Early-out of previously failed thumbnail requests.

If we've failed to grab a thumbnail for a particular content id, don't
try again to avoid lots of wasted file access and computation.

BUG=892320

Change-Id: I9d980c0cce1644f2c0da3d8699c5e25e220af8f0
Reviewed-on: https://chromium-review.googlesource.com/c/1263416
Commit-Queue: David Trainor <dtrainor@chromium.org>
Reviewed-by: Xing Liu <xingliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597336}

chrome/android/java/src/org/chromium/chrome/browser/widget/ThumbnailProviderImpl.java

@@ -7,6 +7,7 @@ package org.chromium.chrome.browser.widget;
 import android.graphics.Bitmap;
 import android.support.annotation.NonNull;
 import android.support.annotation.Nullable;
+import android.support.v4.util.LruCache;
 import android.text.TextUtils;
 
 import org.chromium.base.DiscardableReferencePool;
@@ -33,8 +34,21 @@ public class ThumbnailProviderImpl implements ThumbnailProvider, ThumbnailStorag
     /** 5 MB of thumbnails should be enough for everyone. */
     private static final int MAX_CACHE_BYTES = 5 * ConversionUtils.BYTES_PER_MEGABYTE;
 
+    /**
+     * Helper object to store in the LruCache when we don't really need a value but can't use null.
+     */
+    private static final Object NO_BITMAP_PLACEHOLDER = new Object();
+
     private BitmapCache mBitmapCache;
 
+    /**
+     * Tracks a set of Content Ids where thumbnail generation or retrieval failed.  This should
+     * prevent making subsequent (potentially expensive) thumbnail generation requests when there
+     * would be no point.
+     */
+    private LruCache<String /* Content Id */, Object /* Placeholder */> mNoBitmapCache =
+            new LruCache<>(100);
+
     /** Queue of files to retrieve thumbnails for. */
     private final Deque<ThumbnailRequest> mRequestQueue = new ArrayDeque<>();
 
@@ -70,6 +84,11 @@ public class ThumbnailProviderImpl implements ThumbnailProvider, ThumbnailStorag
             return;
         }
 
+        if (mNoBitmapCache.get(request.getContentId()) != null) {
+            request.onThumbnailRetrieved(request.getContentId(), null);
+            return;
+        }
+
         Bitmap cachedBitmap = getBitmapFromCache(request.getContentId(), request.getIconSize());
         if (cachedBitmap != null) {
             request.onThumbnailRetrieved(request.getContentId(), cachedBitmap);
@@ -169,8 +188,10 @@ public class ThumbnailProviderImpl implements ThumbnailProvider, ThumbnailStorag
             // fetches of this thumbnail can recognise the key in the cache.
             String key = getKey(contentId, mCurrentRequest.getIconSize());
             mBitmapCache.putBitmap(key, bitmap);
+            mNoBitmapCache.remove(contentId);
             mCurrentRequest.onThumbnailRetrieved(contentId, bitmap);
         } else {
+            mNoBitmapCache.put(contentId, NO_BITMAP_PLACEHOLDER);
             mCurrentRequest.onThumbnailRetrieved(contentId, null);
         }