enha: end session when user kills the App.

sentry-android-core/src/main/AndroidManifest.xml

@@ -8,5 +8,8 @@
       android:name=".SentryInitProvider"
       android:authorities="${applicationId}.SentryInitProvider"
       android:exported="false"/>
+
+    <service android:name=".SentryService"
+      android:exported="false" />
   </application>
 </manifest>

sentry-android-core/src/main/java/io/sentry/android/core/AndroidOptionsInitializer.java

@@ -76,6 +76,14 @@ static void init(
     options.setSerializer(new AndroidSerializer(options.getLogger(), envelopeReader));
 
     options.setTransportGate(new AndroidTransportGate(context, options.getLogger()));
+
+    try {
+      context.startService(SentryService.getIntent(context));
+    } catch (IllegalStateException e) {
+      // maybe we should make Throwable instead of to be safe than sorry
+      // services have different behaviours across OS versioms eg foreground etc
+      options.getLogger().log(SentryLevel.ERROR, e, "SentryService can't be started.");
+    }
   }
 
   private static void installDefaultIntegrations(

sentry-android-core/src/main/java/io/sentry/android/core/SentryService.java

@@ -0,0 +1,78 @@
+package io.sentry.android.core;
+
+import android.app.Service;
+import android.content.Context;
+import android.content.Intent;
+import android.os.IBinder;
+import androidx.annotation.NonNull;
+import io.sentry.core.HubAdapter;
+import io.sentry.core.hints.DiskFlushNotification;
+import io.sentry.core.hints.Flushable;
+import io.sentry.core.hints.SessionEnd;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.TimeUnit;
+import org.jetbrains.annotations.ApiStatus;
+import org.jetbrains.annotations.Nullable;
+
+/**
+ * This is a best effort to end a session during onTaskRemoved (user swiped up the App. aka killed
+ * it. It needs to be public.
+ */
+@ApiStatus.Internal
+public final class SentryService extends Service {
+
+  @Override
+  public @Nullable IBinder onBind(Intent intent) {
+    return null;
+  }
+
+  @Override
+  public int onStartCommand(Intent intent, int flags, int startId) {
+    super.onStartCommand(intent, flags, startId);
+    // START_NOT_STICKY, we don't want to recreate the service nor the AppContext.
+    return START_NOT_STICKY;
+  }
+
+  @Override
+  public void onTaskRemoved(Intent rootIntent) {
+    // TODO: get logger and flush timeout from options thru onStartCommand.intent
+    final TaskRemovedHint hint = new TaskRemovedHint(15000);
+    HubAdapter.getInstance().endSession(hint);
+
+    hint.waitFlush();
+
+    super.onTaskRemoved(rootIntent);
+  }
+
+  @NonNull
+  public static Intent getIntent(Context context) {
+    return new Intent(context, SentryService.class);
+  }
+
+  private static final class TaskRemovedHint
+      implements DiskFlushNotification, Flushable, SessionEnd {
+
+    private final CountDownLatch latch;
+    private final long flushTimeoutMillis;
+
+    TaskRemovedHint(final long flushTimeoutMillis) {
+      this.flushTimeoutMillis = flushTimeoutMillis;
+      latch = new CountDownLatch(1);
+    }
+
+    @Override
+    public boolean waitFlush() {
+      try {
+        return latch.await(flushTimeoutMillis, TimeUnit.MILLISECONDS);
+      } catch (InterruptedException e) {
+        Thread.currentThread().interrupt();
+      }
+      return false;
+    }
+
+    @Override
+    public void markFlushed() {
+      latch.countDown();
+    }
+  }
+}

sentry-core/src/main/java/io/sentry/core/Hub.java

@@ -218,7 +218,7 @@ public void startSession() {
   }
 
   @Override
-  public void endSession() {
+  public void endSession(final @Nullable Object hint) {
     if (!isEnabled()) {
       options
           .getLogger()
@@ -228,7 +228,7 @@ public void endSession() {
       if (item != null) {
         final Session previousSession = item.scope.endSession();
         if (previousSession != null) {
-          item.client.captureSession(previousSession, new SessionEndHint());
+          item.client.captureSession(previousSession, hint);
         }
       } else {
         options.getLogger().log(SentryLevel.FATAL, "Stack peek was null when endSession");

sentry-core/src/main/java/io/sentry/core/HubAdapter.java

@@ -48,8 +48,8 @@ public void startSession() {
   }
 
   @Override
-  public void endSession() {
-    Sentry.endSession();
+  public void endSession(@Nullable Object hint) {
+    Sentry.getCurrentHub().endSession(hint);
   }
 
   @Override

sentry-core/src/main/java/io/sentry/core/IHub.java

@@ -1,5 +1,6 @@
 package io.sentry.core;
 
+import io.sentry.core.hints.SessionEndHint;
 import io.sentry.core.protocol.SentryId;
 import io.sentry.core.protocol.User;
 import java.util.List;
@@ -96,7 +97,11 @@ default SentryId captureException(Throwable throwable) {
   void startSession();
 
   /** Ends the current session */
-  void endSession();
+  default void endSession() {
+    endSession(new SessionEndHint());
+  }
+
+  void endSession(@Nullable Object hint);
 
   /** Flushes out the queue for up to timeout seconds and disable the Hub. */
   void close();

sentry-core/src/main/java/io/sentry/core/NoOpHub.java

@@ -44,7 +44,7 @@ public SentryId captureException(Throwable throwable, @Nullable Object hint) {
   public void startSession() {}
 
   @Override
-  public void endSession() {}
+  public void endSession(@Nullable Object hint) {}
 
   @Override
   public void close() {}

sentry-core/src/main/java/io/sentry/core/transport/AsyncConnection.java

@@ -370,6 +370,11 @@ public void run() {
 
       sessionCache.store(envelope, hint);
 
+      if (hint instanceof DiskFlushNotification) {
+        ((DiskFlushNotification) hint).markFlushed();
+        options.getLogger().log(SentryLevel.DEBUG, "Disk flush envelope fired.");
+      }
+
       // we only flush a session update to the disk, but not to the network
       if (hint instanceof SessionUpdate) {
         options