Gorse version

master (HEAD b7e6aa9, 2026-04-16). Reproduces on Go 1.26.2 / darwin-arm64 and the 16-goroutine CI path added in #1232.

Describe the bug

#1232 added per-user locks to Dataset.AddFeedback to prevent concurrent append to userFeedback[userIndex], but two other writes in the same function remain outside the lock scope:

1. dataset/dataset.go:237 — d.itemFeedback[itemIndex] = append(d.itemFeedback[itemIndex], userIndex) is executed before any lock is acquired. Concurrent goroutines that share an itemIndex race on the slice header.
2. dataset/dataset.go:242 — d.numFeedback++ is protected only by a per-user lock. Two goroutines holding different user locks still race on the shared counter.

The current layout only avoids the userFeedback[userIndex] race; goroutines that write feedback for different users but share an item (a common case when loading data in parallel) still collide.

To Reproduce

// dataset/race_repro_test.go
package dataset

import (
    "fmt"
    "sync"
    "testing"
    "time"

    "github.com/gorse-io/gorse/storage/data"
    "github.com/stretchr/testify/assert"
)

func TestAddFeedbackRace(t *testing.T) {
    const numUsers, numItems = 16, 32
    ds := NewDataset(time.Now(), numUsers, numItems)
    for i := 0; i < numUsers; i++ {
        ds.AddUser(data.User{UserId: fmt.Sprintf("user%d", i)})
    }
    for j := 0; j < numItems; j++ {
        ds.AddItem(data.Item{ItemId: fmt.Sprintf("item%d", j)})
    }

    var wg sync.WaitGroup
    ts := time.Unix(1700000000, 0)
    for u := 0; u < numUsers; u++ {
        wg.Add(1)
        go func(userIdx int) {
            defer wg.Done()
            for j := 0; j < numItems; j++ {
                ds.AddFeedback(fmt.Sprintf("user%d", userIdx), fmt.Sprintf("item%d", j), ts)
            }
        }(u)
    }
    wg.Wait()
    assert.Equal(t, numUsers*numItems, ds.CountFeedback())
}

Run go test -race -run TestAddFeedbackRace -count=1 ./dataset/...:

==================
WARNING: DATA RACE
Write at 0x00c00004a2d4 by goroutine 21:
  dataset.(*Dataset).AddFeedback()
      /.../dataset/dataset.go:237 +0x76c
Previous write at 0x00c00004a2d4 by goroutine 29:
  dataset.(*Dataset).AddFeedback()
      /.../dataset/dataset.go:237 +0x76c
==================
--- FAIL: TestAddFeedbackRace (0.02s)
    Error: expected 512  actual: 437
    testing.go:1712: race detected during execution of test

So in addition to the race detector firing, 75 of 512 (14.6%) numFeedback increments are silently lost under contention. The incorrect numFeedback also leaks into CountFeedback, which is consumed by progress reporting and by downstream validation in the parallel loader.

Expected behavior

• go test -race is clean.
• ds.CountFeedback() equals the number of successful AddFeedback calls, independent of concurrency.

Additional context / suggested fix

A minimal correction mirrors the existing per-user lock pattern:

func (d *Dataset) AddFeedback(userId, itemId string, timestamp time.Time) {
    userIndex := d.userDict.Add(userId)
    itemIndex := d.itemDict.Add(itemId)

    d.itemDict.Lock(itemIndex)
    d.itemFeedback[itemIndex] = append(d.itemFeedback[itemIndex], userIndex)
    d.itemDict.Unlock(itemIndex)

    d.userDict.Lock(userIndex)
    d.userFeedback[userIndex] = append(d.userFeedback[userIndex], itemIndex)
    d.timestamps[userIndex]   = append(d.timestamps[userIndex], timestamp)
    d.userDict.Unlock(userIndex)

    atomic.AddInt64(&d.numFeedback, 1) // change field type to int64
}

Happy to send the PR with the regression test above if the approach looks good.