ImageReader: Add discardFreeBuffers method

This method (currently @hide) discards all cached buffers for
the ImageReader, which can be useful to free up memory at the
cost of reallocationt time if more buffers are needed later.

It has no effect on acquired Images, filled Images waiting to be
acquired, or buffers being actively used by the source rendering to the
Surface.

Bug: 28695173
Change-Id: I427a8ba4ad73a267083f843dbf228bfc53a730d9

media/java/android/media/ImageReader.java

@@ -520,6 +520,31 @@ public class ImageReader implements AutoCloseable {
         }
     }
 
+    /**
+     * Discard any free buffers owned by this ImageReader.
+     *
+     * <p>
+     * Generally, the ImageReader caches buffers for reuse once they have been
+     * allocated, for best performance. However, sometimes it may be important to
+     * release all the cached, unused buffers to save on memory.
+     * </p>
+     * <p>
+     * Calling this method will discard all free cached buffers. This does not include any buffers
+     * associated with Images acquired from the ImageReader, any filled buffers waiting to be
+     * acquired, and any buffers currently in use by the source rendering buffers into the
+     * ImageReader's Surface.
+     * <p>
+     * The ImageReader continues to be usable after this call, but may need to reallocate buffers
+     * when more buffers are needed for rendering.
+     * </p>
+     * @hide
+     */
+    public void discardFreeBuffers() {
+        synchronized (mCloseLock) {
+            nativeDiscardFreeBuffers();
+        }
+    }
+
     @Override
     protected void finalize() throws Throwable {
         try {
@@ -872,6 +897,7 @@ public class ImageReader implements AutoCloseable {
     private synchronized native void nativeReleaseImage(Image i);
     private synchronized native Surface nativeGetSurface();
     private synchronized native int nativeDetachImage(Image i);
+    private synchronized native void nativeDiscardFreeBuffers();
 
     /**
      * @return A return code {@code ACQUIRE_*}

media/jni/android_media_ImageReader.cpp

@@ -611,6 +611,23 @@ static jint ImageReader_detachImage(JNIEnv* env, jobject thiz, jobject image) {
     return OK;
 }
 
+static void ImageReader_discardFreeBuffers(JNIEnv* env, jobject thiz) {
+    ALOGV("%s:", __FUNCTION__);
+    JNIImageReaderContext* ctx = ImageReader_getContext(env, thiz);
+    if (ctx == NULL) {
+        jniThrowException(env, "java/lang/IllegalStateException", "ImageReader was already closed");
+        return;
+    }
+
+    BufferItemConsumer* bufferConsumer = ctx->getBufferConsumer();
+    status_t res = bufferConsumer->discardFreeBuffers();
+    if (res != OK) {
+        ALOGE("Buffer discard failed: %s (%d)", strerror(-res), res);
+        jniThrowRuntimeException(env,
+                "nativeDicardFreebuffers failed");
+    }
+}
+
 static jobject ImageReader_getSurface(JNIEnv* env, jobject thiz)
 {
     ALOGV("%s: ", __FUNCTION__);
@@ -773,6 +790,7 @@ static const JNINativeMethod gImageReaderMethods[] = {
     {"nativeImageSetup",       "(Landroid/media/Image;)I",   (void*)ImageReader_imageSetup },
     {"nativeGetSurface",       "()Landroid/view/Surface;",   (void*)ImageReader_getSurface },
     {"nativeDetachImage",      "(Landroid/media/Image;)I",   (void*)ImageReader_detachImage },
+    {"nativeDiscardFreeBuffers", "()V",                      (void*)ImageReader_discardFreeBuffers }
 };
 
 static const JNINativeMethod gImageMethods[] = {