fuzz: target CDBWrapper #34887

1. 
   andrewtoth commented at 1:12 am on March 21, 2026: contributor

   Inspired by #34866 (comment).

   We currently don’t have a dedicated harness targeting CDBWrapper. OSS-Fuzz has a rudimentary harness for levelDB which fails, so doesn’t appear maintained.

   This PR adds a harness targeting CDBWrapper against an in-memory oracle to verify correctness.

   A DeterministicEnv wraps levelDB’s memenv to eliminate non-determinism by capturing background compaction and running it at fuzzer-chosen points. The fuzzer also controls the cache_bytes and max_file_size sizes so that small values trigger memtable flushes and compaction.

2. DrahtBot added the label Fuzzing on Mar 21, 2026
3. 
   DrahtBot commented at 1:12 am on March 21, 2026: contributor

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4. DrahtBot added the label CI failed on Mar 21, 2026
5. andrewtoth force-pushed on Mar 21, 2026
6. DrahtBot removed the label CI failed on Mar 21, 2026
7. andrewtoth force-pushed on Mar 21, 2026
8. andrewtoth force-pushed on Mar 21, 2026
9. andrewtoth force-pushed on Mar 21, 2026
10. DrahtBot added the label CI failed on Mar 22, 2026
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12. andrewtoth force-pushed on Mar 22, 2026
13. DrahtBot removed the label CI failed on Mar 22, 2026
14. dbwrapper: make max_file_size a configurable DBParams field

    Useful for fuzzing different values.

    9e444624bb
15. dbwrapper: accept optional testing leveldb::Env in DBParams

    Allow callers to inject a custom leveldb::Env via DBParams::testing_env,
    which takes priority over the memory_only in-memory environment. This
    enables fuzz harnesses to supply a deterministic environment.

    e903203536
16. andrewtoth force-pushed on Mar 29, 2026
17. 
    marcofleon commented at 3:22 pm on April 2, 2026: contributor

    Concept ACK

    Nice differential fuzz test. I’m running it now and I’ll do a more thorough review soon.

18. test: add fuzz harness for CDBWrapper

    Introduces a libFuzzer harness that exercises CDBWrapper operations
    against a std::map oracle, with a DeterministicEnv that captures LevelDB
    background compaction for single-threaded determinism.

    b967e255e7
19. andrewtoth force-pushed on Apr 11, 2026
20. 
    andrewtoth commented at 3:10 pm on April 11, 2026: contributor

    Improved the assertions around iterators. This now exhausts the DB iterator completely instead of using LIMITED_WHILE, and asserts that the oracle iterator is also exhausted, ensuring no entries are missing.

    Also added coverage for failing to deserialize a value by reading with a type that always throws on deserialization.

    git diff 430f357f7e304336d2b61f72d2bc5439dea557d8..b967e255e7620b8366ec1aeb8932138455be8701

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andrewtoth DrahtBot marcofleon

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