devicescale: Read ~/.config/cinnamon-monitors.xml on Cinnamon

Updates #1307

internal/devicescale/impl_unix.go

@@ -19,8 +19,10 @@
 package devicescale
 
 import (
+	"encoding/xml"
 	"os"
 	"os/exec"
+	"path/filepath"
 	"regexp"
 	"strconv"
 	"strings"
@@ -58,6 +60,8 @@ func currentDesktop() desktop {
 var gsettingsRe = regexp.MustCompile(`\Auint32 (\d+)\s*\z`)
 
 func gnomeScale() float64 {
+	// TODO: Should 'monitors.xml' be loaded?
+
 	out, err := exec.Command("gsettings", "get", "org.gnome.desktop.interface", "scaling-factor").Output()
 	if err != nil {
 		if err == exec.ErrNotFound {
@@ -76,7 +80,65 @@ func gnomeScale() float64 {
 	return float64(s)
 }
 
+type xmlBool bool
+
+func (b *xmlBool) UnmarshalXML(d *xml.Decoder, start xml.StartElement) error {
+	var s string
+	if err := d.DecodeElement(&s, &start); err != nil {
+		return err
+	}
+	*b = xmlBool(s == "yes")
+	return nil
+}
+
+func cinnamonScaleFromXML() (float64, error) {
+	type cinnamonMonitors struct {
+		XMLName       xml.Name `xml:"monitors"`
+		Version       string   `xml:"version,attr"`
+		Configuration struct {
+			BaseScale float64 `xml:"base_scale"`
+			Output    []struct {
+				Scale   float64 `xml:"scale"`
+				Primary xmlBool `xml:"primary"`
+			} `xml:"output"`
+		} `xml:"configuration"`
+	}
+
+	home, err := os.UserHomeDir()
+	if err != nil {
+		return 0, err
+	}
+	f, err := os.Open(filepath.Join(home, ".config", "cinnamon-monitors.xml"))
+	if err != nil {
+		return 0, err
+	}
+	defer f.Close()
+
+	d := xml.NewDecoder(f)
+
+	var monitors cinnamonMonitors
+	if err = d.Decode(&monitors); err != nil {
+		return 0, err
+	}
+
+	scale := monitors.Configuration.BaseScale
+	for _, v := range monitors.Configuration.Output {
+		// TODO: Get the monitor at the specified position.
+		if v.Primary {
+			if v.Scale != 0.0 {
+				scale = v.Scale
+			}
+			break
+		}
+	}
+	return scale, nil
+}
+
 func cinnamonScale() float64 {
+	if s, err := cinnamonScaleFromXML(); err == nil && s > 0 {
+		return s
+	}
+
 	out, err := exec.Command("gsettings", "get", "org.cinnamon.desktop.interface", "scaling-factor").Output()
 	if err != nil {
 		if err == exec.ErrNotFound {