Allow a read of length 0 in InputStreamAdapter #28751 (#28759)

akka-stream-tests/src/test/scala/akka/stream/io/InputStreamSinkSpec.scala

@@ -153,7 +153,7 @@ class InputStreamSinkSpec extends StreamSpec(UnboundedMailboxConfig) {
       an[IllegalArgumentException] shouldBe thrownBy(inputStream.read(buf, -1, 2))
       an[IllegalArgumentException] shouldBe thrownBy(inputStream.read(buf, 0, 5))
       an[IllegalArgumentException] shouldBe thrownBy(inputStream.read(new Array[Byte](0), 0, 1))
-      an[IllegalArgumentException] shouldBe thrownBy(inputStream.read(buf, 0, 0))
+      an[IllegalArgumentException] shouldBe thrownBy(inputStream.read(buf, 0, -1))
       inputStream.close()
     }
 
@@ -275,6 +275,18 @@ class InputStreamSinkSpec extends StreamSpec(UnboundedMailboxConfig) {
       }
       thrown.getCause should ===(error)
     }
+
+    "a read of length 0 should not request bytes from upstream" in assertAllStagesStopped {
+      val (probe, inputStream) = TestSource.probe[ByteString].toMat(StreamConverters.asInputStream())(Keep.both).run()
+      probe.ensureSubscription()
+      probe.expectRequest()
+
+      inputStream.read(new Array[Byte](byteString.size), 0, 0) should ===(0)
+      probe.expectNoMessage()
+
+      inputStream.close()
+      probe.expectCancellation()
+    }
   }
 
 }

akka-stream/src/main/scala/akka/stream/impl/io/InputStreamSinkStage.scala

@@ -148,9 +148,11 @@ private[stream] object InputStreamSinkStage {
   override def read(a: Array[Byte], begin: Int, length: Int): Int = {
     require(a.length > 0, "array size must be >= 0")
     require(begin >= 0, "begin must be >= 0")
-    require(length > 0, "length must be > 0")
+    require(length >= 0, "length must be >= 0")
     require(begin + length <= a.length, "begin + length must be smaller or equal to the array length")
 
+    if (length == 0) 0
+    else
       executeIfNotClosed(() =>
         if (isStageAlive) {
           detachedChunk match {