempDir()
+	writeFixture(t, dir, "AC", map[string]string{"type": "Mains"})
+	writeFixture(t, dir, "BAT0", map[string]string{"type": "Battery"})
+	writeFixture(t, dir, "BAT1", map[string]string{"type": "Battery", "scope": "System"})
+	writeFixture(t, dir, "hidpp_battery_0", map[string]string{"type": "Battery", "scope": "Device"})
+	writeFixture(t, dir, "ucsi-source-psy-USBC000:001", map[string]string{"type": "USB", "scope": "Device"})
+
+	orig := PowerSupplyBase
+	PowerSupplyBase = dir
+	defer func() { PowerSupplyBase = orig }()
+
+	got := ListBatteries()
+	want := []string{"BAT0", "BAT1"}
+	if len(got) != len(want) {
+		t.Fatalf("ListBatteries() = %v, want %v", got, want)
+	}
+	for i := range want {
+		if got[i] != want[i] {
+			t.Fatalf("ListBatteries() = %v, want %v", got, want)
+		}
+	}
+}
+
+func TestIsDeviceScoped(t *testing.T) {
+	dir := t.TempDir()
+	writeFixture(t, dir, "BAT0", map[string]string{"type": "Battery"})
+	writeFixture(t, dir, "hidpp_battery_0", map[string]string{"type": "Battery", "scope": "Device"})
+	writeFixture(t, dir, "BAT1", map[string]string{"type": "Battery", "scope": "System"})
+
+	orig := PowerSupplyBase
+	PowerSupplyBase = dir
+	defer func() { PowerSupplyBase = orig }()
+
+	tests := []struct {
+		name string
+		want bool
+	}{
+		{"BAT0", false},
+		{"BAT1", false},
+		{"hidpp_battery_0", true},
+		{"nonexistent", false},
+	}
+	for _, tt := range tests {
+		if got := IsDeviceScoped(tt.name); got != tt.want {
+			t.Errorf("IsDeviceScoped(%q) = %v, want %v", tt.name, got, tt.want)
+		}
+	}
+}