runtime: request(n) in stream ingest fix

 - change `StreamIngest` to request one message at
   time as `request(n)` does not always deliver what
   you ask for on some servers and can cause message
   flow to halt. Therefore, we switch back to a more
   predictable approach where we request more
   messages when queue size is less than `prefetchN`.

 - adjust tests for new fetch strategy.

 - Note: We might optimize internals at a later point
   if this approach is too slow, but now correctness
   trumps speed.

runtime/src/main/scala/client/Fs2ClientCall.scala

@@ -106,8 +106,8 @@ class Fs2ClientCall[F[_], Request, Response] private[client] (
 
   private def mkStreamListenerR(md: Metadata): Resource[F, Fs2StreamClientCallListener[F, Response]] = {
 
-    val acquire = start(Fs2StreamClientCallListener.create[F, Response](request, dispatcher, options.prefetchN), md) <*
-      request(options.prefetchN)
+    val acquire =
+      start(Fs2StreamClientCallListener.create[F, Response](request, dispatcher, options.prefetchN), md) <* request(1)
 
     val release = handleExitCase(cancelSucceed = true)
 

runtime/src/main/scala/client/StreamIngest.scala

@@ -24,7 +24,7 @@ package grpc
 package client
 
 import cats.implicits._
-import cats.effect.kernel.{Concurrent, Ref}
+import cats.effect.Concurrent
 import cats.effect.std.Queue
 
 private[client] trait StreamIngest[F[_], T] {
@@ -39,39 +39,33 @@ private[client] object StreamIngest {
       request: Int => F[Unit],
       prefetchN: Int
   ): F[StreamIngest[F, T]] =
-    (Concurrent[F].ref(prefetchN), Queue.unbounded[F, Either[GrpcStatus, T]])
-      .mapN((d, q) => create[F, T](request, prefetchN, d, q))
+    Queue
+      .unbounded[F, Either[GrpcStatus, T]]
+      .map(q => create[F, T](request, prefetchN, q))
 
   def create[F[_], T](
       request: Int => F[Unit],
       prefetchN: Int,
-      demand: Ref[F, Int],
       queue: Queue[F, Either[GrpcStatus, T]]
   )(implicit F: Concurrent[F]): StreamIngest[F, T] = new StreamIngest[F, T] {
 
+    val limit: Int =
+      math.max(1, prefetchN)
+
+    val ensureMessages: F[Unit] =
+      queue.size.flatMap(qs => request(1).whenA(qs < limit))
+
     def onMessage(msg: T): F[Unit] =
-      decreaseDemandBy(1) *> queue.offer(msg.asRight)
+      queue.offer(msg.asRight) *> ensureMessages
 
     def onClose(status: GrpcStatus): F[Unit] =
       queue.offer(status.asLeft)
 
-    def ensureMessages(nextWhenEmpty: Int): F[Unit] =
-      (demand.get, queue.size).mapN((cd, qs) => fetch(nextWhenEmpty).whenA((cd + qs) < 1)).flatten
-
-    def decreaseDemandBy(n: Int): F[Unit] =
-      demand.update(d => math.max(d - n, 0))
-
-    def increaseDemandBy(n: Int): F[Unit] =
-      demand.update(_ + n)
-
-    def fetch(n: Int): F[Unit] =
-      request(n) *> increaseDemandBy(n)
-
     val messages: Stream[F, T] = {
 
       val run: F[Option[T]] =
         queue.take.flatMap {
-          case Right(v) => v.some.pure[F] <* ensureMessages(prefetchN)
+          case Right(v) => ensureMessages *> v.some.pure[F]
           case Left(GrpcStatus(status, trailers)) =>
             if (!status.isOk) F.raiseError(status.asRuntimeException(trailers))
             else none[T].pure[F]

runtime/src/test/scala/client/ClientSuite.scala

@@ -189,7 +189,7 @@ class ClientSuite extends Fs2GrpcSuite {
     tc.tick()
     assertEquals(result.value, Some(Success(List(1, 2, 3))))
     assertEquals(dummy.messagesSent.size, 1)
-    assertEquals(dummy.requested, 2)
+    assertEquals(dummy.requested, 3)
 
   }
 
@@ -215,7 +215,7 @@ class ClientSuite extends Fs2GrpcSuite {
 
     assertEquals(result.value, Some(Success(List(1, 2))))
     assertEquals(dummy.messagesSent.size, 1)
-    assertEquals(dummy.requested, 1)
+    assertEquals(dummy.requested, 2)
   }
 
   runTest0("stream to streamingToStreaming") { (tc, io, d) =>
@@ -243,7 +243,7 @@ class ClientSuite extends Fs2GrpcSuite {
     tc.tick()
     assertEquals(result.value, Some(Success(List(1, 2, 3))))
     assertEquals(dummy.messagesSent.size, 5)
-    assertEquals(dummy.requested, 2)
+    assertEquals(dummy.requested, 3)
 
   }
 
@@ -308,7 +308,7 @@ class ClientSuite extends Fs2GrpcSuite {
       Status.INTERNAL
     )
     assertEquals(dummy.messagesSent.size, 5)
-    assertEquals(dummy.requested, 2)
+    assertEquals(dummy.requested, 3)
 
   }
 

runtime/src/test/scala/client/StreamIngestSuite.scala

@@ -44,9 +44,10 @@ class StreamIngestSuite extends CatsEffectSuite with CatsEffectFunFixtures {
     }
 
     run(prefetchN = 1, takeN = 1, expectedReq = 1, expectedCount = 1) *>
-      run(prefetchN = 2, takeN = 1, expectedReq = 0, expectedCount = 1) *>
-      run(prefetchN = 1024, takeN = 1024, expectedReq = 1024, expectedCount = 1024) *>
-      run(prefetchN = 1024, takeN = 1023, expectedReq = 0, expectedCount = 1023)
+      run(prefetchN = 2, takeN = 1, expectedReq = 2, expectedCount = 1) *>
+      run(prefetchN = 2, takeN = 2, expectedReq = 3, expectedCount = 2) *>
+      run(prefetchN = 1024, takeN = 1024, expectedReq = 2047, expectedCount = 1024) *>
+      run(prefetchN = 1024, takeN = 1023, expectedReq = 2046, expectedCount = 1023)
 
   }