Fix 'fs.io.readInputStreamGeneric' overallocation of underlying buffers

build.sbt

@@ -236,6 +236,10 @@ ThisBuild / mimaBinaryIssueFilters ++= Seq(
   ),
   ProblemFilters.exclude[InheritedNewAbstractMethodProblem](
     "fs2.io.file.Files.openSeekableByteChannel"
+  ),
+  // package-private method: #3318
+  ProblemFilters.exclude[IncompatibleMethTypeProblem](
+    "fs2.io.package.readInputStreamGeneric"
   )
 )
 

io/jvm/src/main/scala/fs2/io/ioplatform.scala

@@ -43,10 +43,10 @@ private[fs2] trait ioplatform extends iojvmnative {
       F.pure(System.in),
       F.delay(new Array[Byte](bufSize)),
       false
-    ) { (is, buf) =>
+    ) { (is, buf, off) =>
       F.async[Int] { cb =>
         F.delay {
-          val task: Runnable = () => cb(Right(is.read(buf)))
+          val task: Runnable = () => cb(Right(is.read(buf, off, buf.length - off)))
           stdinExecutor.submit(task)
         }.map { fut =>
           Some(F.delay {

io/jvm/src/test/scala/fs2/io/IoPlatformSuite.scala

@@ -40,6 +40,42 @@ class IoPlatformSuite extends Fs2Suite {
   // This suite runs for a long time, this avoids timeouts in CI.
   override def munitIOTimeout: Duration = 2.minutes
 
+  group("readInputStream") {
+    test("reuses internal buffer on smaller chunks") {
+      forAllF { (bytes: Array[Byte], chunkSize0: Int) =>
+        val chunkSize = (chunkSize0 % 20).abs + 1
+        fs2.Stream
+          .chunk(Chunk.array(bytes))
+          .chunkN(chunkSize / 3 + 1)
+          .unchunks
+          .covary[IO]
+          // we know that '.toInputStream' reads by chunk
+          .through(fs2.io.toInputStream)
+          .flatMap(is => io.readInputStream(IO(is), chunkSize))
+          .chunks
+          .zipWithPrevious
+          .assertForall {
+            case (None, _)                        => true // skip first element
+            case (_, _: Chunk.Singleton[_])       => true // skip singleton bytes
+            case (Some(_: Chunk.Singleton[_]), _) => true // skip singleton bytes
+            case (Some(Chunk.ArraySlice(bs1, o1, l1)), Chunk.ArraySlice(bs2, o2, _)) =>
+              {
+                // if first slice buffer is not 'full'
+                (bs1.length != (o1 + l1)) &&
+                // we expect that next slice will wrap same buffer
+                ((bs2 eq bs1) && (o2 == o1 + l1))
+              } || {
+                // if first slice buffer is 'full'
+                (bs2.length == (o1 + l1)) &&
+                // we expect new buffer allocated for next slice
+                ((bs2 ne bs1) && (o2 == 0))
+              }
+            case _ => false // unexpected chunk subtype
+          }
+      }
+    }
+  }
+
   group("readOutputStream") {
     test("writes data and terminates when `f` returns") {
       forAllF { (bytes: Array[Byte], chunkSize0: Int) =>

io/shared/src/main/scala/fs2/io/io.scala

@@ -45,7 +45,7 @@ package object io extends ioplatform {
       fis,
       F.delay(new Array[Byte](chunkSize)),
       closeAfterUse
-    )((is, buf) => F.blocking(is.read(buf)))
+    )((is, buf, off) => F.blocking(is.read(buf, off, buf.length - off)))
 
   private[io] def readInputStreamCancelable[F[_]](
       fis: F[InputStream],
@@ -57,7 +57,7 @@ package object io extends ioplatform {
       fis,
       F.delay(new Array[Byte](chunkSize)),
       closeAfterUse
-    )((is, buf) => F.blocking(is.read(buf)).cancelable(cancel))
+    )((is, buf, off) => F.blocking(is.read(buf, off, buf.length - off)).cancelable(cancel))
 
   /** Reads all bytes from the specified `InputStream` with a buffer size of `chunkSize`.
     * Set `closeAfterUse` to false if the `InputStream` should not be closed after use.
@@ -76,31 +76,28 @@ package object io extends ioplatform {
       fis,
       F.pure(new Array[Byte](chunkSize)),
       closeAfterUse
-    )((is, buf) => F.blocking(is.read(buf)))
+    )((is, buf, off) => F.blocking(is.read(buf, off, buf.length - off)))
 
-  private def readBytesFromInputStream[F[_]](is: InputStream, buf: Array[Byte])(
-      read: (InputStream, Array[Byte]) => F[Int]
+  private def readBytesFromInputStream[F[_]](is: InputStream, buf: Array[Byte], offset: Int)(
+      read: (InputStream, Array[Byte], Int) => F[Int]
   )(implicit
       F: Sync[F]
-  ): F[Option[Chunk[Byte]]] =
-    read(is, buf).map { numBytes =>
+  ): F[Option[(Chunk[Byte], Option[(Array[Byte], Int)])]] =
+    read(is, buf, offset).map { numBytes =>
       if (numBytes < 0) None
-      else if (numBytes == 0) Some(Chunk.empty)
-      else if (numBytes < buf.size) Some(Chunk.array(buf, 0, numBytes))
-      else Some(Chunk.array(buf))
+      else if (offset + numBytes == buf.size) Some(Chunk.array(buf, offset, numBytes) -> None)
+      else Some(Chunk.array(buf, offset, numBytes) -> Some(buf -> (offset + numBytes)))
     }
 
   private[fs2] def readInputStreamGeneric[F[_]](
       fis: F[InputStream],
       buf: F[Array[Byte]],
       closeAfterUse: Boolean
-  )(read: (InputStream, Array[Byte]) => F[Int])(implicit F: Sync[F]): Stream[F, Byte] = {
-    def useIs(is: InputStream) =
-      Stream
-        .eval(buf.flatMap(b => readBytesFromInputStream(is, b)(read)))
-        .repeat
-        .unNoneTerminate
-        .flatMap(c => Stream.chunk(c))
+  )(read: (InputStream, Array[Byte], Int) => F[Int])(implicit F: Sync[F]): Stream[F, Byte] = {
+    def useIs(is: InputStream) = Stream.unfoldChunkEval(Option.empty[(Array[Byte], Int)]) {
+      case None              => buf.flatMap(b => readBytesFromInputStream(is, b, 0)(read))
+      case Some((b, offset)) => readBytesFromInputStream(is, b, offset)(read)
+    }
 
     if (closeAfterUse)
       Stream.bracket(fis)(is => Sync[F].blocking(is.close())).flatMap(useIs)