Bound MongoDB sort memory in lifecycle current listing

[delthas]

Summary

When lifecycle v2 indexes are used for current listings, MongoDB must
re-sort results by _id in memory because the indexes are ordered by
value.last-modified. Without a cursor .limit(), this SORT stage
collects all matching documents before returning any — which can
exceed the 100MB memLimit and spill to disk.

On systems with low disk space (the scenario described in BB-753),
this disk spill fails, breaking lifecycle processing entirely. The
problem is not index creation failing, but index usage triggering
an unbounded sort that spills to a full disk.

Fix

Add a cursor limit based on maxScannedLifecycleListingEntries
(default 10,000) in DelimiterCurrent.genMDParamsV0(). This flows
through to MongoReadStream which already supports options.limit
on the cursor.

With the limit, MongoDB uses a bounded top-k heap sort — keeping only
~10,000 documents in memory regardless of total matches. We use
maxScannedLifecycleListingEntries rather than maxKeys because it
counts documents scanned (not results), which maps directly to cursor
documents regardless of bucket key format (v0 or v1).

Impact

Tested on a 100k-object bucket with explain("executionStats"):

	Without .limit()	With .limit(1001)
nReturned	448,497	1,001
totalDataSizeSorted	478 MB	1 MB
usedDisk	true (spilled!)	false

Issue: ARSN-565

[bert-e]

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[claude]

LGTM

The change is well-targeted: adding params.limit = this.maxKeys + 1 bounds the MongoDB in-memory sort to a top-k heap, preventing the 100MB memLimit spill. The limit is applied post-filter in MongoDB (after lastModified and dataStoreName predicates), so the count is correct. genMDParamsV1() delegates to genMDParamsV0(), so both versioning formats benefit. maxKeys defaults to 1000 and can never be undefined/NaN, so limit is always a valid positive integer.

Review by Claude Code

[bert-e]

Waiting for approval

The following approvals are needed before I can proceed with the merge:

• the author

• 2 peers

[claude]

• Import in delimiterCurrent.ts goes through the barrel index file instead of directly importing from ../../versioning/constants, risking circular dependencies and inconsistent with every other file in lib/algos/list/.
  • Import directly from ../../versioning/constants as other files in the same directory do.

Review by Claude Code

[claude]

LGTM

The fix correctly bounds MongoDB's in-memory sort by adding .limit(maxKeys + 1) for v1 bucket format. Key observations:

- The beforeDate and excludedDataStoreName filters are applied server-side in MongoDB (via readStream.js query construction) before the cursor limit, so no valid documents are lost.
- The client-side filtering in addContents() is redundant/defensive — the limit interacts safely with it.
- The v1-only guard is appropriate since v0 interleaves master/version keys.

Review by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.37%. Comparing base (eefeb8b) to head (617c843).
⚠️ Report is 3 commits behind head on development/8.4.

Additional details and impacted files

@@               Coverage Diff                @@
##           development/8.4    #2603   +/-   ##
================================================
  Coverage            73.37%   73.37%           
================================================
  Files                  222      222           
  Lines                18160    18162    +2     
  Branches              3761     3786   +25     
================================================
+ Hits                 13324    13326    +2     
  Misses                4831     4831           
  Partials                 5        5           

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[claude]

LGTM

Review by Claude Code

[claude]

LGTM

The fix correctly bounds the MongoDB cursor via params.limit in genMDParamsV0(), which flows through mainStreamParams to MongoReadStream where it is applied as cursor.limit(). The use of maxScannedLifecycleListingEntries + 1 is appropriate — it matches the existing scan-limit semantics and the +1 preserves truncation detection. Test coverage is updated accordingly.

Review by Claude Code

[francoisferrand]

is this on the right branch?
ideally we may want this in zenko 2.13, which is still on backbeat 9.1, which is using artesca 8.2.26.
→ this is a long way from we should assess if there is a risk to go from 8.2.26 to 8.2.49...
→ how confident are we in this change? can we just put it in prod ASAP, or should we be slightly more careful?

[claude]

LGTM

The fix correctly bounds the MongoDB cursor via params.limit = maxScannedLifecycleListingEntries + 1 in genMDParamsV0(), which flows through to MongoReadStream's existing .limit() support. The +1 to detect truncation is correct. The guard if (this.maxScannedLifecycleListingEntries) properly skips when the value is undefined, 0, or falsy. Both unit and functional tests cover the new behavior, including ordering verification.

Review by Claude Code

[claude]

LGTM

The cursor limit is correctly applied in genMDParamsV0(), which is also called by the V1 path via genMDParamsV1() → listingParamsMasterKeysV0ToV1(), so the limit propagates to both bucket formats. The falsiness guard on maxScannedLifecycleListingEntries correctly handles undefined/null/0. Unit and functional tests cover the new behavior well.

Review by Claude Code

[claude]

LGTM

Review by Claude Code

[bert-e]

Jira issue not found

The Jira issue ARSN-565 was not found.

[bert-e]

Waiting for approval

The following approvals are needed before I can proceed with the merge:

• the author

• 2 peers

[delthas]

/approve

[bert-e]

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[bert-e]

I have successfully merged the changeset of this pull request
into targetted development branches:

• ✔️ development/8.4

The following branches have NOT changed:

• development/6.4
• development/7.10
• development/7.4
• development/7.70
• development/8.1
• development/8.2
• development/8.3

Please check the status of the associated issue ARSN-565.

Goodbye delthas.