Merge pull request #538 from jordiolivares/bugfix/fix-deadlock

Fix problem with fibers being cancelled due to timeout and causing fetch deadlock

Author: juanpedromoreno

fetch/src/main/scala/datasource.scala

@@ -19,7 +19,7 @@ package fetch
 import cats.data.NonEmptyList
 import cats.effect._
 import cats.effect.implicits._
-import cats.effect.std.Queue
+import cats.effect.std.{Queue, Supervisor}
 import cats.kernel.{Hash => H}
 import cats.syntax.all._
 
@@ -75,8 +75,10 @@ object DataSource {
       implicit F: Temporal[F]
   ): F[List[T]] = {
     Ref[F].of(List.empty[T]).flatMap { ref =>
-      val takeAndBuffer = queue.take.flatMap { x =>
-        ref.updateAndGet(list => x :: list)
+      val takeAndBuffer = F.uncancelable { poll =>
+        poll(queue.take).flatMap { x =>
+          ref.updateAndGet(list => x :: list)
+        }
       }
       val bufferUntilNumElements = takeAndBuffer.iterateUntil { buffer =>
         buffer.size == maxElements
@@ -112,14 +114,16 @@ object DataSource {
   ): Resource[F, DataSource[F, I, A]] = {
     type Callback = Either[Throwable, Option[A]] => Unit
     for {
-      queue <- Resource.eval(Queue.unbounded[F, (I, Callback)])
+      queue      <- Resource.eval(Queue.unbounded[F, (I, Callback)])
+      supervisor <- Supervisor[F]
       workerFiber = upToWithin(
         queue,
         dataSource.maxBatchSize.getOrElse(Int.MaxValue),
         delayPerBatch
-      ).flatMap {
-        case Nil => F.start(F.unit)
-        case x =>
+      ).flatMap { x =>
+        if (x.isEmpty) {
+          supervisor.supervise(F.unit)
+        } else {
           val asMap        = x.groupBy(_._1).mapValues(callbacks => callbacks.map(_._2))
           val batchResults = dataSource.batch(NonEmptyList.fromListUnsafe(asMap.keys.toList))
           val resultsHaveBeenSent = batchResults.map { results =>
@@ -132,7 +136,8 @@ object DataSource {
               callbacks.foreach(cb => cb(Left(ex)))
             }
           }
-          F.start(fiberWork)
+          supervisor.supervise(fiberWork)
+        }
       }.foreverM[Unit]
       _ <- F.background(workerFiber)
     } yield {