Avoid additional buffer copy in userspace

Directly send the data from MediaCodec buffers to the LocalSocket,
without an intermediate copy in userspace.

server/src/main/java/com/genymobile/scrcpy/DesktopConnection.java

@@ -5,9 +5,9 @@ import android.net.LocalSocket;
 import android.net.LocalSocketAddress;
 
 import java.io.Closeable;
+import java.io.FileDescriptor;
 import java.io.IOException;
 import java.io.InputStream;
-import java.io.OutputStream;
 import java.nio.charset.StandardCharsets;
 
 public final class DesktopConnection implements Closeable {
@@ -18,14 +18,14 @@ public final class DesktopConnection implements Closeable {
 
     private final LocalSocket socket;
     private final InputStream inputStream;
-    private final OutputStream outputStream;
+    private final FileDescriptor fd;
 
     private final ControlEventReader reader = new ControlEventReader();
 
     private DesktopConnection(LocalSocket socket) throws IOException {
         this.socket = socket;
         inputStream = socket.getInputStream();
-        outputStream = socket.getOutputStream();
+        fd = socket.getFileDescriptor();
     }
 
     private static LocalSocket connect(String abstractName) throws IOException {
@@ -78,11 +78,11 @@ public final class DesktopConnection implements Closeable {
         buffer[DEVICE_NAME_FIELD_LENGTH + 1] = (byte) width;
         buffer[DEVICE_NAME_FIELD_LENGTH + 2] = (byte) (height >> 8);
         buffer[DEVICE_NAME_FIELD_LENGTH + 3] = (byte) height;
-        outputStream.write(buffer, 0, buffer.length);
+        IO.writeFully(fd, buffer, 0, buffer.length);
     }
 
-    public OutputStream getOutputStream() {
-        return outputStream;
+    public FileDescriptor getFd() {
+        return fd;
     }
 
     public ControlEvent receiveControlEvent() throws IOException {

server/src/main/java/com/genymobile/scrcpy/IO.java

@@ -0,0 +1,31 @@
+package com.genymobile.scrcpy;
+
+import android.system.ErrnoException;
+import android.system.Os;
+import android.system.OsConstants;
+
+import java.io.FileDescriptor;
+import java.io.IOException;
+import java.nio.ByteBuffer;
+
+public class IO {
+    private IO() {
+        // not instantiable
+    }
+
+    public static void writeFully(FileDescriptor fd, ByteBuffer from) throws IOException {
+        while (from.hasRemaining()) {
+            try {
+                Os.write(fd, from);
+            } catch (ErrnoException e) {
+                if (e.errno != OsConstants.EINTR) {
+                    throw new IOException(e);
+                }
+            }
+        }
+    }
+
+    public static void writeFully(FileDescriptor fd, byte[] buffer, int offset, int len) throws IOException {
+        writeFully(fd, ByteBuffer.wrap(buffer, offset, len));
+    }
+}

server/src/main/java/com/genymobile/scrcpy/ScreenEncoder.java

@@ -9,8 +9,8 @@ import android.media.MediaFormat;
 import android.os.IBinder;
 import android.view.Surface;
 
+import java.io.FileDescriptor;
 import java.io.IOException;
-import java.io.OutputStream;
 import java.nio.ByteBuffer;
 import java.util.concurrent.atomic.AtomicBoolean;
 
@@ -48,7 +48,7 @@ public class ScreenEncoder implements Device.RotationListener {
         return rotationChanged.getAndSet(false);
     }
 
-    public void streamScreen(Device device, OutputStream outputStream) throws IOException {
+    public void streamScreen(Device device, FileDescriptor fd) throws IOException {
         MediaFormat format = createFormat(bitRate, frameRate, iFrameInterval);
         device.setRotationListener(this);
         boolean alive;
@@ -64,7 +64,7 @@ public class ScreenEncoder implements Device.RotationListener {
                 setDisplaySurface(display, surface, contentRect, videoRect);
                 codec.start();
                 try {
-                    alive = encode(codec, outputStream);
+                    alive = encode(codec, fd);
                 } finally {
                     codec.stop();
                     destroyDisplay(display);
@@ -77,9 +77,7 @@ public class ScreenEncoder implements Device.RotationListener {
         }
     }
 
-    private boolean encode(MediaCodec codec, OutputStream outputStream) throws IOException {
-        @SuppressWarnings("checkstyle:MagicNumber")
-        byte[] buf = new byte[bitRate / 8]; // may contain up to 1 second of video
+    private boolean encode(MediaCodec codec, FileDescriptor fd) throws IOException {
         boolean eof = false;
         MediaCodec.BufferInfo bufferInfo = new MediaCodec.BufferInfo();
         while (!consumeRotationChange() && !eof) {
@@ -91,15 +89,8 @@ public class ScreenEncoder implements Device.RotationListener {
                     break;
                 }
                 if (outputBufferId >= 0) {
-                    ByteBuffer outputBuffer = codec.getOutputBuffer(outputBufferId);
-                    while (outputBuffer.hasRemaining()) {
-                        int remaining = outputBuffer.remaining();
-                        int len = Math.min(buf.length, remaining);
-                        // the outputBuffer is probably direct (it has no underlying array), and LocalSocket does not expose channels,
-                        // so we must copy the data locally to write them manually to the output stream
-                        outputBuffer.get(buf, 0, len);
-                        outputStream.write(buf, 0, len);
-                    }
+                    ByteBuffer codecBuffer = codec.getOutputBuffer(outputBufferId);
+                    IO.writeFully(fd, codecBuffer);
                 }
             } finally {
                 if (outputBufferId >= 0) {

server/src/main/java/com/genymobile/scrcpy/Server.java

@@ -21,7 +21,7 @@ public final class Server {
 
             try {
                 // synchronous
-                screenEncoder.streamScreen(device, connection.getOutputStream());
+                screenEncoder.streamScreen(device, connection.getFd());
             } catch (IOException e) {
                 // this is expected on close
                 Ln.d("Screen streaming stopped");