fix(storage): a spilled value over ~4 MB reads back instead of OverflowBroken (moon#1201)

[TinDang97]

Fixes #1201.

Bug. A value too big for one 4 KB page is stored as a chain of overflow pages, each holding about 4 KB. build_overflow_chain writes chains of any length. read_overflow_chain refused any chain longer than a fixed 1000 pages (about 4.03 MB), so a larger spilled value was written intact but could never be read back. The key stayed indexed and every read failed with OverflowBroken, which clients see as IOERR.

In the v0.8.10 release soak this happened to consumer-group streams: the pending list keeps growing without XACK, so the streams passed 4 MB.

• IOERR on 3966 XADD runs and 3966 XREADGROUP runs.
• On disk the failing chains are intact, with 1003 to 1007 pages each.

Not a regression. The 1000-page limit dates from PR #43. v0.8.9 has the same bug, but there the failed read was a silent miss (XLEN 0, GET nil). #1004 turned that miss into an IOERR, which is why it only now shows up as an error.

Fix. The walk is bounded by the file's own page count. A chain that doesn't loop visits each page at most once, so it can't be longer than the file; a looping chain always exceeds that count. No on-disk format change: files written by any earlier version read back.

Tests (red on unfixed code, green with the fix, red again with only the bound reverted):

• kv_page::test_overflow_chain_longer_than_1000_pages_reads_back_1201
• cold_read::test_cold_read_overflow_chain_longer_than_1000_pages_1201: spills 4.5 MB and reads it back.
• kv_page::test_overflow_chain_cycle_is_rejected_1201: the cycle guard still works.

Repro (1 shard, --maxmemory 48mb, AOF on, a 20k-entry stream with a 20k-entry pending list, plus a 4.5 MB string):

Build	Stream	4.5 MB string
main	IOERR on XLEN, XADD and XREADGROUP	IOERR
fixed	XLEN 20000, stream intact	GET returns the exact bytes
v0.8.9	XLEN 0, XADD silently starts a new stream	nil

Known, separate: reading a multi-MB cold value back into memory can push the server over maxmemory, which is the #1156 class.

[coderabbitai]

Navigate logical layers of code changes, visualize relationships, and explore their blast radius.

Note

Currently processing new changes in this PR. This may take a few minutes, please wait...

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Advanced

Run ID: 5024eeca-ff88-4f3d-b53f-0bde478697ad

📥 Commits

Reviewing files that changed from the base of the PR and between 843611f and 67d8239.

📒 Files selected for processing (3)

• CHANGELOG.md
• src/persistence/kv_page.rs
• src/storage/tiered/cold_read.rs

 ___________________________________________________________________________________________
< Optimism is an occupational hazard of programming; feedback is the treatment. - Kent Beck >
 -------------------------------------------------------------------------------------------
  \
   \   \
        \ /\
        ( )
      .( o ).

✨ Finishing Touches

📝 Generate docstrings

• Commit to this branch
• Create a new PR

🧪 Generate unit tests (beta)

• Commit to this branch
• Create a new PR

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands.}

[qodo-code-review]

ⓘ Qodo reviews are paused because the subscription is no longer active. Ask your workspace admin to reactivate the subscription to resume reviews. Manage billing