Gregory Chanan created SOLR-8152:
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Summary: Overseer Task Processor/Queue can miss responses, leading
to timeouts
Key: SOLR-8152
URL: https://issues.apache.org/jira/browse/SOLR-8152
Project: Solr
Issue Type: Bug
Components: SolrCloud
Reporter: Gregory Chanan
Assignee: Gregory Chanan
I noticed some jenkins reports of timeouts in the
TestConfigSetsAPIExclusivityTest, which seemed strange given the amount of work
to be done is small and the timeout generous at 300 seconds.
I added some statistics gathering and started beasting the test and sure
enough, some tests reported tasks taking slightly more than 300 seconds, while
most tests ran with a maximum task run of less than a second. This suggested
something was hanging until the timeout.
Some investigation lead to this code:
https://github.com/apache/lucene-solr/blob/80a73535b20debb1717c6f7f11e08fc311833c88/solr/core/src/java/org/apache/solr/cloud/OverseerTaskQueue.java#L179-L194
There appears to be a few issues here:
{code}
String path = createData(dir + "/" + PREFIX, data,
CreateMode.PERSISTENT_SEQUENTIAL);
String watchID = createData(
dir + "/" + response_prefix + path.substring(path.lastIndexOf("-") +
1),
null, CreateMode.EPHEMERAL);
Object lock = new Object();
LatchWatcher watcher = new LatchWatcher(lock);
synchronized (lock) {
if (zookeeper.exists(watchID, watcher, true) != null) {
watcher.await(timeout);
}
}
{code}
For one, the request object is created before the response object. If the
request is quickly picked up and processed, two things can happen:
1) The response is written before the watch is set, which means we wait until
the timeout even though the response is ready. This will still pass the test
because the response is available, the client will just wait needlessly.
2) The response is attempted to be written before the response node is even
created. The fact that the response node doesn't exist is ignored:
https://github.com/apache/lucene-solr/blob/80a73535b20debb1717c6f7f11e08fc311833c88/solr/core/src/java/org/apache/solr/cloud/OverseerTaskQueue.java#L92-L94
In this case, the task is processed but the client will actually see a failure
because there is no response.
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