Fix FileChannel leak when request fails before FileMultipartPart gets written, close AsyncHttpClient#1418

Motivation:

We currently eagerly open FileChannel in FileMultipartPart prior to
trying to send the request.
If we can’t even try to send the body, eg because connection fails, we
never close this FileChannel.

Modification:

Lazily open FileChannel when writing.

Result:

No more file descriptor leak

Author: slandelle

client/src/main/java/org/asynchttpclient/request/body/multipart/part/FileMultipartPart.java

@@ -16,9 +16,7 @@
 import static org.asynchttpclient.util.MiscUtils.closeSilently;
 import io.netty.buffer.ByteBuf;
 
-import java.io.FileInputStream;
-import java.io.FileNotFoundException;
-import java.io.IOException;
+import java.io.*;
 import java.nio.channels.FileChannel;
 import java.nio.channels.WritableByteChannel;
 
@@ -27,19 +25,26 @@
 
 public class FileMultipartPart extends FileLikeMultipartPart<FilePart> {
 
-    private final FileChannel channel;
+    private FileChannel channel;
     private final long length;
     private long position = 0L;
 
-    @SuppressWarnings("resource")
+    private FileChannel getChannel() throws IOException {
+        if (channel == null) {
+            channel = new RandomAccessFile(part.getFile(), "r").getChannel();
+        }
+        return channel;
+    }
+
     public FileMultipartPart(FilePart part, byte[] boundary) {
         super(part, boundary);
-        try {
-            channel = new FileInputStream(part.getFile()).getChannel();
-        } catch (FileNotFoundException e) {
-            throw new IllegalArgumentException("File part doesn't exist: " + part.getFile().getAbsolutePath(), e);
+        File file = part.getFile();
+        if (!file.exists()) {
+            throw new IllegalArgumentException("File part doesn't exist: " + file.getAbsolutePath());
+        } else if (!file.canRead()) {
+            throw new IllegalArgumentException("File part can't be read: " + file.getAbsolutePath());
         }
-        length = part.getFile().length();
+        length = file.length();
     }
 
     @Override
@@ -50,7 +55,7 @@ protected long getContentLength() {
     @Override
     protected long transferContentTo(ByteBuf target) throws IOException {
         // can return -1 if file is empty or FileChannel was closed
-        int transferred = target.writeBytes(channel, target.writableBytes());
+        int transferred = target.writeBytes(getChannel(), target.writableBytes());
         if (transferred > 0) {
             position += transferred;
         }
@@ -66,8 +71,9 @@ protected long transferContentTo(ByteBuf target) throws IOException {
     @Override
     protected long transferContentTo(WritableByteChannel target) throws IOException {
         // WARN: don't use channel.position(), it's always 0 here
-        // from FileChannel javadoc: "This method does not modify this channel's position."
-        long transferred = channel.transferTo(position, BodyChunkedInput.DEFAULT_CHUNK_SIZE, target);
+        // from FileChannel javadoc: "This method does not modify this channel's
+        // position."
+        long transferred = getChannel().transferTo(position, BodyChunkedInput.DEFAULT_CHUNK_SIZE, target);
         if (transferred > 0) {
             position += transferred;
         }