[jira] [Resolved] (SOLR-16871) Race condition for coordinator node init

["Patson Luk (Jira)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/SOLR-16871?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Patson Luk resolved SOLR-16871.
-------------------------------
    Resolution: Fixed

> Race condition for coordinator node init
> ----------------------------------------
>
>                 Key: SOLR-16871
>                 URL: https://issues.apache.org/jira/browse/SOLR-16871
>             Project: Solr
>          Issue Type: Bug
>          Components: SolrCloud
>            Reporter: Patson Luk
>            Priority: Major
>          Time Spent: 3h 50m
>  Remaining Estimate: 0h
>
> From a unit test case [that issue concurrent select queries to coordinator nodes|https://github.com/cowpaths/fullstory-solr/blob/e4226eb8fa2afb01d7615f7faea01f71b144cd58/solr/core/src/test/org/apache/solr/search/TestCoordinatorRole.java#L486], it’s found that there could be 3 race condition issues:
> 1. If multiple concurrent requests find the synthetic collection is not yet created, they might all attempt to create the synthetic collection. This could trigger SolrException on `collection already exists`
> 2. Similarly, if multiple concurrent requests find there’s no replica of the synthetic collection for current node (multiple coordinator node scenario), then CoordinatorHttpSolrCall#addReplica could be invoked multiple times. This should not trigger any exception, but would create multiple replicas for the same node in the synthetic collection
> 3. The existing logic [here|https://github.com/cowpaths/fullstory-solr/blob/6c8531f08301a291478502c262499abed0d5075c/solr/core/src/java/org/apache/solr/servlet/CoordinatorHttpSolrCall.java#L102] assumes if syntheticColl.getReplicas(solrCall.cores.getZkController().getNodeName()) returns non empty result, then the following call in [here|https://github.com/cowpaths/fullstory-solr/blob/6c8531f08301a291478502c262499abed0d5075c/solr/core/src/java/org/apache/solr/servlet/CoordinatorHttpSolrCall.java#L112] should return a core. Unfortunately, the first call can return a non empty list but with a DOWN replica if another request is in the progress of creating such replica. In this case, the solrCall.getCoreByCollection(syntheticCollectionName, isPreferLeader) would call super.getCoreByCollection at [here|https://github.com/cowpaths/fullstory-solr/blob/6c8531f08301a291478502c262499abed0d5075c/solr/core/src/java/org/apache/solr/servlet/CoordinatorHttpSolrCall.java#L69] which would return a null (since super impl only returns active replica). So CoordinatorHttpSolrCall#getCoreByCollection would end up calling CoordinatorHttpSolrCall#getCore , introducing an infinite loop and cause stack overflow



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