Throttle loading subscriptions feed to avoid YouTube rate limits

app/src/main/java/org/schabi/newpipe/local/feed/service/FeedLoadManager.kt

@@ -90,6 +90,10 @@ class FeedLoadManager(private val context: Context) {
             else -> feedDatabaseManager.outdatedSubscriptionsForGroup(groupId, outdatedThreshold)
         }
 
+        // like `currentProgress`, but counts the number of extractions that have begun, so they
+        // can be properly throttled every once in a while (see doOnNext below)
+        val extractionCount = AtomicInteger()
+
         return outdatedSubscriptions
             .take(1)
             .doOnNext {
@@ -105,6 +109,13 @@ class FeedLoadManager(private val context: Context) {
             .observeOn(Schedulers.io())
             .flatMap { Flowable.fromIterable(it) }
             .takeWhile { !cancelSignal.get() }
+            .doOnNext {
+                // throttle extractions once every BATCH_SIZE to avoid being throttled
+                val previousCount = extractionCount.getAndIncrement()
+                if (previousCount != 0 && previousCount % BATCH_SIZE == 0) {
+                    Thread.sleep(DELAY_BETWEEN_BATCHES_MILLIS.random())
+                }
+            }
             .parallel(PARALLEL_EXTRACTIONS, PARALLEL_EXTRACTIONS * 2)
             .runOn(Schedulers.io(), PARALLEL_EXTRACTIONS * 2)
             .filter { !cancelSignal.get() }
@@ -328,7 +339,19 @@ class FeedLoadManager(private val context: Context) {
         /**
          * How many extractions will be running in parallel.
          */
-        private const val PARALLEL_EXTRACTIONS = 6
+        private const val PARALLEL_EXTRACTIONS = 3
+
+        /**
+         * How many extractions to perform before waiting [DELAY_BETWEEN_BATCHES_MILLIS] to avoid
+         * being rate limited
+         */
+        private const val BATCH_SIZE = 50
+
+        /**
+         * Wait a random delay in this range once every [BATCH_SIZE] extractions to avoid being
+         * rate limited
+         */
+        private val DELAY_BETWEEN_BATCHES_MILLIS = (6000L..12000L)
 
         /**
          * Number of items to buffer to mass-insert in the database.