Merge pull request #3299 from lhns/feature/file-handle-seekable-byte-…

…channel

FileHandle from SeekableByteChannel

build.sbt

@@ -233,6 +233,9 @@ ThisBuild / mimaBinaryIssueFilters ++= Seq(
   ),
   ProblemFilters.exclude[ReversedMissingMethodProblem](
     "fs2.concurrent.Channel.closeWithElement"
+  ),
+  ProblemFilters.exclude[InheritedNewAbstractMethodProblem](
+    "fs2.io.file.Files.openSeekableByteChannel"
   )
 )
 

io/jvm-native/src/main/scala/fs2/io/file/FileHandlePlatform.scala

@@ -24,7 +24,7 @@ package io
 package file
 
 import java.nio.ByteBuffer
-import java.nio.channels.{FileChannel, FileLock}
+import java.nio.channels.{FileChannel, FileLock, SeekableByteChannel}
 import java.nio.file.{OpenOption, Path => JPath}
 
 import cats.effect.kernel.{Async, Resource, Sync}
@@ -119,4 +119,59 @@ private[file] trait FileHandleCompanionPlatform {
       override def write(bytes: Chunk[Byte], offset: Long): F[Int] =
         F.blocking(chan.write(bytes.toByteBuffer, offset))
     }
+
+  /** Creates a `FileHandle[F]` from a `java.nio.channels.SeekableByteChannel`. Because a `SeekableByteChannel` doesn't provide all the functionalities required by `FileHandle` some features like locking will be unavailable. */
+  private[file] def makeFromSeekableByteChannel[F[_]](
+      chan: SeekableByteChannel,
+      unsupportedOperationException: => Throwable
+  )(implicit F: Sync[F]): FileHandle[F] =
+    new FileHandle[F] {
+      type Lock = Unit
+
+      override def force(metaData: Boolean): F[Unit] =
+        F.raiseError(unsupportedOperationException)
+
+      override def lock: F[Lock] =
+        F.raiseError(unsupportedOperationException)
+
+      override def lock(position: Long, size: Long, shared: Boolean): F[Lock] =
+        F.raiseError(unsupportedOperationException)
+
+      override def read(numBytes: Int, offset: Long): F[Option[Chunk[Byte]]] =
+        F.blocking {
+          val buf = ByteBuffer.allocate(numBytes)
+          val len = chan.synchronized {
+            // don't set position on sequential operations because not all file systems support setting the position
+            if (chan.position() != offset) { chan.position(offset); () }
+            chan.read(buf)
+          }
+          if (len < 0) None
+          else if (len == 0) Some(Chunk.empty)
+          else Some(Chunk.array(buf.array, 0, len))
+        }
+
+      override def size: F[Long] =
+        F.blocking(chan.size)
+
+      override def truncate(size: Long): F[Unit] =
+        F.blocking { chan.truncate(size); () }
+
+      override def tryLock: F[Option[Lock]] =
+        F.raiseError(unsupportedOperationException)
+
+      override def tryLock(position: Long, size: Long, shared: Boolean): F[Option[Lock]] =
+        F.raiseError(unsupportedOperationException)
+
+      override def unlock(f: Lock): F[Unit] =
+        F.unit
+
+      override def write(bytes: Chunk[Byte], offset: Long): F[Int] =
+        F.blocking {
+          chan.synchronized {
+            // don't set position on sequential operations because not all file systems support setting the position
+            if (chan.position() != offset) { chan.position(offset); () }
+            chan.write(bytes.toByteBuffer)
+          }
+        }
+    }
 }

io/jvm-native/src/main/scala/fs2/io/file/FilesPlatform.scala

@@ -26,7 +26,7 @@ package file
 import cats.effect.kernel.{Async, Resource, Sync}
 import cats.syntax.all._
 
-import java.nio.channels.FileChannel
+import java.nio.channels.{FileChannel, SeekableByteChannel}
 import java.nio.file.{Files => JFiles, Path => JPath, _}
 import java.nio.file.attribute.{
   BasicFileAttributeView,
@@ -47,6 +47,12 @@ private[file] trait FilesPlatform[F[_]] extends DeprecatedFilesApi[F] { self: Fi
   /** Creates a `FileHandle` for the supplied NIO `FileChannel`. JVM only. */
   def openFileChannel(channel: F[FileChannel]): Resource[F, FileHandle[F]]
 
+  /** Creates a `FileHandle` for the supplied NIO `SeekableByteChannel`. Because a `SeekableByteChannel` doesn't provide all the functionalities required by `FileHandle` some features like locking will be unavailable. JVM only. */
+  def openSeekableByteChannel(
+      channel: F[SeekableByteChannel],
+      unsupportedOperationException: => Throwable
+  ): Resource[F, FileHandle[F]]
+
   /** Gets the contents of the specified directory whose paths match the supplied glob pattern.
     *
     * Example glob patterns: `*.scala`, `*.{scala,java}`
@@ -314,11 +320,25 @@ private[file] trait FilesCompanionPlatform {
     def open(path: Path, flags: Flags): Resource[F, FileHandle[F]] =
       openFileChannel(
         Sync[F].blocking(FileChannel.open(path.toNioPath, flags.value.map(_.option): _*))
-      )
+      ).recoverWith { case unsupportedOperationException: UnsupportedOperationException =>
+        // not all file systems support file channels
+        openSeekableByteChannel(
+          Sync[F].blocking(JFiles.newByteChannel(path.toNioPath, flags.value.map(_.option): _*)),
+          unsupportedOperationException
+        )
+      }
 
     def openFileChannel(channel: F[FileChannel]): Resource[F, FileHandle[F]] =
       Resource.make(channel)(ch => Sync[F].blocking(ch.close())).map(ch => FileHandle.make(ch))
 
+    def openSeekableByteChannel(
+        channel: F[SeekableByteChannel],
+        unsupportedOperationException: => Throwable
+    ): Resource[F, FileHandle[F]] =
+      Resource
+        .make(channel)(ch => Sync[F].blocking(ch.close()))
+        .map(ch => FileHandle.makeFromSeekableByteChannel(ch, unsupportedOperationException))
+
     def realPath(path: Path): F[Path] =
       Sync[F].blocking(Path.fromNioPath(path.toNioPath.toRealPath()))
 

io/jvm/src/test/scala/fs2/io/file/JvmFilesSuite.scala

@@ -62,4 +62,23 @@ class JvmFilesSuite extends Fs2Suite with BaseFileSuite {
     }
   }
 
+  test("read from SeekableByteChannel") {
+    Stream
+      .resource(
+        tempFile
+          .evalMap(modify)
+          .flatMap(path =>
+            Files[IO].openSeekableByteChannel(
+              IO.blocking(JFiles.newByteChannel(path.toNioPath, Flag.Read.option)),
+              new UnsupportedOperationException()
+            )
+          )
+          .map(new ReadCursor[IO](_, 0))
+      )
+      .flatMap(_.readAll(4096).void.stream)
+      .compile
+      .toList
+      .assertEquals(List[Byte](0, 1, 2, 3))
+  }
+
 }