David Turner created LUCENE-9138:

             Summary: Behaviour of concurrent calls to IndexInput#clone is 
                 Key: LUCENE-9138
                 URL: https://issues.apache.org/jira/browse/LUCENE-9138
             Project: Lucene - Core
          Issue Type: Improvement
          Components: core/store
    Affects Versions: 8.4
            Reporter: David Turner

I think this is a documentation issue, rather than anything actually wrong, but 
need expert guidance to propose a fix.

The Javadocs for {{IndexInput#clone}} warn that it is not thread safe:
   * <p>This method is NOT thread safe, so if the current \{@code IndexInput}
   * is being used by one thread while \{@code clone} is called by another,
   * disaster could strike.
  public IndexInput clone() {
However, there are places where {{clone()}} may be called concurrently. For 
instance I believe {{SegmentReader#getFieldsReader}} clones an {{IndexInput}} 
and requires no extra synchronization. I think this comment is supposed to mean 
that you should not {{clone()}} an {{IndexInput}} while you're _reading or 
seeking from it_ concurrently, but the precise guarantees aren't totally clear.
Furthermore there's no mention of the threadsafety of {{slice()}}. and there 
seem to be similar concurrent usages of it in e.g. 
{{Lucene80DocValuesProducer}}. Does this have the same guarantees as 

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