[8.18] Remove DocumentSubsetBitsetCache locking (#133681) (#133707)

docs/changelog/133681.yaml

@@ -0,0 +1,6 @@
+pr: 133681
+summary: Remove `DocumentSubsetBitsetCache` locking
+area: Authorization
+type: bug
+issues:
+ - 132842

x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/security/authz/accesscontrol/DocumentSubsetBitsetCache.java

@@ -31,26 +31,21 @@
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.common.unit.ByteSizeValue;
 import org.elasticsearch.common.util.concurrent.ConcurrentCollections;
-import org.elasticsearch.common.util.concurrent.ReleasableLock;
 import org.elasticsearch.core.Nullable;
 import org.elasticsearch.core.TimeValue;
 import org.elasticsearch.lucene.util.BitSets;
 import org.elasticsearch.lucene.util.MatchAllBitSet;
-import org.elasticsearch.threadpool.ThreadPool;
 
 import java.io.Closeable;
 import java.io.IOException;
 import java.util.List;
 import java.util.Map;
 import java.util.Objects;
-import java.util.Optional;
 import java.util.Set;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ExecutionException;
-import java.util.concurrent.ExecutorService;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicLong;
-import java.util.concurrent.locks.ReentrantReadWriteLock;
 
 /**
  * This is a cache for {@link BitSet} instances that are used with the {@link DocumentSubsetReader}.
@@ -81,6 +76,8 @@
  */
 public final class DocumentSubsetBitsetCache implements IndexReader.ClosedListener, Closeable, Accountable {
 
+    private static final Logger logger = LogManager.getLogger(DocumentSubsetBitsetCache.class);
+
     /**
      * The TTL defaults to 2 hours. We default to a large cache size ({@link #CACHE_SIZE_SETTING}), and aggressively
      * expire unused entries so that the cache does not hold on to memory unnecessarily.
@@ -102,40 +99,15 @@ public final class DocumentSubsetBitsetCache implements IndexReader.ClosedListen
 
     private static final BitSet NULL_MARKER = new FixedBitSet(0);
 
-    private static final Logger logger = LogManager.getLogger(DocumentSubsetBitsetCache.class);
-
-    /**
-     * When a {@link BitSet} is evicted from {@link #bitsetCache}, we need to also remove it from {@link #keysByIndex}.
-     * We use a {@link ReentrantReadWriteLock} to control atomicity here - the "read" side represents potential insertions to the
-     * {@link #bitsetCache}, the "write" side represents removals from {@link #keysByIndex}.
-     * The risk (that {@link Cache} does not provide protection for) is that an entry is removed from the cache, and then immediately
-     * re-populated, before we process the removal event. To protect against that we need to check the state of the {@link #bitsetCache}
-     * but we need exclusive ("write") access while performing that check and updating the values in {@link #keysByIndex}.
-     */
-    private final ReleasableLock cacheEvictionLock;
-    private final ReleasableLock cacheModificationLock;
-    private final ExecutorService cleanupExecutor;
-
     private final long maxWeightBytes;
     private final Cache<BitsetCacheKey, BitSet> bitsetCache;
     private final Map<IndexReader.CacheKey, Set<BitsetCacheKey>> keysByIndex;
     private final AtomicLong cacheFullWarningTime;
 
-    public DocumentSubsetBitsetCache(Settings settings, ThreadPool threadPool) {
-        this(settings, threadPool.executor(ThreadPool.Names.GENERIC));
-    }
-
     /**
      * @param settings The global settings object for this node
-     * @param cleanupExecutor An executor on which the cache cleanup tasks can be run. Due to the way the cache is structured internally,
-     *                        it is sometimes necessary to run an asynchronous task to synchronize the internal state.
      */
-    protected DocumentSubsetBitsetCache(Settings settings, ExecutorService cleanupExecutor) {
-        final ReentrantReadWriteLock readWriteLock = new ReentrantReadWriteLock();
-        this.cacheEvictionLock = new ReleasableLock(readWriteLock.writeLock());
-        this.cacheModificationLock = new ReleasableLock(readWriteLock.readLock());
-        this.cleanupExecutor = cleanupExecutor;
-
+    public DocumentSubsetBitsetCache(Settings settings) {
         final TimeValue ttl = CACHE_TTL_SETTING.get(settings);
         this.maxWeightBytes = CACHE_SIZE_SETTING.get(settings).getBytes();
         this.bitsetCache = CacheBuilder.<BitsetCacheKey, BitSet>builder()
@@ -150,8 +122,8 @@ protected DocumentSubsetBitsetCache(Settings settings, ExecutorService cleanupEx
     }
 
     @Override
-    public void onClose(IndexReader.CacheKey ownerCoreCacheKey) {
-        final Set<BitsetCacheKey> keys = keysByIndex.remove(ownerCoreCacheKey);
+    public void onClose(IndexReader.CacheKey indexKey) {
+        final Set<BitsetCacheKey> keys = keysByIndex.remove(indexKey);
         if (keys != null) {
             // Because this Set has been removed from the map, and the only update to the set is performed in a
             // Map#compute call, it should not be possible to get a concurrent modification here.
@@ -163,24 +135,17 @@ public void onClose(IndexReader.CacheKey ownerCoreCacheKey) {
      * Cleanup (synchronize) the internal state when an object is removed from the primary cache
      */
     private void onCacheEviction(RemovalNotification<BitsetCacheKey, BitSet> notification) {
-        final BitsetCacheKey bitsetKey = notification.getKey();
-        final IndexReader.CacheKey indexKey = bitsetKey.index;
-        if (keysByIndex.getOrDefault(indexKey, Set.of()).contains(bitsetKey) == false) {
-            // If the bitsetKey isn't in the lookup map, then there's nothing to synchronize
-            return;
-        }
-        // We push this to a background thread, so that it reduces the risk of blocking searches, but also so that the lock management is
-        // simpler - this callback is likely to take place on a thread that is actively adding something to the cache, and is therefore
-        // holding the read ("update") side of the lock. It is not possible to upgrade a read lock to a write ("eviction") lock, but we
-        // need to acquire that lock here.
-        cleanupExecutor.submit(() -> {
-            try (ReleasableLock ignored = cacheEvictionLock.acquire()) {
-                // it's possible for the key to be back in the cache if it was immediately repopulated after it was evicted, so check
-                if (bitsetCache.get(bitsetKey) == null) {
-                    // key is no longer in the cache, make sure it is no longer in the lookup map either.
-                    Optional.ofNullable(keysByIndex.get(indexKey)).ifPresent(set -> set.remove(bitsetKey));
-                }
-            }
+        final BitsetCacheKey cacheKey = notification.getKey();
+        final IndexReader.CacheKey indexKey = cacheKey.indexKey;
+        // the key is *probably* no longer in the cache, so make sure it is no longer in the lookup map.
+        // note: rather than locking (which destroys our throughput), we're erring on the side of tidying the keysByIndex
+        // structure even if some other racing thread has already added a new bitset into the cache for this same key.
+        // the keysByIndex structure is used in onClose (our notification from lucene that a segment has become inaccessible),
+        // so we might end up failing to *eagerly* invalidate a bitset -- the consequence of that would be temporarily higher
+        // memory use (the bitset will not be accessed, and it will still be invalidated eventually for size or ttl reasons).
+        keysByIndex.computeIfPresent(indexKey, (ignored, keys) -> {
+            keys.remove(cacheKey);
+            return keys.isEmpty() ? null : keys;
         });
     }
 
@@ -231,41 +196,39 @@ public BitSet getBitSet(final Query query, final LeafReaderContext context) thro
         final IndexReader.CacheKey indexKey = coreCacheHelper.getKey();
         final BitsetCacheKey cacheKey = new BitsetCacheKey(indexKey, query);
 
-        try (ReleasableLock ignored = cacheModificationLock.acquire()) {
-            final BitSet bitSet = bitsetCache.computeIfAbsent(cacheKey, ignore1 -> {
-                // This ensures all insertions into the set are guarded by ConcurrentHashMap's atomicity guarantees.
-                keysByIndex.compute(indexKey, (ignore2, set) -> {
-                    if (set == null) {
-                        set = ConcurrentCollections.newConcurrentSet();
-                    }
-                    set.add(cacheKey);
-                    return set;
-                });
-                final BitSet result = computeBitSet(query, context);
-                if (result == null) {
-                    // A cache loader is not allowed to return null, return a marker object instead.
-                    return NULL_MARKER;
-                }
-                final long bitSetBytes = result.ramBytesUsed();
-                if (bitSetBytes > this.maxWeightBytes) {
-                    logger.warn(
-                        "built a DLS BitSet that uses [{}] bytes; the DLS BitSet cache has a maximum size of [{}] bytes;"
-                            + " this object cannot be cached and will need to be rebuilt for each use;"
-                            + " consider increasing the value of [{}]",
-                        bitSetBytes,
-                        maxWeightBytes,
-                        CACHE_SIZE_SETTING.getKey()
-                    );
-                } else if (bitSetBytes + bitsetCache.weight() > maxWeightBytes) {
-                    maybeLogCacheFullWarning();
+        final BitSet bitSet = bitsetCache.computeIfAbsent(cacheKey, ignore1 -> {
+            // This ensures all insertions into the set are guarded by ConcurrentHashMap's atomicity guarantees.
+            keysByIndex.compute(indexKey, (ignore2, keys) -> {
+                if (keys == null) {
+                    keys = ConcurrentCollections.newConcurrentSet();
                 }
-                return result;
+                keys.add(cacheKey);
+                return keys;
             });
-            if (bitSet == NULL_MARKER) {
-                return null;
-            } else {
-                return bitSet;
+            final BitSet result = computeBitSet(query, context);
+            if (result == null) {
+                // A cache loader is not allowed to return null, return a marker object instead.
+                return NULL_MARKER;
             }
+            final long bitSetBytes = result.ramBytesUsed();
+            if (bitSetBytes > this.maxWeightBytes) {
+                logger.warn(
+                    "built a DLS BitSet that uses [{}] bytes; the DLS BitSet cache has a maximum size of [{}] bytes;"
+                        + " this object cannot be cached and will need to be rebuilt for each use;"
+                        + " consider increasing the value of [{}]",
+                    bitSetBytes,
+                    maxWeightBytes,
+                    CACHE_SIZE_SETTING.getKey()
+                );
+            } else if (bitSetBytes + bitsetCache.weight() > maxWeightBytes) {
+                maybeLogCacheFullWarning();
+            }
+            return result;
+        });
+        if (bitSet == NULL_MARKER) {
+            return null;
+        } else {
+            return bitSet;
         }
     }
 
@@ -323,11 +286,11 @@ public Map<String, Object> usageStats() {
     }
 
     private static class BitsetCacheKey {
-        final IndexReader.CacheKey index;
+        final IndexReader.CacheKey indexKey;
         final Query query;
 
-        private BitsetCacheKey(IndexReader.CacheKey index, Query query) {
-            this.index = index;
+        private BitsetCacheKey(IndexReader.CacheKey indexKey, Query query) {
+            this.indexKey = indexKey;
             this.query = query;
         }
 
@@ -340,41 +303,59 @@ public boolean equals(Object other) {
                 return false;
             }
             final BitsetCacheKey that = (BitsetCacheKey) other;
-            return Objects.equals(this.index, that.index) && Objects.equals(this.query, that.query);
+            return Objects.equals(this.indexKey, that.indexKey) && Objects.equals(this.query, that.query);
         }
 
         @Override
         public int hashCode() {
-            return Objects.hash(index, query);
+            return Objects.hash(indexKey, query);
         }
 
         @Override
         public String toString() {
-            return getClass().getSimpleName() + "(" + index + "," + query + ")";
+            return getClass().getSimpleName() + "(" + indexKey + "," + query + ")";
         }
     }
 
     /**
-     * This method verifies that the two internal data structures ({@link #bitsetCache} and {@link #keysByIndex}) are consistent with one
-     * another. This method is only called by tests.
+     * This test-only method verifies that the two internal data structures ({@link #bitsetCache} and {@link #keysByIndex}) are consistent
+     * with one another.
      */
+    // visible for testing
     void verifyInternalConsistency() {
-        this.bitsetCache.keys().forEach(bck -> {
-            final Set<BitsetCacheKey> set = this.keysByIndex.get(bck.index);
-            if (set == null) {
-                throw new IllegalStateException(
-                    "Key [" + bck + "] is in the cache, but there is no entry for [" + bck.index + "] in the lookup map"
-                );
-            }
-            if (set.contains(bck) == false) {
+        verifyInternalConsistencyCacheToKeys();
+        verifyInternalConsistencyKeysToCache();
+    }
+
+    /**
+     * This test-only method iterates over the {@link #bitsetCache} and checks that {@link #keysByIndex} is consistent with it.
+     */
+    // visible for testing
+    void verifyInternalConsistencyCacheToKeys() {
+        bitsetCache.keys().forEach(cacheKey -> {
+            final Set<BitsetCacheKey> keys = keysByIndex.get(cacheKey.indexKey);
+            if (keys == null || keys.contains(cacheKey) == false) {
                 throw new IllegalStateException(
-                    "Key [" + bck + "] is in the cache, but the lookup entry for [" + bck.index + "] does not contain that key"
+                    "Key [" + cacheKey + "] is in the cache, but the lookup entry for [" + cacheKey.indexKey + "] does not contain that key"
                 );
             }
         });
-        this.keysByIndex.values().stream().flatMap(Set::stream).forEach(bck -> {
-            if (this.bitsetCache.get(bck) == null) {
-                throw new IllegalStateException("Key [" + bck + "] is in the lookup map, but is not in the cache");
+    }
+
+    /**
+     * This test-only method iterates over the {@link #keysByIndex} and checks that {@link #bitsetCache} is consistent with it.
+     */
+    // visible for testing
+    void verifyInternalConsistencyKeysToCache() {
+        keysByIndex.forEach((indexKey, keys) -> {
+            if (keys == null || keys.isEmpty()) {
+                throw new IllegalStateException("The lookup entry for [" + indexKey + "] is null or empty");
+            } else {
+                keys.forEach(cacheKey -> {
+                    if (bitsetCache.get(cacheKey) == null) {
+                        throw new IllegalStateException("Key [" + cacheKey + "] is in the lookup map, but is not in the cache");
+                    }
+                });
             }
         });
     }