When XWiki stops, the indexer refuses new entries (an entry being queued at that moment is either processed or refused, never left behind) and processes what is already queued, bounded by the new solr.indexer.shutdownTimeout property (in milliseconds, 30000 by default; 0 drops the queued entries immediately, which is the previous behaviour).
After the timeout, typically with a Solr server which does not answer, the remaining entries are dropped with a warning giving their number, and the indexer stops sending anything to the Solr server; the stop then always waits for the whole timeout. The Solr clients are only guaranteed to be usable while the drain runs.
When the requests fail immediately (a refused connection for example), the queues are processed within the timeout with one error per failed request, followed by a warning giving the number of failed requests.
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