index: prevent race by calling ‘CustomInit’ prior setting ‘synced’ flag #27720

1. 
   furszy commented at 4:08 pm on May 22, 2023: member

   Decoupled from #27607.

   Fixed a potential race condition in master (not possible so far) that could become an actual issue soon. Where the index’s CustomAppend method could be called (from BlockConnected) before its CustomInit method, causing the index to try to update itself before it is initialized.

   This could happen because we set the index m_synced flag (which enables BlockConnected events) before calling to the child class init function (CustomInit). So, for example, the block filter index could process a block before initialize the next filter position field and end up overwriting the first stored filter.

   This race was introduced in https://github.com/bitcoin/bitcoin/commit/bef4e405f3de2718dfee279a9abff4daf016da26 from #25494.

2. index: prevent race by calling 'CustomInit' prior setting 'synced' flag

   The 'm_synced' flag enables 'BlockConnected' events to be processed by
   the index. If we set the flag before calling 'CustomInit', we could be
   dispatching a block connected event to an uninitialized index child
   class.
   
   e.g. BlockFilterIndex, initializes the next filter position
   inside 'CustomInit'. So, if `CustomInit` is not called prior receiving
   the block event, the index will use 'next_filter_position=0' which
   overwrites the first filter in disk.

   3126454dcf
3. 
   DrahtBot commented at 4:08 pm on May 22, 2023: contributor

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   Type	Reviewers
   ACK	mzumsande, TheCharlatan, achow101

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   Conflicts

   Reviewers, this pull request conflicts with the following ones:

   • #27596 (assumeutxo (2) by jamesob)

   If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

4. DrahtBot added the label UTXO Db and Indexes on May 22, 2023
5. 
   mzumsande commented at 10:17 pm on May 23, 2023: contributor

   Code review ACK 3126454dcfa1dd29bb66500d5f2b5261684d6c58
6. fanquake requested review from ryanofsky on May 24, 2023
7. TheCharlatan approved
8. 
   TheCharlatan commented at 11:38 am on May 25, 2023: contributor

   Nice, ACK 3126454dcfa1dd29bb66500d5f2b5261684d6c58
9. fanquake assigned ryanofsky on May 25, 2023
10. 
    achow101 commented at 5:45 pm on May 31, 2023: member

    ACK 3126454dcfa1dd29bb66500d5f2b5261684d6c58
11. achow101 merged this on May 31, 2023
12. achow101 closed this on May 31, 2023

    ---

13. sidhujag referenced this in commit 9ec40ffad2 on May 31, 2023
14. furszy deleted the branch on Feb 3, 2024
15. bitcoin locked this on Feb 2, 2025

Contributors
furszy DrahtBot mzumsande TheCharlatan achow101

Review Requested
ryanofsky

Labels
UTXO Db and Indexes