NookGL4plus: use persistent root shell for warmth to avoid per-call su overhead

Spawning a new `su` process per setWarmth call and blocking on waitFor()
could cause sluggish slider response when the user makes rapid changes.
Keep a single long-lived root shell open and pipe echo commands through it.
The shell is (re)started on demand if it has exited.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: backcountrymountains

app/src/main/java/org/koreader/launcher/device/lights/NookGL4plusController.kt

@@ -4,10 +4,11 @@ import android.app.Activity
 import android.provider.Settings
 import android.util.Log
 import org.koreader.launcher.device.LightsInterface
+import java.io.PrintWriter
 
 /* Controller for Nook Glowlight 4 Plus (bnrv1300) on Android 8.1.
  * Brightness via Settings.System (SELinux blocks direct sysfs writes from the app process).
- * Warmth via 'su -c echo' to sysfs color file (SELinux blocks direct ioctl from the app process).
+ * Warmth via a persistent root shell to avoid per-call su process overhead during slider use.
  * see https://github.com/koreader/koreader/issues/14574
  */
 class NookGL4plusController : LightsInterface {
@@ -21,6 +22,40 @@ class NookGL4plusController : LightsInterface {
     }
 
     private var currentWarmth: Int? = null
+    private var suProcess: Process? = null
+    private var suWriter: PrintWriter? = null
+
+    @Synchronized
+    private fun runRoot(cmd: String): Boolean {
+        if (!isShellAlive()) {
+            try {
+                suProcess = Runtime.getRuntime().exec("su")
+                suWriter = PrintWriter(suProcess!!.outputStream, true)
+            } catch (e: Exception) {
+                Log.w(TAG, "failed to start root shell: $e")
+                return false
+            }
+        }
+        return try {
+            suWriter!!.println(cmd)
+            true
+        } catch (e: Exception) {
+            Log.w(TAG, "runRoot write failed: $e")
+            suProcess = null
+            suWriter = null
+            false
+        }
+    }
+
+    private fun isShellAlive(): Boolean {
+        val p = suProcess ?: return false
+        return try {
+            p.exitValue()
+            false
+        } catch (e: IllegalThreadStateException) {
+            true
+        }
+    }
 
     override fun getPlatform(): String = "nook"
     override fun hasFallback(): Boolean = false
@@ -64,16 +99,8 @@ class NookGL4plusController : LightsInterface {
             return
         }
         Log.v(TAG, "Setting warmth to $warmth of $WARMTH_MAX")
-        try {
-            val process = Runtime.getRuntime().exec(arrayOf("su", "-c", "echo $warmth > $COLOR_FILE"))
-            val exitCode = process.waitFor()
-            if (exitCode == 0) {
-                currentWarmth = warmth
-            } else {
-                Log.w(TAG, "su failed (exit $exitCode)")
-            }
-        } catch (e: Exception) {
-            Log.w(TAG, "setWarmth: $e")
+        if (runRoot("echo $warmth > $COLOR_FILE")) {
+            currentWarmth = warmth
         }
     }