Error writing index in case of multiple rating files

[agazzarini]

Describe the bug
In case multiple rating file are found within the raings folder, RRE loads the corresponding corpora multiple times (once for each ratings file). In these scenarios an exception is thrown, probably related with the fact multiple writers try to index data.

To Reproduce
Steps to reproduce the behavior:

1. Add multiple files in the ratings folder
2. Run rre:evaluate

Expected behavior
The evaluation should happen separately for each ratings file.

Observed behavior
The evaluation happens only for the first rating file; after that, the exception below is thrown and the evaluation process stops.

Stacktrace

[ERROR] org.apache.solr.common.SolrException: /private/var/folders/g4/3splwc6j543551z38mnd86zw0000gn/T/1621406750085/main_v1.0/data/index/write.lock
	at org.apache.solr.handler.RequestHandlerBase.handleRequest(RequestHandlerBase.java:217)
	at org.apache.solr.core.SolrCore.execute(SolrCore.java:2576)
	at org.apache.solr.client.solrj.embedded.EmbeddedSolrServer.request(EmbeddedSolrServer.java:227)
	at org.apache.solr.client.solrj.SolrRequest.process(SolrRequest.java:211)
	at io.sease.rre.search.api.impl.ApacheSolr.load(ApacheSolr.java:129)
	at io.sease.rre.core.Engine.lambda$prepareData$15(Engine.java:471)
	at java.base/java.util.stream.ForEachOps$ForEachOp$OfRef.accept(ForEachOps.java:183)
	at java.base/java.util.stream.ReferencePipeline$11$1.accept(ReferencePipeline.java:442)
	at java.base/java.util.ArrayList.forEach(ArrayList.java:1540)
	at java.base/java.util.stream.SortedOps$RefSortingSink.end(SortedOps.java:395)
	at java.base/java.util.stream.Sink$ChainedReference.end(Sink.java:258)
	at java.base/java.util.stream.Sink$ChainedReference.end(Sink.java:258)
	at java.base/java.util.stream.Sink$ChainedReference.end(Sink.java:258)
	at java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:485)
	at java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:474)
	at java.base/java.util.stream.ForEachOps$ForEachOp.evaluateSequential(ForEachOps.java:150)
	at java.base/java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateSequential(ForEachOps.java:173)
	at java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
	at java.base/java.util.stream.ReferencePipeline.forEach(ReferencePipeline.java:497)
	at io.sease.rre.core.Engine.prepareData(Engine.java:469)
	at io.sease.rre.core.Engine.lambda$evaluate$5(Engine.java:236)

[agazzarini]

Temporary workaround: add all queries/query groups/topics in a single ratings file.