[LUCENE-3866] Make CompositeReader.getSequentialSubReaders() and the corresponding IndexReaderContext methods return unmodifiable List<R extends IndexReader>

• Type: Improvement
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 4.0-ALPHA, 6.0
• Lucene Fields: New
• Assignee: Uwe Schindler
• Reporter: Uwe Schindler
• Created: 13/Mar/12 09:27
• Updated: 28/Aug/22 13:10
• Resolved: 19/Jun/12 07:39

Description

Since Lucene 2.9 we have the method getSequentialSubReader() returning IndexReader[]. Based on hardly-to-debug errors in user's code, Robert and me realized that returning an array from a public API is an anti-pattern. If the array is intended to be modifiable (like byte[] in terms,...), it is fine to use arrays in public APIs, but not, if the array must be protected from modification. As IndexReaders are 100% unmodifiable in trunk code (no deletions,...), the only possibility to corrumpt the reader is by modifying the array returned by getSequentialSubReaders(). We should prevent this.

The same theoretically applies to FieldCache, too - but the party that is afraid of performance problems is too big to fight against that

For getSequentialSubReaders there is no performance problem at all. The binary search of reader-ids inside BaseCompositeReader can still be done with the internal protected array, but public APIs should expose only a unmodifiable List. The same applies to leaves() and children() in IndexReaderContext. This change to list would also allow to make CompositeReader and CompositeReaderContext Iterable<R extends IndexReader>, so some loops would look nice.

Issue Links

• blocks: LUCENE-4152 add one-syllable method to IndexReader enumerate subreaders
• blocks: LUCENE-4155 Move & hide ReaderSlice and BitSlice classes (and possibly others) to oal.index package; move ReaderUtil to oal.index