Use a direct buffer when available while accepting Bitmap from Mojo.

This saves a copy.

Bug: None
Change-Id: Id48a8cd618527f640af44dc3f7807877213f5fa5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2597001Reviewed-by: Oksana Zhuravlova <oksamyt@chromium.org>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Commit-Queue: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840802}

mojo/public/java/base/src/org/chromium/mojo_base/BigBufferUtil.java

@@ -18,6 +18,33 @@ import java.nio.ByteBuffer;
 public final class BigBufferUtil {
     public static final int MAX_INLINE_ARRAY_SIZE = 64 * 1024;
 
+    /**
+     * A mapping to a BigBuffer.
+     *
+     * If it is backed by shared memory, this will be a direct mapping which must be closed and will
+     * be invalid thereafter. The simplest way to do this is by using try-with-resources.
+     */
+    public static class Mapping implements AutoCloseable {
+        private final SharedBufferHandle mHandle;
+        private final ByteBuffer mBuffer;
+
+        Mapping(SharedBufferHandle handle, ByteBuffer buffer) {
+            mHandle = handle;
+            mBuffer = buffer;
+        }
+
+        public ByteBuffer getBuffer() {
+            return mBuffer;
+        }
+
+        @Override
+        public void close() {
+            if (mHandle != null) {
+                mHandle.unmap(mBuffer);
+            }
+        }
+    }
+
     // Retrives a copy of the buffer's contents regardless of what type was backing it (i.e. array
     // or shared memory).
     public static byte[] getBytesFromBigBuffer(BigBuffer buffer) {
@@ -34,6 +61,20 @@ public final class BigBufferUtil {
         }
     }
 
+    // Opens a mapping to an existing buffer for direct reading, without a copy.
+    // This must be used with a try-with-resources so that close is called to prevent a leak.
+    // The direct buffer must not be used after close is called.
+    public static Mapping map(BigBuffer buffer) {
+        if (buffer.which() == BigBuffer.Tag.Bytes) {
+            return new Mapping(null, ByteBuffer.wrap(buffer.getBytes()));
+        } else {
+            BigBufferSharedMemoryRegion region = buffer.getSharedMemory();
+            ByteBuffer byteBuffer =
+                    region.bufferHandle.map(0, region.size, SharedBufferHandle.MapFlags.NONE);
+            return new Mapping(region.bufferHandle, byteBuffer);
+        }
+    }
+
     // Creates a new mojom.BigBuffer for IPC from a set of bytes. If the byte array is larger than
     // MAX_INLINE_ARRAY_SIZE, shared memory will be used instead of an inline array.
     public static BigBuffer createBigBufferFromBytes(byte[] bytes) {

services/shape_detection/android/java/src/org/chromium/shape_detection/BitmapUtils.java

@@ -32,16 +32,16 @@ public class BitmapUtils {
             return null;
         }
 
-        ByteBuffer imageBuffer =
-                ByteBuffer.wrap(BigBufferUtil.getBytesFromBigBuffer(bitmapData.pixelData));
-        if (imageBuffer.capacity() <= 0) {
-            return null;
+        try (BigBufferUtil.Mapping mapping = BigBufferUtil.map(bitmapData.pixelData)) {
+            ByteBuffer imageBuffer = mapping.getBuffer();
+            if (imageBuffer.capacity() <= 0) {
+                return null;
+            }
+
+            Bitmap bitmap = Bitmap.createBitmap(width, height, Bitmap.Config.ARGB_8888);
+            bitmap.copyPixelsFromBuffer(imageBuffer);
+            return bitmap;
         }
-
-        Bitmap bitmap = Bitmap.createBitmap(width, height, Bitmap.Config.ARGB_8888);
-        bitmap.copyPixelsFromBuffer(imageBuffer);
-
-        return bitmap;
     }
 
     public static Frame convertToFrame(org.chromium.skia.mojom.BitmapN32 bitmapData) {