android: Use the default scheduler for short-term events

Using AlarmManager has quite some overhead, so we use our regular
scheduler for events that are to be executed in the near future.

src/frontends/android/app/src/main/jni/libandroidbridge/backend/android_scheduler.c

@@ -21,6 +21,12 @@
 #include <processing/jobs/callback_job.h>
 #include <threading/mutex.h>
 
+/**
+ * Threshold in milliseconds up to which the default scheduler is used.
+ * This includes the roaming events (100 ms) and initial retransmits.
+ */
+#define DEFAULT_SCHEDULER_THRESHOLD 3000
+
 typedef struct private_scheduler_t private_scheduler_t;
 
 /**
@@ -52,6 +58,11 @@ struct private_scheduler_t {
 	 * Mutex to safely access the scheduled jobs.
 	 */
 	mutex_t *mutex;
+
+	/**
+	 * Default scheduler used for short-term events.
+	 */
+	scheduler_t *default_scheduler;
 };
 
 /**
@@ -137,6 +148,14 @@ METHOD(scheduler_t, schedule_job_ms, void,
 	entry_t *entry = NULL;
 	jstring jid;
 
+	/* use the default scheduler for short-term events */
+	if (ms <= DEFAULT_SCHEDULER_THRESHOLD)
+	{
+		this->default_scheduler->schedule_job_ms(this->default_scheduler,
+												 job, ms);
+		return;
+	}
+
 	androidjni_attach_thread(&env);
 	jid = allocate_id(this, env);
 	if (!jid)
@@ -213,6 +232,8 @@ METHOD(scheduler_t, flush, void,
 	JNIEnv *env;
 	jmethodID method_id;
 
+	this->default_scheduler->flush(this->default_scheduler);
+
 	this->mutex->lock(this->mutex);
 	this->jobs->destroy_function(this->jobs, destroy_entry);
 	this->jobs = hashtable_create(hashtable_hash_str, hashtable_equals_str, 16);
@@ -247,6 +268,7 @@ METHOD(scheduler_t, destroy, void,
 		(*env)->DeleteGlobalRef(env, this->cls);
 	}
 	androidjni_detach_thread();
+	this->default_scheduler->destroy(this->default_scheduler);
 	this->mutex->destroy(this->mutex);
 	this->jobs->destroy(this->jobs);
 	free(this);
@@ -255,7 +277,7 @@ METHOD(scheduler_t, destroy, void,
 /*
  * Described in header
  */
-scheduler_t *android_scheduler_create(jobject context)
+scheduler_t *android_scheduler_create(jobject context, scheduler_t *scheduler)
 {
 	private_scheduler_t *this;
 	JNIEnv *env;
@@ -272,6 +294,7 @@ scheduler_t *android_scheduler_create(jobject context)
 			.flush = _flush,
 			.destroy = _destroy,
 		},
+		.default_scheduler = scheduler,
 		.jobs = hashtable_create(hashtable_hash_str, hashtable_equals_str, 16),
 		.mutex = mutex_create(MUTEX_TYPE_DEFAULT),
 	);

src/frontends/android/app/src/main/jni/libandroidbridge/backend/android_scheduler.h

@@ -28,9 +28,14 @@
 /**
  * Create an Android-specific scheduler_t implementation.
  *
+ * The given scheduler is used for short-term events. We can't destroy it anyway
+ * because of the scheduler job operating on it, and this way we can use it to
+ * avoid the overhead of broadcasts for some events.
+ *
  * @param context	Context object
+ * @param scheduler	the default scheduler used as fallback
  * @return			scheduler_t instance
  */
-scheduler_t *android_scheduler_create(jobject context);
+scheduler_t *android_scheduler_create(jobject context, scheduler_t *scheduler);
 
 #endif /** ANDROID_SCHEDULER_H_ @}*/

src/frontends/android/app/src/main/jni/libandroidbridge/charonservice.c

@@ -94,11 +94,6 @@ struct private_charonservice_t {
 	 * Sockets that were bypassed and we keep track for
 	 */
 	linked_list_t *sockets;
-
-	/**
-	 * Default scheduler if we don't use it
-	 */
-	scheduler_t *default_scheduler;
 };
 
 /**
@@ -580,15 +575,6 @@ static void charonservice_init(JNIEnv *env, jobject service, jobject builder,
 	);
 	charonservice = &this->public;
 
-	if (android_sdk_version >= ANDROID_MARSHMALLOW)
-	{
-		/* use a custom scheduler so the app is woken when jobs have to run.
-		 * we can't destroy the default scheduler here due to the scheduler
-		 * job that's operating on it, so we stash it away until later */
-		this->default_scheduler = lib->scheduler;
-		lib->scheduler = android_scheduler_create(service);
-	}
-
 	lib->plugins->add_static_features(lib->plugins, "androidbridge", features,
 									  countof(features), TRUE, NULL, NULL);
 
@@ -615,7 +601,6 @@ static void charonservice_deinit(JNIEnv *env)
 {
 	private_charonservice_t *this = (private_charonservice_t*)charonservice;
 
-	DESTROY_IF(this->default_scheduler);
 	this->network_manager->destroy(this->network_manager);
 	this->sockets->destroy(this->sockets);
 	this->builder->destroy(this->builder);
@@ -661,6 +646,12 @@ JNI_METHOD(CharonVpnService, initializeCharon, jboolean,
 		return FALSE;
 	}
 
+	if (android_sdk_version >= ANDROID_MARSHMALLOW)
+	{
+		/* use a custom scheduler so the app is woken when jobs have to run */
+		lib->scheduler = android_scheduler_create(this, lib->scheduler);
+	}
+
 	/* set options before initializing other libraries that might read them */
 	logfile = androidjni_convert_jstring(env, jlogfile);