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_r362036658
########## 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; + } + + /** + * Checks if a given object is a directory placeholder and should be ignored. + * + * Adapted from org.jets3t.service.model.StorageObject.isDirectoryPlaceholder(). Does not include the check for + * legacy JetS3t directory placeholder objects, since it is based on content-type, which isn't available in an + * S3ObjectSummary. + */ + private static boolean isDirectoryPlaceholder(final S3ObjectSummary objectSummary) + { + // Recognize "standard" directory place-holder indications used by Amazon's AWS Console and Panic's Transmit. + if (objectSummary.getKey().endsWith("/") && objectSummary.getSize() == 0) { + return true; + } + + // Recognize s3sync.rb directory placeholders by MD5/ETag value. + if ("d66759af42f282e1ba19144df2d405d0".equals(objectSummary.getETag())) { Review comment: It's from `org.jets3t.service.model.StorageObject.isDirectoryPlaceholder` (mentioned in the javadoc for this method). Sources are at https://github.com/mondain/jets3t/blob/master/jets3t/src/main/java/org/jets3t/service/model/StorageObject.java. I did it this way for these two reasons: - Minimal diff from the original function, in case someone wants to compare them visually and see what we changed. - I didn't see a benefit to putting `"d66759af42f282e1ba19144df2d405d0"` into a constant: it won't change, it won't be used in any other locations, and it's adequately explained by the comment above the line it appears in, so the usual benefits of pulling magic values out into constants don't apply. Although actually, maybe this is moot, since it might be reasonable to just remove this block. I don't think we have any particular need to support the `s3sync.rb` style directory placeholders. The other two kinds are much more common. It looks like `s3sync.rb` hasn't been maintained in many years, according to http://s3sync.net/wiki.html. What do you think? ---------------------------------------------------------------- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. For queries about this service, please contact Infrastructure at: [email protected] With regards, Apache Git Services --------------------------------------------------------------------- To unsubscribe, e-mail: [email protected] For additional commands, e-mail: [email protected]
