[LUCENE-2355] Refactor Directory/Multi/SegmentReader creation/reopening/cloning/closing

• Type: Improvement
• Status: Open
• Priority: Major
• Resolution: Unresolved
• Component/s: core/index
• Lucene Fields: New
• Assignee: Unassigned
• Reporter: Earwin Burrfoot
• Created: 28/Mar/10 19:39
• Updated: 28/Aug/22 12:23

Description

*Reader lifecycle evolved over time to become some heavily tangled mess. It's hard to understand what's going on there, it's even harder to add some fields/logic while ensuring that all possible code paths preserve these fields/interact with the logic properly. While some of said mess is justified by the task at hand, a big part is just badly done copypaste and can be removed.

I am currently refactoring this and intended to open an issue with a working patch, but the task winded up somewhat bigger than I expected, so I'm opening it earlier to track stuff encountered/changed/fixed.
The list is by no means exhaustive.

• an iteration to create SRs is copypasted several times, one of them (IW) with wrong iteration bound
• it is also overly complex and can be folded for create/reopen cases
• readers sent to IndexReaderWarmer are termindexless/docstoreless on some occasions
• it is possible to clone() your way to readwrite NRT reader
• IndexDeletionPolicy is not always preserved through clones/reopens
• cloned readers share CoreReaders and, consequently, updated termsIndex/docStores
• threadlocal versions of fieldsReader/termsVector are bound to SR, not CoreReaders and thus are recreated on clone/reopen
• double-initialization for some fields (someone got lost and did this to be sure I guess), stupid assert checks ( qwe = new(); assert qwe != null )
• SR is not always recreated when compound status of underlying segment changes
• deleting already deleted doc marks deletions dirty and rewrites them
• lots of synchronization is done around Reader, while it can be narrowed down to norms/deletions/whatever

I did some structural modifications:

• CompositeReader extracts common code from DirectoryReader and MultiReader (complete)
• ReadonlyDirectoryReader and ReadonlySegmentReader are dead, MutableD/SReaders are introduced and carry all modification logic/fields (DR complete, SR in progress)
• WriterBackedReader encapsulates NRT reader logic (complete)
• CoreReaders split into CoreReaders, DocStores, TermInfos. All of these are immutable and SR is cloned when you need to change its mode (in progress)

Issue Links

• relates to: LUCENE-2802 DirectoryReader ignores NRT SegmentInfos in #isOptimized()