fix(likes): make updateLike atomic with a FOR UPDATE row lock

[Reggiebro]

Problem

updateLike in src/services/likeService.ts had a TOCTOU race: it read the whole like JSON column, mutated the array in memory, then wrote it back with no locking. Two concurrent toggles both read the same array, and the second blind write clobbers the first — silently dropping likes, and leaving likeCount inconsistent with the array actually stored.

Request A reads:  { likes: [user1] }
Request B reads:  { likes: [user1] }
Request A writes: { likes: [user1, user2] }
Request B writes: { likes: [user1, user3] }  -> user2's like is lost

Fix

Run the read-compute-write inside a single interactive transaction that first takes a SELECT ... FOR UPDATE row lock on the item. Concurrent requests now serialise on that lock instead of overwriting each other, and likeCount is always derived from the same array that gets written.

await lockLikeRow(tx, model, itemId);          // SELECT ... FOR UPDATE
const result = await findItem(tx, model, itemId);
// ...compute newLikes...
await applyLike(tx, model, itemId, newLikes, likeCount);

This is the same pattern as the merged credits/submissions race-condition fix (#1411) — interactive $transaction plus a FOR UPDATE row lock.

Notes

• updateLike's public signature is unchanged, so src/pages/api/submission/like.ts, api/pow/like.ts and api/grant-application/like.ts need no changes.
• likeCount is computed from newLikes (the array that is stored) rather than tracked separately, so it can't drift.
• The locked table name comes from a fixed internal map, never from user input; itemId stays a bound parameter.

Closes #1435

Summary by CodeRabbit

• Bug Fixes
  • Improved reliability when liking or unliking submissions, proofs of work, and grant applications, particularly when multiple actions occur at the same time. Like changes are applied consistently, and displayed totals reflect the resulting likes, reducing the chance that concurrent interactions overwrite another person’s like or show an incorrect count.

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• src/services/likeService.ts

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• src/services/likeService.ts

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Walkthrough

updateLike now locks the item row and performs the read, like toggle, and update in a transaction. Model-specific reads use findFirstOrThrow. The service derives likeCount from the resulting like array and reports whether the user was previously absent.

Changes

Like update flow

Layer / File(s)	Summary
Lock and apply like update src/services/likeService.ts	Helpers lock, read, and update supported items. updateLike runs these operations in a transaction, toggles the user’s entry, derives likeCount from the resulting array, and sets likesIncremented based on prior membership.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix · Severity of issue fixed: Medium

Merge Risk: ⚪ Minimal · up to 9509c

The like-update flow appears ready to merge after normal checks; no confirmed blocking risk remains.

Pre-merge checks: 3 · 2

Failed checks: 2 warnings

Check name	Status	Explanation	Resolution
Linked Issues check		Issue [#1435] requires the lost-like race fix and proposes an automated Promise.all concurrency test. src/services/likeService.ts now runs the read, toggle, and write in one interactive transactio…	Add an automated test for [#1435]. Invoke concurrent updateLike calls with Promise.all for the same item and distinct users. Verify that both users remain in the stored like array and that likeCount equals the array length.
Docstring Coverage		Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 4 functions across 1 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 3 passed

Check name	Status	Explanation
Description Check		Check skipped - CodeRabbit’s high-level summary is enabled.
Title check		The title clearly and concisely describes the main change: making like updates atomic by using a FOR UPDATE row lock.
Out of Scope Changes check		The reviewed change is limited to src/services/likeService.ts. The helper extraction, transaction client usage, row lock, and combined like update directly support the race-condition fix in [#1435].…

Full details: Linked Issues check

Explanation

Issue [#1435] requires the lost-like race fix and proposes an automated Promise.all concurrency test. src/services/likeService.ts now runs the read, toggle, and write in one interactive transaction. It locks a fixed table row with FOR UPDATE and derives likeCount from newLikes. The reviewed changes contain no concurrency test, so the automated test requirement remains unmet.

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_{A rabbit guards the likes in rows,
With locks in place, the data flows.
A toggle turns, the count is right,
Each user’s choice is held in sight.
I thump and hop beneath the moon,
Safe likes arrive, both late and soon.
Then nibble greens and hum a tune.}

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

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Actionable comments posted: 1

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• 🪄 Fix comments: Fix CodeRabbit comments on this PR

🤖 Prompt to fix review comments

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Inline comments:
Review comments at @src/services/likeService.ts:
- Around line 111-113: Update the likesIncremented calculation in the
like-toggle response to compare the new like count with the previously read
likes array, such as resLikes, rather than the potentially stale
result.likeCount; preserve the toggle’s correct like-versus-unlike outcome when
the stored count has drifted.

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• src/services/likeService.ts

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