[GitHub] [druid] gianm commented on a change in pull request #9098: S3: Improvements to prefix listing (including fix for an infinite loop)

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

gianm commented on a change in pull request #9098: S3: Improvements to prefix listing (including fix for an infinite loop)
URL: https://github.com/apache/druid/pull/9098#discussion_r362031593
 
 

 ##########
 File path: extensions-core/s3-extensions/src/main/java/org/apache/druid/storage/s3/LazyObjectSummariesIterator.java
 ##########
 @@ -0,0 +1,162 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.apache.druid.storage.s3;
+
+import com.amazonaws.services.s3.model.ListObjectsV2Request;
+import com.amazonaws.services.s3.model.ListObjectsV2Result;
+import com.amazonaws.services.s3.model.S3ObjectSummary;
+import org.apache.druid.java.util.common.RE;
+
+import java.net.URI;
+import java.util.Iterator;
+import java.util.NoSuchElementException;
+
+/**
+ * Iterator class used by {@link S3Utils#lazyObjectSummaryIterator}.
+ *
+ * Walks a set of prefixes, returning all objects underneath them except for directory placeholders.
+ */
+public class LazyObjectSummariesIterator implements Iterator<S3ObjectSummary>
+{
+  private final ServerSideEncryptingAmazonS3 s3Client;
+  private final Iterator<URI> prefixesIterator;
+  private final int maxListingLength;
+
+  private ListObjectsV2Request request;
+  private ListObjectsV2Result result;
+  private Iterator<S3ObjectSummary> objectSummaryIterator;
+  private S3ObjectSummary currentObjectSummary;
+
+  LazyObjectSummariesIterator(
+      final ServerSideEncryptingAmazonS3 s3Client,
+      final Iterable<URI> prefixes,
+      final int maxListingLength
+  )
+  {
+    this.s3Client = s3Client;
+    this.prefixesIterator = prefixes.iterator();
+    this.maxListingLength = maxListingLength;
+
+    prepareNextRequest();
+    fetchNextBatch();
+    advanceObjectSummary();
+  }
+
+  private void prepareNextRequest()
+  {
+    final URI currentUri = prefixesIterator.next();
+    final String currentBucket = currentUri.getAuthority();
+    final String currentPrefix = S3Utils.extractS3Key(currentUri);
+
+    request = new ListObjectsV2Request()
+        .withBucketName(currentBucket)
+        .withPrefix(currentPrefix)
+        .withMaxKeys(maxListingLength);
+  }
+
+  private void fetchNextBatch()
+  {
+    try {
+      result = S3Utils.retryS3Operation(() -> s3Client.listObjectsV2(request));
+      request.setContinuationToken(result.getNextContinuationToken());
+      objectSummaryIterator = result.getObjectSummaries().iterator();
+    }
+    catch (Exception e) {
+      throw new RE(
+          e,
+          "Failed to get object summaries from S3 bucket[%s], prefix[%s]",
+          request.getBucketName(),
+          request.getPrefix()
+      );
+    }
+  }
+
+  /**
+   * Advance objectSummaryIterator to the next non-placeholder, updating "currentObjectSummary".
+   */
+  private void advanceObjectSummary()
+  {
+    while (objectSummaryIterator.hasNext() || result.isTruncated() || prefixesIterator.hasNext()) {
+      while (objectSummaryIterator.hasNext()) {
+        currentObjectSummary = objectSummaryIterator.next();
+
+        if (!isDirectoryPlaceholder(currentObjectSummary)) {
+          return;
+        }
+      }
+
+      // Exhausted "objectSummaryIterator" without finding a non-placeholder.
+      if (result.isTruncated()) {
+        fetchNextBatch();
+      } else if (prefixesIterator.hasNext()) {
+        prepareNextRequest();
+        fetchNextBatch();
+      }
+    }
+
+    // Truly nothing left to read.
+    currentObjectSummary = null;
+  }
+
+  @Override
+  public boolean hasNext()
+  {
+    return currentObjectSummary != null;
+  }
+
+  @Override
+  public S3ObjectSummary next()
+  {
+    if (currentObjectSummary == null) {
+      throw new NoSuchElementException();
+    }
+
+    final S3ObjectSummary retVal = currentObjectSummary;
+    advanceObjectSummary();
+    return retVal;
+  }
 
 Review comment:
   I'll move it, because I agree with this particular point, but IMO style nits should either be encoded into an automated style checker or written down somewhere centrally agreed-upon. Otherwise they can create friction in the review process. Maybe we can come up with a nice way as a community to handle this.

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