Restore optimized JVM I/O functions in coil-network-ktor3. (#2478)

coil-network-ktor3/src/jvmCommonMain/kotlin/coil3/network/ktor3/internal/utils.jvmCommon.kt

@@ -1,37 +1,26 @@
 package coil3.network.ktor3.internal
 
 import io.ktor.utils.io.ByteReadChannel
-import io.ktor.utils.io.core.readFully
-import io.ktor.utils.io.core.remaining
-import io.ktor.utils.io.readRemaining
+import io.ktor.utils.io.copyTo
+import java.io.RandomAccessFile
 import okio.BufferedSink
 import okio.FileSystem
 import okio.Path
 
-// TODO: Use JVM-optimized implementations from Ktor 2 once
-//  https://youtrack.jetbrains.com/issue/KTOR-7220 is fixed.
-
 internal actual suspend fun ByteReadChannel.writeTo(sink: BufferedSink) {
-    val buffer = ByteArray(OKIO_BUFFER_SIZE)
-
-    while (!isClosedForRead) {
-        val packet = readRemaining(buffer.size.toLong())
-        if (packet.exhausted()) break
-
-        // TODO: Figure out how to remove 'buffer' and read directly into 'sink'.
-        val bytesRead = packet.remaining.toInt()
-        packet.readFully(buffer, 0, bytesRead)
-        sink.write(buffer, 0, bytesRead)
-    }
-
-    closedCause?.let { throw it }
+    copyTo(sink)
 }
 
 internal actual suspend fun ByteReadChannel.writeTo(fileSystem: FileSystem, path: Path) {
-    fileSystem.write(path) {
-        writeTo(this)
+    if (fileSystem === FileSystem.SYSTEM) {
+        // Fast path: normal jvm File, write to FileChannel directly.
+        RandomAccessFile(path.toFile(), "rw").use {
+            copyTo(it.channel)
+        }
+    } else {
+        // Slow path: cannot guarantee a "real" file.
+        fileSystem.write(path) {
+            copyTo(this)
+        }
     }
 }
-
-// Okio uses 8 KB internally.
-private const val OKIO_BUFFER_SIZE = 8 * 1024