ui: replace GlobalScope with a hand-rolled CoroutineScope

GlobalScope has numerous problems[1] that make it unfit for
use in most applications and making it behave correctly requires
an excessive amount of verbosity that's alleviated simply by using
any other scope. Since we run multiple operations in the context
of the application's lifecycle, introduce a new scope that is created
when our application is, and cancelled upon its termination.

While at it, make the scope default to Dispatchers.IO to reduce pressure
on the UI event loop. Tasks requiring access to the UI thread appropriately
switch context making the change completely safe.

1: https://medium.com/@elizarov/the-reason-to-avoid-globalscope-835337445abc

Signed-off-by: Harsh Shandilya <me@msfjarvis.dev>

6 files changed, 25 insertions, 16 deletions

diff --git a/ui/src/main/java/com/wireguard/android/Application.kt b/ui/src/main/java/com/wireguard/android/Application.kt
index f63e6270..537d5416 100644
--- a/ui/src/main/java/com/wireguard/android/Application.kt
+++ b/ui/src/main/java/com/wireguard/android/Application.kt
@@ -24,14 +24,17 @@ import com.wireguard.android.util.ModuleLoader
 import com.wireguard.android.util.RootShell
 import com.wireguard.android.util.ToolsInstaller
 import kotlinx.coroutines.CompletableDeferred
+import kotlinx.coroutines.CoroutineScope
 import kotlinx.coroutines.Dispatchers
-import kotlinx.coroutines.GlobalScope
+import kotlinx.coroutines.Job
+import kotlinx.coroutines.cancel
 import kotlinx.coroutines.launch
 import java.lang.ref.WeakReference
 import java.util.Locale
 
 class Application : android.app.Application(), OnSharedPreferenceChangeListener {
     private val futureBackend = CompletableDeferred<Backend>()
+    private val coroutineScope = CoroutineScope(Job() + Dispatchers.Main.immediate)
     private var backend: Backend? = null
     private lateinit var moduleLoader: ModuleLoader
     private lateinit var rootShell: RootShell
@@ -98,7 +101,7 @@ class Application : android.app.Application(), OnSharedPreferenceChangeListener
         }
         tunnelManager = TunnelManager(FileConfigStore(applicationContext))
         tunnelManager.onCreate()
-        GlobalScope.launch(Dispatchers.IO) {
+        coroutineScope.launch(Dispatchers.IO) {
             try {
                 backend = determineBackend()
                 futureBackend.complete(backend!!)
@@ -116,6 +119,7 @@ class Application : android.app.Application(), OnSharedPreferenceChangeListener
 
     override fun onTerminate() {
         sharedPreferences.unregisterOnSharedPreferenceChangeListener(this)
+        coroutineScope.cancel()
         super.onTerminate()
     }
 
@@ -146,6 +150,9 @@ class Application : android.app.Application(), OnSharedPreferenceChangeListener
 
         @JvmStatic
         fun getTunnelManager() = get().tunnelManager
+
+        @JvmStatic
+        fun getCoroutineScope() = get().coroutineScope
     }
 
     init {
diff --git a/ui/src/main/java/com/wireguard/android/BootShutdownReceiver.kt b/ui/src/main/java/com/wireguard/android/BootShutdownReceiver.kt
index 70899a0c..203350fe 100644
--- a/ui/src/main/java/com/wireguard/android/BootShutdownReceiver.kt
+++ b/ui/src/main/java/com/wireguard/android/BootShutdownReceiver.kt
@@ -9,13 +9,12 @@ import android.content.Context
 import android.content.Intent
 import android.util.Log
 import com.wireguard.android.backend.WgQuickBackend
-import kotlinx.coroutines.Dispatchers
-import kotlinx.coroutines.GlobalScope
+import com.wireguard.android.util.applicationScope
 import kotlinx.coroutines.launch
 
 class BootShutdownReceiver : BroadcastReceiver() {
     override fun onReceive(context: Context, intent: Intent) {
-        GlobalScope.launch(Dispatchers.Main.immediate) {
+        applicationScope.launch {
             if (Application.getBackend() !is WgQuickBackend) return@launch
             val action = intent.action ?: return@launch
             val tunnelManager = Application.getTunnelManager()
diff --git a/ui/src/main/java/com/wireguard/android/QuickTileService.kt b/ui/src/main/java/com/wireguard/android/QuickTileService.kt
index 5989499f..cb740d18 100644
--- a/ui/src/main/java/com/wireguard/android/QuickTileService.kt
+++ b/ui/src/main/java/com/wireguard/android/QuickTileService.kt
@@ -20,9 +20,8 @@ import com.wireguard.android.activity.MainActivity
 import com.wireguard.android.activity.TunnelToggleActivity
 import com.wireguard.android.backend.Tunnel
 import com.wireguard.android.model.ObservableTunnel
+import com.wireguard.android.util.applicationScope
 import com.wireguard.android.widget.SlashDrawable
-import kotlinx.coroutines.Dispatchers
-import kotlinx.coroutines.GlobalScope
 import kotlinx.coroutines.launch
 
 /**
@@ -57,7 +56,7 @@ class QuickTileService : TileService() {
                     tile.icon = if (tile.icon == iconOn) iconOff else iconOn
                     tile.updateTile()
                 }
-                GlobalScope.launch(Dispatchers.Main.immediate) {
+                applicationScope.launch {
                     try {
                         tunnel!!.setStateAsync(Tunnel.State.TOGGLE)
                         updateTile()
diff --git a/ui/src/main/java/com/wireguard/android/model/ObservableTunnel.kt b/ui/src/main/java/com/wireguard/android/model/ObservableTunnel.kt
index 89ad9a67..8bf8bf76 100644
--- a/ui/src/main/java/com/wireguard/android/model/ObservableTunnel.kt
+++ b/ui/src/main/java/com/wireguard/android/model/ObservableTunnel.kt
@@ -11,9 +11,9 @@ import com.wireguard.android.BR
 import com.wireguard.android.backend.Statistics
 import com.wireguard.android.backend.Tunnel
 import com.wireguard.android.databinding.Keyed
+import com.wireguard.android.util.applicationScope
 import com.wireguard.config.Config
 import kotlinx.coroutines.Dispatchers
-import kotlinx.coroutines.GlobalScope
 import kotlinx.coroutines.launch
 import kotlinx.coroutines.withContext
 
@@ -74,7 +74,7 @@ class ObservableTunnel internal constructor(
         get() {
             if (field == null)
             // Opportunistically fetch this if we don't have a cached one, and rely on data bindings to update it eventually
-                GlobalScope.launch(Dispatchers.Main.immediate) {
+                applicationScope.launch {
                     try {
                         manager.getTunnelConfig(this@ObservableTunnel)
                     } catch (e: Throwable) {
@@ -110,7 +110,7 @@ class ObservableTunnel internal constructor(
         get() {
             if (field == null || field?.isStale != false)
             // Opportunistically fetch this if we don't have a cached one, and rely on data bindings to update it eventually
-                GlobalScope.launch(Dispatchers.Main.immediate) {
+                applicationScope.launch {
                     try {
                         manager.getTunnelStatistics(this@ObservableTunnel)
                     } catch (e: Throwable) {
diff --git a/ui/src/main/java/com/wireguard/android/model/TunnelManager.kt b/ui/src/main/java/com/wireguard/android/model/TunnelManager.kt
index 1f9bc6c0..f4a0f1b4 100644
--- a/ui/src/main/java/com/wireguard/android/model/TunnelManager.kt
+++ b/ui/src/main/java/com/wireguard/android/model/TunnelManager.kt
@@ -22,10 +22,10 @@ import com.wireguard.android.backend.Statistics
 import com.wireguard.android.backend.Tunnel
 import com.wireguard.android.configStore.ConfigStore
 import com.wireguard.android.databinding.ObservableSortedKeyedArrayList
+import com.wireguard.android.util.applicationScope
 import com.wireguard.config.Config
 import kotlinx.coroutines.CompletableDeferred
 import kotlinx.coroutines.Dispatchers
-import kotlinx.coroutines.GlobalScope
 import kotlinx.coroutines.SupervisorJob
 import kotlinx.coroutines.async
 import kotlinx.coroutines.awaitAll
@@ -101,7 +101,7 @@ class TunnelManager(private val configStore: ConfigStore) : BaseObservable() {
     }
 
     fun onCreate() {
-        GlobalScope.launch(Dispatchers.Main.immediate) {
+        applicationScope.launch {
             try {
                 onTunnelsLoaded(withContext(Dispatchers.IO) { configStore.enumerate() }, withContext(Dispatchers.IO) { getBackend().runningTunnelNames })
             } catch (e: Throwable) {
@@ -122,7 +122,7 @@ class TunnelManager(private val configStore: ConfigStore) : BaseObservable() {
     }
 
     private fun refreshTunnelStates() {
-        GlobalScope.launch(Dispatchers.Main.immediate) {
+        applicationScope.launch {
             try {
                 val running = withContext(Dispatchers.IO) { getBackend().runningTunnelNames }
                 for (tunnel in tunnelMap)
@@ -139,7 +139,7 @@ class TunnelManager(private val configStore: ConfigStore) : BaseObservable() {
         val previouslyRunning = getSharedPreferences().getStringSet(KEY_RUNNING_TUNNELS, null)
                 ?: return
         if (previouslyRunning.isEmpty()) return
-        GlobalScope.launch(Dispatchers.Main.immediate) {
+        applicationScope.launch {
             withContext(Dispatchers.IO) {
                 try {
                     tunnelMap.filter { previouslyRunning.contains(it.name) }.map { async(SupervisorJob()) { setTunnelState(it, Tunnel.State.UP) } }.awaitAll()
@@ -232,7 +232,7 @@ class TunnelManager(private val configStore: ConfigStore) : BaseObservable() {
                 else -> return
             }
             val tunnelName = intent.getStringExtra("tunnel") ?: return
-            GlobalScope.launch(Dispatchers.Main.immediate) {
+            applicationScope.launch {
                 val tunnels = manager.getTunnels()
                 val tunnel = tunnels[tunnelName] ?: return@launch
                 manager.setTunnelState(tunnel, state)
diff --git a/ui/src/main/java/com/wireguard/android/util/Extensions.kt b/ui/src/main/java/com/wireguard/android/util/Extensions.kt
index c8892295..22ef8bd7 100644
--- a/ui/src/main/java/com/wireguard/android/util/Extensions.kt
+++ b/ui/src/main/java/com/wireguard/android/util/Extensions.kt
@@ -11,6 +11,7 @@ import androidx.annotation.AttrRes
 import androidx.fragment.app.Fragment
 import androidx.lifecycle.lifecycleScope
 import androidx.preference.Preference
+import com.wireguard.android.Application
 import com.wireguard.android.activity.SettingsActivity
 import kotlinx.coroutines.CoroutineScope
 
@@ -24,6 +25,9 @@ fun Fragment.requireTargetFragment(): Fragment {
     return requireNotNull(targetFragment) { "A target fragment should always be set for $this" }
 }
 
+val Any.applicationScope: CoroutineScope
+    get() = Application.getCoroutineScope()
+
 val Preference.activity: SettingsActivity
     get() = context as? SettingsActivity ?: throw IllegalStateException("Failed to resolve SettingsActivity")