The Tor Browser: mobile/android/base/favicons/cache/FaviconCache.java@b8a032363ba2 (annotated)

mobile/android/base/favicons/cache/FaviconCache.java@b8a032363ba2 (annotated)

mobile/android/base/favicons/cache/FaviconCache.java

Thu, 22 Jan 2015 13:21:57 +0100

author: Michael Schloh von Bennewitz <michael@schloh.com>
date: Thu, 22 Jan 2015 13:21:57 +0100
branch: TOR_BUG_9701
changeset 15: b8a032363ba2
permissions: -rw-r--r--

Incorporate requested changes from Mozilla in review:
https://bugzilla.mozilla.org/show_bug.cgi?id=1123480#c6

 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 package org.mozilla.gecko.favicons.cache;
 import android.graphics.Bitmap;
 import android.util.Log;
 import org.mozilla.gecko.favicons.Favicons;
 import java.util.Iterator;
 import java.util.LinkedList;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.Semaphore;
 import java.util.concurrent.atomic.AtomicInteger;
 /**
  * Implements a Least-Recently-Used cache for Favicons, keyed by Favicon URL.
  *
  * When a favicon at a particular URL is decoded, it will yield one or more bitmaps.
  * While in memory, these bitmaps are stored in a list, sorted in ascending order of size, in a
  * FaviconsForURL object.
  * The collection of FaviconsForURL objects currently in the cache is stored in backingMap, keyed
  * by favicon URL.
  *
  * A second map exists for permanent cache entries -- ones that are never expired. These entries
  * are assumed to be disjoint from those in the normal cache, and this map is checked first.
  *
  * FaviconsForURL provides a method for obtaining the smallest icon larger than a given size - the
  * most appropriate icon for a particular size.
  * It also distinguishes between "primary" favicons (Ones that have merely been extracted from a
  * file downloaded from the website) and "secondary" favicons (Ones that have been computed locally
  * as resized versions of primary favicons.).
  *
  * FaviconsForURL is also responsible for storing URL-specific, as opposed to favicon-specific,
  * information. For the purposes of this cache, the simplifying assumption that the dominant colour
  * for all favicons served from a particular favicon URL shall be the same is made. (To violate this
  * would mandate serving an ICO or similar file with multiple radically different images in it - an
  * ill-advised and extremely uncommon use-case, for sure.)
  * The dominant colour information is updated as the element is being added to the cache - typically
  * on the background thread.
  * Also present here are the download timestamp and isFailed flag. Upon failure, the flag is set.
  * A constant exists in this file to specify the maximum time permitted between failures before
  * a retry is again permitted.
  *
  * TODO: Expiry of Favicons from the favicon database cache is not implemented. (Bug 914296)
  *
  * A typical request to the cache will consist of a Favicon URL and a target size. The FaviconsForURL
  * object for that URL will be obtained, queried for a favicon matching exactly the needed size, and
  * if successful, the result is returned.
  * If unsuccessful, the object is asked to return the smallest available primary favicon larger than
  * the target size. If this step works, the result is downscaled to create a new secondary favicon,
  * which is then stored (So subsequent requests will succeed at the first step) and returned.
  * If that step fails, the object finally walks backwards through its sequence of favicons until it
  * finds the largest primary favicon smaller than the target. This is then upscaled by a maximum of
  * 2x towards the target size, and the result cached and returned as above.
  *
  * The bitmaps themselves are encapsulated inside FaviconCacheElement objects. These objects contain,
  * as well as the bitmap, a pointer to the encapsulating FaviconsForURL object (Used by the LRU
  * culler), the size of the encapsulated image, a flag indicating if this is a primary favicon, and
  * a flag indicating if the entry is invalid.
  * All FaviconCacheElement objects are tracked in the ordering LinkedList. This is used to record
  * LRU information about FaviconCacheElements. In particular, the most recently used FaviconCacheElement
  * will be at the start of the list, the least recently used at the end of the list.
  *
  * When the cache runs out of space, it removes FaviconCacheElements starting from the end of the list
  * until a sufficient amount of space has been freed.
  * When a secondary favicon is removed in this way, it is simply deleted from its parent FaviconsForURLs
  * object's list of available favicons.
  * The backpointer field on the FaviconCacheElement is used to remove the element from the encapsulating
  * FaviconsForURL object, when this is required.
  * When a primary favicon is removed, its invalid flag is set to true and its bitmap payload is set
  * to null (So it is available for freeing by the garbage collector). This reduces the memory footprint
  * of the icon to essentially zero, but keeps track of which primary favicons exist for this favicon
  * URL.
  * If a subsequent request comes in for that favicon URL, it is then known that a primary of those
  * dimensions is available, just that it is not in the cache. The system is then able to load the
  * primary back into the cache from the database (Where the entirety of the initially encapsulating
  * container-formatted image file is stored).
  * If this were not done, then when processing requests after the culling of primary favicons it would
  * be impossible to distinguish between the nonexistence of a primary and the nonexistence of a primary
  * in the cache without querying the database.
  *
  * The implementation is safe to use from multiple threads and, while is it not entirely strongly
  * consistent all of the time, you almost certainly don't care.
  * The thread-safety implementation used is approximately MRSW with semaphores. An extra semaphore
  * is used to grant mutual exclusion over reordering operations from reader threads (Who thus gain
  * a quasi-writer status to do such limited mutation as is necessary).
  *
  * Reads which race with writes are liable to not see the ongoing write. The cache may return a
  * stale or now-removed value to the caller. Returned values are never invalid, even in the face
  * of concurrent reading and culling.
  */
 public class FaviconCache {
     private static final String LOGTAG = "FaviconCache";
     // The number of spaces to allocate for favicons in each node.
     private static final int NUM_FAVICON_SIZES = 4;
     // Dimensions of the largest favicon to store in the cache. Everything is downscaled to this.
     public final int maxCachedWidth;
     // Retry failed favicons after 20 minutes.
     public static final long FAILURE_RETRY_MILLISECONDS = 1000 * 60 * 20;
     // Map relating Favicon URLs with objects representing decoded favicons.
     // Since favicons may be container formats holding multiple icons, the underlying type holds a
     // sorted list of bitmap payloads in ascending order of size. The underlying type may be queried
     // for the least larger payload currently present.
     private final ConcurrentHashMap<String, FaviconsForURL> backingMap = new ConcurrentHashMap<String, FaviconsForURL>();
     // And the same, but never evicted.
     private final ConcurrentHashMap<String, FaviconsForURL> permanentBackingMap = new ConcurrentHashMap<String, FaviconsForURL>();
     // A linked list used to implement a queue, defining the LRU properties of the cache. Elements
     // contained within the various FaviconsForURL objects are held here, the least recently used
     // of which at the end of the list. When space needs to be reclaimed, the appropriate bitmap is
     // culled.
     private final LinkedList<FaviconCacheElement> ordering = new LinkedList<FaviconCacheElement>();
     // The above structures, if used correctly, enable this cache to exhibit LRU semantics across all
     // favicon payloads in the system, as well as enabling the dynamic selection from the cache of
     // the primary bitmap most suited to the requested size (in cases where multiple primary bitmaps
     // are provided by the underlying file format).
     // Current size, in bytes, of the bitmap data present in the LRU cache.
     private final AtomicInteger currentSize = new AtomicInteger(0);
     // The maximum quantity, in bytes, of bitmap data which may be stored in the cache.
     private final int maxSizeBytes;
     // Tracks the number of ongoing read operations. Enables the first one in to lock writers out and
     // the last one out to let them in.
     private final AtomicInteger ongoingReads = new AtomicInteger(0);
     // Used to ensure transaction fairness - each txn acquires and releases this as the first operation.
     // The effect is an orderly, inexpensive ordering enforced on txns to prevent writer starvation.
     private final Semaphore turnSemaphore = new Semaphore(1);
     // A deviation from the usual MRSW solution - this semaphore is used to guard modification to the
     // ordering map. This allows for read transactions to update the most-recently-used value without
     // needing to take out the write lock.
     private final Semaphore reorderingSemaphore = new Semaphore(1);
     // The semaphore one must acquire in order to perform a write.
     private final Semaphore writeLock = new Semaphore(1);
     /**
      * Called by txns performing only reads as they start. Prevents writer starvation with a turn
      * semaphore and locks writers out if this is the first concurrent reader txn starting up.
      */
     private void startRead() {
         turnSemaphore.acquireUninterruptibly();
         turnSemaphore.release();
         if (ongoingReads.incrementAndGet() == 1) {
             // First one in. Wait for writers to finish and lock them out.
             writeLock.acquireUninterruptibly();
         }
     }
     /**
      * Called by transactions performing only reads as they finish. Ensures that if this is the last
      * concluding read transaction then then writers are subsequently allowed in.
      */
     private void finishRead() {
         if (ongoingReads.decrementAndGet() == 0) {
             writeLock.release();
         }
     }
     /**
      * Called by writer transactions upon start. Ensures fairness and then obtains the write lock.
      * Upon return, no other txns will be executing concurrently.
      */
     private void startWrite() {
         turnSemaphore.acquireUninterruptibly();
         writeLock.acquireUninterruptibly();
     }
     /**
      * Called by a concluding write transaction - unlocks the structure.
      */
     private void finishWrite() {
         turnSemaphore.release();
         writeLock.release();
     }
     public FaviconCache(int maxSize, int maxWidthToCache) {
         maxSizeBytes = maxSize;
         maxCachedWidth = maxWidthToCache;
     }
     /**
      * Determine if the provided favicon URL is marked as a failure (Has failed to load before -
      * such icons get blacklisted for a time to prevent us endlessly retrying.)
      *
      * @param faviconURL Favicon URL to check if failed in memcache.
      * @return true if this favicon is blacklisted, false otherwise.
      */
     public boolean isFailedFavicon(String faviconURL) {
         if (faviconURL == null) {
             return true;
         }
         startRead();
         try {
             // If we don't have it in the cache, it certainly isn't a known failure.
             // Non-evictable favicons are never failed, so we don't need to
             // check permanentBackingMap.
             if (!backingMap.containsKey(faviconURL)) {
                 return false;
             }
             FaviconsForURL container = backingMap.get(faviconURL);
             // If the has failed flag is not set, it's certainly not a known failure.
             if (!container.hasFailed) {
                 return false;
             }
             final long failureTimestamp = container.downloadTimestamp;
             // Calculate elapsed time since the failing download.
             final long failureDiff = System.currentTimeMillis() - failureTimestamp;
             // If the expiry is still in effect, return. Otherwise, continue and unmark the failure.
             if (failureDiff < FAILURE_RETRY_MILLISECONDS) {
                 return true;
             }
         } catch (Exception unhandled) {
             Log.e(LOGTAG, "FaviconCache exception!", unhandled);
             return true;
         }  finally {
             finishRead();
         }
         startWrite();
         // If the entry is no longer failed, remove the record of it from the cache.
         try {
             recordRemoved(backingMap.remove(faviconURL));
             return false;
         } finally {
             finishWrite();
         }
     }
     /**
      * Mark the indicated page URL as a failed Favicon until the provided time.
      *
      * @param faviconURL Page URL for which a Favicon load has failed.
      */
     public void putFailed(String faviconURL) {
         startWrite();
         try {
             FaviconsForURL container = new FaviconsForURL(0, true);
             recordRemoved(backingMap.put(faviconURL, container));
         } finally {
             finishWrite();
         }
     }
     /**
      * Fetch a Favicon for the given URL as close as possible to the size provided.
      * If an icon of the given size is already in the cache, it is returned.
      * If an icon of the given size is not in the cache but a larger unscaled image does exist in
      * the cache, we downscale the larger image to the target size and cache the result.
      * If there is no image of the required size, null is returned.
      *
      * @param faviconURL The URL for which a Favicon is desired. Must not be null.
      * @param targetSize The size of the desired favicon.
      * @return A favicon of the requested size for the requested URL, or null if none cached.
      */
     public Bitmap getFaviconForDimensions(String faviconURL, int targetSize) {
         if (faviconURL == null) {
             Log.e(LOGTAG, "You passed a null faviconURL to getFaviconForDimensions. Don't.");
             return null;
         }
         boolean shouldComputeColour = false;
         boolean wasPermanent = false;
         FaviconsForURL container;
         final Bitmap newBitmap;
         startRead();
         try {
             container = permanentBackingMap.get(faviconURL);
             if (container == null) {
                 container = backingMap.get(faviconURL);
                 if (container == null) {
                     // We don't have it!
                     return null;
                 }
             } else {
                 wasPermanent = true;
             }
             FaviconCacheElement cacheElement;
             // If targetSize is -1, it means we want the largest possible icon.
             int cacheElementIndex = (targetSize == -1) ? -1 : container.getNextHighestIndex(targetSize);
             // cacheElementIndex now holds either the index of the next least largest bitmap from
             // targetSize, or -1 if targetSize > all bitmaps.
             if (cacheElementIndex != -1) {
                 // If cacheElementIndex is not the sentinel value, then it is a valid index into favicons.
                 cacheElement = container.favicons.get(cacheElementIndex);
                 if (cacheElement.invalidated) {
                     return null;
                 }
                 // If we found exactly what we wanted - we're done.
                 if (cacheElement.imageSize == targetSize) {
                     setMostRecentlyUsedWithinRead(cacheElement);
                     return cacheElement.faviconPayload;
                 }
             } else {
                 // We requested an image larger than all primaries. Set the element to start the search
                 // from to the element beyond the end of the array, so the search runs backwards.
                 cacheElementIndex = container.favicons.size();
             }
             // We did not find exactly what we wanted, but now have set cacheElementIndex to the index
             // where what we want should live in the list. We now request the next least larger primary
             // from the cache. We will downscale this to our target size.
             // If there is no such primary, we'll upscale the next least smaller one instead.
             cacheElement = container.getNextPrimary(cacheElementIndex);
             if (cacheElement == null) {
                 // The primary has been invalidated! Fail! Need to get it back from the database.
                 return null;
             }
             if (targetSize == -1) {
                 // We got the biggest primary, so that's what we'll return.
                 return cacheElement.faviconPayload;
             }
             // Scaling logic...
             Bitmap largestElementBitmap = cacheElement.faviconPayload;
             int largestSize = cacheElement.imageSize;
             if (largestSize >= targetSize) {
                 // The largest we have is larger than the target - downsize to target.
                 newBitmap = Bitmap.createScaledBitmap(largestElementBitmap, targetSize, targetSize, true);
             } else {
                 // Our largest primary is smaller than the desired size. Upscale by a maximum of 2x.
                 // largestSize now reflects the maximum size we can upscale to.
                 largestSize *= 2;
                 if (largestSize >= targetSize) {
                     // Perfect! We can upscale by less than 2x and reach the needed size. Do it.
                     newBitmap = Bitmap.createScaledBitmap(largestElementBitmap, targetSize, targetSize, true);
                 } else {
                     // We don't have enough information to make the target size look nonterrible. Best effort:
                     newBitmap = Bitmap.createScaledBitmap(largestElementBitmap, largestSize, largestSize, true);
                     shouldComputeColour = true;
                 }
             }
         } catch (Exception unhandled) {
             // Handle any exception thrown and return the locks to a sensible state.
             // Flag to prevent finally from doubly-unlocking.
             Log.e(LOGTAG, "FaviconCache exception!", unhandled);
             return null;
         } finally {
             finishRead();
         }
         startWrite();
         try {
             if (shouldComputeColour) {
                 // And since we failed, we'll need the dominant colour.
                 container.ensureDominantColor();
             }
             // While the image might not actually BE that size, we set the size field to the target
             // because this is the best image you can get for a request of that size using the Favicon
             // information provided by this website.
             // This way, subsequent requests hit straight away.
             FaviconCacheElement newElement = container.addSecondary(newBitmap, targetSize);
             if (!wasPermanent) {
                 if (setMostRecentlyUsedWithinWrite(newElement)) {
                     currentSize.addAndGet(newElement.sizeOf());
                 }
             }
         } finally {
             finishWrite();
         }
         return newBitmap;
     }
     /**
      * Query the cache for the dominant colour stored for the Favicon URL provided, if any.
      *
      * @param key The URL of the Favicon for which a dominant colour is desired.
      * @return The cached dominant colour, or null if none is cached.
      */
     public int getDominantColor(String key) {
         startRead();
         try {
             FaviconsForURL element = permanentBackingMap.get(key);
             if (element == null) {
                 element = backingMap.get(key);
             }
             if (element == null) {
                 Log.w(LOGTAG, "Cannot compute dominant color of non-cached favicon. Cache fullness " +
                               currentSize.get() + '/' + maxSizeBytes);
                 return 0xFFFFFF;
             }
             return element.ensureDominantColor();
         } finally {
             finishRead();
         }
     }
     /**
      * Remove all payloads stored in the given container from the LRU cache.
      * Must be called while holding the write lock.
      *
      * @param wasRemoved The container to purge from the cache.
      */
     private void recordRemoved(FaviconsForURL wasRemoved) {
         // If there was an existing value, strip it from the insertion-order cache.
         if (wasRemoved == null) {
             return;
         }
         int sizeRemoved = 0;
         for (FaviconCacheElement e : wasRemoved.favicons) {
             sizeRemoved += e.sizeOf();
             ordering.remove(e);
         }
         currentSize.addAndGet(-sizeRemoved);
     }
     private Bitmap produceCacheableBitmap(Bitmap favicon) {
         // Never cache the default Favicon, or the null Favicon.
         if (favicon == Favicons.defaultFavicon || favicon == null) {
             return null;
         }
         // Some sites serve up insanely huge Favicons (Seen 512x512 ones...)
         // While we want to cache nice big icons, we apply a limit based on screen density for the
         // sake of space.
         if (favicon.getWidth() > maxCachedWidth) {
             return Bitmap.createScaledBitmap(favicon, maxCachedWidth, maxCachedWidth, true);
         }
         return favicon;
     }
     /**
      * Set an existing element as the most recently used element. Intended for use from read transactions. While
      * write transactions may safely use this method, it will perform slightly worse than its unsafe counterpart below.
      *
      * @param element The element that is to become the most recently used one.
      * @return true if this element already existed in the list, false otherwise. (Useful for preventing multiple-insertion.)
      */
     private boolean setMostRecentlyUsedWithinRead(FaviconCacheElement element) {
         reorderingSemaphore.acquireUninterruptibly();
         try {
             boolean contained = ordering.remove(element);
             ordering.offer(element);
             return contained;
         } finally {
             reorderingSemaphore.release();
         }
     }
     /**
      * Functionally equivalent to setMostRecentlyUsedWithinRead, but operates without taking the reordering semaphore.
      * Only safe for use when called from a write transaction, or there is a risk of concurrent modification.
      *
      * @param element The element that is to become the most recently used one.
      * @return true if this element already existed in the list, false otherwise. (Useful for preventing multiple-insertion.)
      */
     private boolean setMostRecentlyUsedWithinWrite(FaviconCacheElement element) {
         boolean contained = ordering.remove(element);
         ordering.offer(element);
         return contained;
     }
     /**
      * Add the provided bitmap to the cache as the only available primary for this URL.
      * Should never be called with scaled Favicons. The input is assumed to be an unscaled Favicon.
      *
      * @param faviconURL The URL of the Favicon being stored.
      * @param aFavicon The Favicon to store.
      */
     public void putSingleFavicon(String faviconURL, Bitmap aFavicon) {
         Bitmap favicon = produceCacheableBitmap(aFavicon);
         if (favicon == null) {
             return;
         }
         // Create a fresh container for the favicons associated with this URL. Allocate extra slots
         // in the underlying ArrayList in case multiple secondary favicons are later created.
         // Currently set to the number of favicon sizes used in the UI, plus 1, at time of writing.
         // Ought to be  tuned as things change for maximal performance.
         FaviconsForURL toInsert = new FaviconsForURL(NUM_FAVICON_SIZES);
         // Create the cache element for the single element we are inserting, and configure it.
         FaviconCacheElement newElement = toInsert.addPrimary(favicon);
         startWrite();
         try {
             // Set the new element as the most recently used one.
             setMostRecentlyUsedWithinWrite(newElement);
             currentSize.addAndGet(newElement.sizeOf());
             // Update the value in the LruCache...
             FaviconsForURL wasRemoved;
             wasRemoved = backingMap.put(faviconURL, toInsert);
             recordRemoved(wasRemoved);
         } finally {
             finishWrite();
         }
         cullIfRequired();
     }
     /**
      * Set the collection of primary favicons for the given URL to the provided collection of bitmaps.
      *
      * @param faviconURL The URL from which the favicons originate.
      * @param favicons A List of favicons decoded from this URL.
      * @param permanently If true, the added favicons are never subject to eviction.
      */
     public void putFavicons(String faviconURL, Iterator<Bitmap> favicons, boolean permanently) {
         // We don't know how many icons we'll have - let's just take a guess.
         FaviconsForURL toInsert = new FaviconsForURL(5 * NUM_FAVICON_SIZES);
         int sizeGained = 0;
         while (favicons.hasNext()) {
             Bitmap favicon = produceCacheableBitmap(favicons.next());
             if (favicon == null) {
                 continue;
             }
             FaviconCacheElement newElement = toInsert.addPrimary(favicon);
             sizeGained += newElement.sizeOf();
         }
         startWrite();
         try {
             if (permanently) {
                 permanentBackingMap.put(faviconURL, toInsert);
                 return;
             }
             for (FaviconCacheElement newElement : toInsert.favicons) {
                 setMostRecentlyUsedWithinWrite(newElement);
             }
             // In the event this insertion is being made to a key that already held a value, the subsequent recordRemoved
             // call will subtract the size of the old value, preventing double-counting.
             currentSize.addAndGet(sizeGained);
             // Update the value in the LruCache...
             recordRemoved(backingMap.put(faviconURL, toInsert));
         } finally {
             finishWrite();
         }
         cullIfRequired();
     }
     /**
      * If cache too large, drop stuff from the cache to get the size back into the acceptable range.
      * Otherwise, do nothing.
      */
     private void cullIfRequired() {
         Log.d(LOGTAG, "Favicon cache fullness: " + currentSize.get() + '/' + maxSizeBytes);
         if (currentSize.get() <= maxSizeBytes) {
             return;
         }
         startWrite();
         try {
             while (currentSize.get() > maxSizeBytes) {
                 // Cull the least recently used element.
                 FaviconCacheElement victim;
                 victim = ordering.poll();
                 currentSize.addAndGet(-victim.sizeOf());
                 victim.onEvictedFromCache();
                 Log.d(LOGTAG, "After cull: " + currentSize.get() + '/' + maxSizeBytes);
             }
         } finally {
             finishWrite();
         }
     }
     /**
      * Purge all elements from the FaviconCache. Handy if you want to reclaim some memory.
      */
     public void evictAll() {
         startWrite();
         // Note that we neither clear, nor track the size of, the permanent map.
         try {
             currentSize.set(0);
             backingMap.clear();
             ordering.clear();
         } finally {
             finishWrite();
         }
     }
 }

The Tor Browser / annotate

mobile/android/base/favicons/cache/FaviconCache.java@b8a032363ba2 (annotated)

mobile/android/base/favicons/cache/FaviconCache.java