ExportWriter should support sorting on more than 4 sort fields

I am a user of Streaming and I am currently trying to use rollups on an 10 dimensional document.
I am unable to get correct results on this query as I am bounded by the limitation of the export handler which supports only 4 sort fields.
I do not see why this needs to be the case, as it could very well be 10 or 20.
My current needs would be satisfied with 10, but one would want to ask why can't it be any decent integer n, beyond which we know performance degrades, but even then it should be caveat emptor.

varunthacker

Code Link:
https://github.com/apache/lucene-solr/blob/19db1df81a18e6eb2cce5be973bf2305d606a9f8/solr/core/src/java/org/apache/solr/handler/ExportWriter.java#L455

Error
null:java.io.IOException: A max of 4 sorts can be specified
at org.apache.solr.handler.ExportWriter.getSortDoc(ExportWriter.java:452)
at org.apache.solr.handler.ExportWriter.writeDocs(ExportWriter.java:228)
at org.apache.solr.handler.ExportWriter.lambda$null$1(ExportWriter.java:219)
at org.apache.solr.common.util.JavaBinCodec.writeIterator(JavaBinCodec.java:664)
at org.apache.solr.common.util.JavaBinCodec.writeKnownType(JavaBinCodec.java:333)
at org.apache.solr.common.util.JavaBinCodec.writeVal(JavaBinCodec.java:223)
at org.apache.solr.common.util.JavaBinCodec$1.put(JavaBinCodec.java:394)
at org.apache.solr.handler.ExportWriter.lambda$null$2(ExportWriter.java:219)
at org.apache.solr.common.util.JavaBinCodec.writeMap(JavaBinCodec.java:437)
at org.apache.solr.common.util.JavaBinCodec.writeKnownType(JavaBinCodec.java:354)
at org.apache.solr.common.util.JavaBinCodec.writeVal(JavaBinCodec.java:223)
at org.apache.solr.common.util.JavaBinCodec$1.put(JavaBinCodec.java:394)
at org.apache.solr.handler.ExportWriter.lambda$write$3(ExportWriter.java:217)
at org.apache.solr.common.util.JavaBinCodec.writeMap(JavaBinCodec.java:437)
at org.apache.solr.handler.ExportWriter.write(ExportWriter.java:215)
at org.apache.solr.core.SolrCore$3.write(SolrCore.java:2601)
at org.apache.solr.response.QueryResponseWriterUtil.writeQueryResponse(QueryResponseWriterUtil.java:49)
at org.apache.solr.servlet.HttpSolrCall.writeResponse(HttpSolrCall.java:809)
at org.apache.solr.servlet.HttpSolrCall.call(HttpSolrCall.java:538)
at org.apache.solr.servlet.SolrDispatchFilter.doFilter(SolrDispatchFilter.java:361)
at org.apache.solr.servlet.SolrDispatchFilter.doFilter(SolrDispatchFilter.java:305)
at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1691)
at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:582)
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:143)
at org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.java:548)
at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:226)
at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1180)
at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:512)
at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:185)
at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1112)
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:141)
at org.eclipse.jetty.server.handler.ContextHandlerCollection.handle(ContextHandlerCollection.java:213)
at org.eclipse.jetty.server.handler.HandlerCollection.handle(HandlerCollection.java:119)
at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:134)
at org.eclipse.jetty.rewrite.handler.RewriteHandler.handle(RewriteHandler.java:335)
at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:134)
at org.eclipse.jetty.server.Server.handle(Server.java:534)
at org.eclipse.jetty.server.HttpChannel.handle(HttpChannel.java:320)
at org.eclipse.jetty.server.HttpConnection.onFillable(HttpConnection.java:251)
at org.eclipse.jetty.io.AbstractConnection$ReadCallback.succeeded(AbstractConnection.java:273)
at org.eclipse.jetty.io.FillInterest.fillable(FillInterest.java:95)
at org.eclipse.jetty.io.SelectChannelEndPoint$2.run(SelectChannelEndPoint.java:93)
at org.eclipse.jetty.util.thread.strategy.ExecuteProduceConsume.executeProduceConsume(ExecuteProduceConsume.java:303)
at org.eclipse.jetty.util.thread.strategy.ExecuteProduceConsume.produceConsume(ExecuteProduceConsume.java:148)
at org.eclipse.jetty.util.thread.strategy.ExecuteProduceConsume.run(ExecuteProduceConsume.java:136)
at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:671)
at org.eclipse.jetty.util.thread.QueuedThreadPool$2.run(QueuedThreadPool.java:589)
at java.lang.Thread.run(Thread.java:745)