From dd9db975606c39c8cb9b691ee81aa1c5982aa628 Mon Sep 17 00:00:00 2001
From: Ferdinand Svehla <ferdinand@fsvehla.at>
Date: Wed, 15 Sep 2021 10:02:55 +0200
Subject: [PATCH] Fix transducer handling of empty arrays

---
 .../zio/json/JsonDecoderPlatformSpecific.scala   | 16 +++++++++++++---
 .../zio/json/DecoderPlatformSpecificSpec.scala   |  9 +++++++++
 2 files changed, 22 insertions(+), 3 deletions(-)

diff --git a/zio-json/jvm/src/main/scala/zio/json/JsonDecoderPlatformSpecific.scala b/zio-json/jvm/src/main/scala/zio/json/JsonDecoderPlatformSpecific.scala
index a114f247a..591ef0d42 100644
--- a/zio-json/jvm/src/main/scala/zio/json/JsonDecoderPlatformSpecific.scala
+++ b/zio-json/jvm/src/main/scala/zio/json/JsonDecoderPlatformSpecific.scala
@@ -53,12 +53,12 @@ trait JsonDecoderPlatformSpecific[A] { self: JsonDecoder[A] =>
     ZTransducer {
       for {
         // format: off
-        runtime    <- ZIO.runtime[Any].toManaged_
+        runtime    <- ZManaged.runtime[Any]
         inQueue    <- Queue.unbounded[Take[Nothing, Char]].toManaged_
         outQueue   <- Queue.unbounded[Take[Throwable, A]].toManaged_
         ended      <- Ref.makeManaged(false)
         reader     <- ZManaged.fromAutoCloseable {
-                        ZIO.effectTotal {
+                        UIO {
                           def readPull: Iterator[Chunk[Char]] =
                             runtime.unsafeRun(inQueue.take)
                               .fold(
@@ -70,13 +70,23 @@ trait JsonDecoderPlatformSpecific[A] { self: JsonDecoder[A] =>
                           new zio.stream.internal.ZReader(Iterator.empty ++ readPull)
                         }
                       }
-        jsonReader <- ZManaged.fromAutoCloseable(ZIO.effectTotal(new WithRetractReader(reader)))
+        jsonReader <- ZManaged.fromAutoCloseable(UIO(new WithRetractReader(reader)))
         process    <- effectBlockingInterrupt {
                         // Exceptions fall through and are pushed into the queue
                         @tailrec def loop(atBeginning: Boolean): Unit = {
                           val nextElem = try {
                             if (atBeginning && delimiter == JsonStreamDelimiter.Array)  {
                               Lexer.char(Nil, jsonReader, '[')
+
+                              jsonReader.nextNonWhitespace() match {
+                                case ']' =>
+                                  // returning empty here instead of falling through, which would
+                                  // attempt to decode a value that we know doesn’t exist.
+                                  return ()
+
+                                case _ =>
+                                  jsonReader.retract()
+                              }
                             } else {
                               delimiter match {
                                 case JsonStreamDelimiter.Newline =>
diff --git a/zio-json/jvm/src/test/scala-2/zio/json/DecoderPlatformSpecificSpec.scala b/zio-json/jvm/src/test/scala-2/zio/json/DecoderPlatformSpecificSpec.scala
index f2b8f2b8d..9afa13584 100644
--- a/zio-json/jvm/src/test/scala-2/zio/json/DecoderPlatformSpecificSpec.scala
+++ b/zio-json/jvm/src/test/scala-2/zio/json/DecoderPlatformSpecificSpec.scala
@@ -197,6 +197,15 @@ object DecoderPlatformSpecificSpec extends DefaultRunnableSpec {
                   assert(xs)(equalTo(Chunk(1001)))
                 }
             },
+            testM("empty array") {
+              ZStream
+                .fromIterable("[]".toSeq)
+                .transduce(JsonDecoder[String].decodeJsonTransducer(JsonStreamDelimiter.Array))
+                .runCollect
+                .map { xs =>
+                  assert(xs)(isEmpty)
+                }
+            },
             testM("decodes multiple elements") {
               ZStream
                 .fromIterable("[ 1001, 1002, 1003 ]".toSeq)