[GitHub] [solr] patsonluk commented on a diff in pull request #909: SOLR-16257: `ZkStateReader` changes to avoid race condition between `collectionWatches` and `watchedCollectionStates`

[GitBox <gi...@apache.org>]

patsonluk commented on code in PR #909:
URL: https://github.com/apache/solr/pull/909#discussion_r903157548


##########
solr/solrj/src/java/org/apache/solr/common/cloud/ZkStateReader.java:
##########
@@ -246,6 +244,90 @@ public boolean canBeRemoved() {
     }
   }
 
+  /**
+   * A ConcurrentHashMap of active watcher by collection name
+   *
+   * <p>Each watcher DocCollectionWatch also contains the latest DocCollection (state) observed
+   */
+  private static class DocCollectionWatches
+      extends ConcurrentHashMap<String, DocCollectionWatch<DocCollectionWatcher>> {
+
+    /**
+     * Gets the DocCollection (state) of the collection which the corresponding watch last observed
+     *
+     * @param collection the collection name to get DocCollection on
+     * @return The last observed DocCollection(state). if null, that means there's no such
+     *     collection.
+     */
+    private DocCollection getDocCollection(String collection) {
+      DocCollectionWatch<DocCollectionWatcher> watch = get(collection);
+      return watch != null ? watch.currentDoc : null;
+    }
+
+    /**
+     * Gets the active collections (collections that exist) being watched
+     *
+     * @return an immutable set of active collection names
+     */
+    private Set<String> activeCollections() {
+      return this.entrySet().stream()
+          .filter(
+              (Entry<String, DocCollectionWatch<DocCollectionWatcher>> entry) ->
+                  entry.getValue().currentDoc != null)
+          .map(Entry::getKey)
+          .collect(Collectors.toUnmodifiableSet());
+    }
+
+    /**
+     * Updates the latest observed DocCollection (state) of the {@link DocCollectionWatch} if the
+     * collection is being watched
+     *
+     * @param collection the collection name
+     * @param newState the new DocCollection (state) observed
+     * @return whether an active watch exists for such collection
+     */
+    private boolean updateDocCollection(String collection, DocCollection newState) {
+      DocCollectionWatch<DocCollectionWatcher> watch = get(collection);
+      if (watch != null) {
+        DocCollection oldState = watch.currentDoc;
+        if (oldState == null && newState == null) {
+          // OK, the collection not yet exist in ZK
+        } else if (oldState == null) {
+          if (log.isDebugEnabled()) {
+            log.debug("Add data for [{}] ver [{}]", collection, newState.getZNodeVersion());
+          }
+          watch.currentDoc = newState;
+        } else if (newState == null) {
+          log.debug("Removing cached collection state for [{}]", collection);
+          watch.currentDoc = null;
+        } else { // both new and old states are non-null
+          int oldCVersion =
+              oldState.getPerReplicaStates() == null ? -1 : oldState.getPerReplicaStates().cversion;
+          int newCVersion =
+              newState.getPerReplicaStates() == null ? -1 : newState.getPerReplicaStates().cversion;
+          if (oldState.getZNodeVersion() < newState.getZNodeVersion()
+              || oldCVersion < newCVersion) {
+            watch.currentDoc = newState;
+            if (log.isDebugEnabled()) {
+              log.debug(
+                  "Updating data for [{}] from [{}] to [{}]",
+                  collection,
+                  oldState.getZNodeVersion(),
+                  newState.getZNodeVersion());
+            }
+          }
+        }
+        return true;
+      } else {
+        return false;
+      }
+    }

Review Comment:
   That's a good point and thank you for the suggestion! 😊 
   
   Yes synchronization is necessary to guarantee higher version is being taken where there are 2 concurrent calls with different versions of `newState`  
   
   Just a really minor nit, perhaps we can even remove the `if (watch != null)` check now was it's `computeIfPresent`?



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