feat: shuffle accounts within same sort group to prevent thundering herd

Add post-sort shuffle for accounts with identical (priority, loadRate,
lastUsedAt) to break deterministic ordering when concurrent requests
read the same scheduler snapshot. Applies to both Antigravity and
OpenAI scheduling paths, plus the sortAccountsByPriorityAndLastUsed
helper.

Keeps upstream CallCount/ModelLoadInfo scheduling intact; shuffle is
additive and only randomises within equivalent-rank groups.

backend/internal/service/gateway_service.go

@@ -1218,6 +1218,7 @@ func (s *GatewayService) SelectAccountWithLoadAwareness(ctx context.Context, gro
 						return a.account.LastUsedAt.Before(*b.account.LastUsedAt)
 					}
 				})
+				shuffleWithinSortGroups(routingAvailable)
 
 				// 4. 尝试获取槽位
 				for _, item := range routingAvailable {
@@ -2027,6 +2028,79 @@ func sortAccountsByPriorityAndLastUsed(accounts []*Account, preferOAuth bool) {
 			return a.LastUsedAt.Before(*b.LastUsedAt)
 		}
 	})
+	shuffleWithinPriorityAndLastUsed(accounts)
+}
+
+// shuffleWithinSortGroups 对排序后的 accountWithLoad 切片，按 (Priority, LoadRate, LastUsedAt) 分组后组内随机打乱。
+// 防止并发请求读取同一快照时，确定性排序导致所有请求命中相同账号。
+func shuffleWithinSortGroups(accounts []accountWithLoad) {
+	if len(accounts) <= 1 {
+		return
+	}
+	i := 0
+	for i < len(accounts) {
+		j := i + 1
+		for j < len(accounts) && sameAccountWithLoadGroup(accounts[i], accounts[j]) {
+			j++
+		}
+		if j-i > 1 {
+			mathrand.Shuffle(j-i, func(a, b int) {
+				accounts[i+a], accounts[i+b] = accounts[i+b], accounts[i+a]
+			})
+		}
+		i = j
+	}
+}
+
+// sameAccountWithLoadGroup 判断两个 accountWithLoad 是否属于同一排序组
+func sameAccountWithLoadGroup(a, b accountWithLoad) bool {
+	if a.account.Priority != b.account.Priority {
+		return false
+	}
+	if a.loadInfo.LoadRate != b.loadInfo.LoadRate {
+		return false
+	}
+	return sameLastUsedAt(a.account.LastUsedAt, b.account.LastUsedAt)
+}
+
+// shuffleWithinPriorityAndLastUsed 对排序后的 []*Account 切片，按 (Priority, LastUsedAt) 分组后组内随机打乱。
+func shuffleWithinPriorityAndLastUsed(accounts []*Account) {
+	if len(accounts) <= 1 {
+		return
+	}
+	i := 0
+	for i < len(accounts) {
+		j := i + 1
+		for j < len(accounts) && sameAccountGroup(accounts[i], accounts[j]) {
+			j++
+		}
+		if j-i > 1 {
+			mathrand.Shuffle(j-i, func(a, b int) {
+				accounts[i+a], accounts[i+b] = accounts[i+b], accounts[i+a]
+			})
+		}
+		i = j
+	}
+}
+
+// sameAccountGroup 判断两个 Account 是否属于同一排序组（Priority + LastUsedAt）
+func sameAccountGroup(a, b *Account) bool {
+	if a.Priority != b.Priority {
+		return false
+	}
+	return sameLastUsedAt(a.LastUsedAt, b.LastUsedAt)
+}
+
+// sameLastUsedAt 判断两个 LastUsedAt 是否相同（精度到秒）
+func sameLastUsedAt(a, b *time.Time) bool {
+	switch {
+	case a == nil && b == nil:
+		return true
+	case a == nil || b == nil:
+		return false
+	default:
+		return a.Unix() == b.Unix()
+	}
 }
 
 // selectByCallCount 从候选账号中选择调用次数最少的账号（Antigravity 专用）

backend/internal/service/openai_gateway_service.go

@@ -580,10 +580,6 @@ func (s *OpenAIGatewayService) SelectAccountWithLoadAwareness(ctx context.Contex
 			}
 		}
 	} else {
-		type accountWithLoad struct {
-			account  *Account
-			loadInfo *AccountLoadInfo
-		}
 		var available []accountWithLoad
 		for _, acc := range candidates {
 			loadInfo := loadMap[acc.ID]
@@ -618,6 +614,7 @@ func (s *OpenAIGatewayService) SelectAccountWithLoadAwareness(ctx context.Contex
 					return a.account.LastUsedAt.Before(*b.account.LastUsedAt)
 				}
 			})
+			shuffleWithinSortGroups(available)
 
 			for _, item := range available {
 				result, err := s.tryAcquireAccountSlot(ctx, item.account.ID, item.account.Concurrency)

backend/internal/service/scheduler_shuffle_test.go

+//go:build unit
+
+package service
+
+import (
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+)
+
+// ============ shuffleWithinSortGroups 测试 ============
+
+func TestShuffleWithinSortGroups_Empty(t *testing.T) {
+	shuffleWithinSortGroups(nil)
+	shuffleWithinSortGroups([]accountWithLoad{})
+}
+
+func TestShuffleWithinSortGroups_SingleElement(t *testing.T) {
+	accounts := []accountWithLoad{
+		{account: &Account{ID: 1, Priority: 1}, loadInfo: &AccountLoadInfo{LoadRate: 10}},
+	}
+	shuffleWithinSortGroups(accounts)
+	require.Equal(t, int64(1), accounts[0].account.ID)
+}
+
+func TestShuffleWithinSortGroups_DifferentGroups_OrderPreserved(t *testing.T) {
+	now := time.Now()
+	earlier := now.Add(-1 * time.Hour)
+
+	accounts := []accountWithLoad{
+		{account: &Account{ID: 1, Priority: 1, LastUsedAt: &earlier}, loadInfo: &AccountLoadInfo{LoadRate: 10}},
+		{account: &Account{ID: 2, Priority: 1, LastUsedAt: &now}, loadInfo: &AccountLoadInfo{LoadRate: 20}},
+		{account: &Account{ID: 3, Priority: 2, LastUsedAt: &earlier}, loadInfo: &AccountLoadInfo{LoadRate: 10}},
+	}
+
+	// 每个元素都属于不同组（Priority 或 LoadRate 或 LastUsedAt 不同），顺序不变
+	for i := 0; i < 20; i++ {
+		cpy := make([]accountWithLoad, len(accounts))
+		copy(cpy, accounts)
+		shuffleWithinSortGroups(cpy)
+		require.Equal(t, int64(1), cpy[0].account.ID)
+		require.Equal(t, int64(2), cpy[1].account.ID)
+		require.Equal(t, int64(3), cpy[2].account.ID)
+	}
+}
+
+func TestShuffleWithinSortGroups_SameGroup_Shuffled(t *testing.T) {
+	now := time.Now()
+	// 同一秒的时间戳视为同一组
+	sameSecond := time.Unix(now.Unix(), 0)
+	sameSecond2 := time.Unix(now.Unix(), 500_000_000) // 同一秒但不同纳秒
+
+	accounts := []accountWithLoad{
+		{account: &Account{ID: 1, Priority: 1, LastUsedAt: &sameSecond}, loadInfo: &AccountLoadInfo{LoadRate: 10}},
+		{account: &Account{ID: 2, Priority: 1, LastUsedAt: &sameSecond2}, loadInfo: &AccountLoadInfo{LoadRate: 10}},
+		{account: &Account{ID: 3, Priority: 1, LastUsedAt: &sameSecond}, loadInfo: &AccountLoadInfo{LoadRate: 10}},
+	}
+
+	// 多次执行，验证所有 ID 都出现在第一个位置（说明确实被打乱了）
+	seen := map[int64]bool{}
+	for i := 0; i < 100; i++ {
+		cpy := make([]accountWithLoad, len(accounts))
+		copy(cpy, accounts)
+		shuffleWithinSortGroups(cpy)
+		seen[cpy[0].account.ID] = true
+		// 无论怎么打乱，所有 ID 都应在候选中
+		ids := map[int64]bool{}
+		for _, a := range cpy {
+			ids[a.account.ID] = true
+		}
+		require.True(t, ids[1] && ids[2] && ids[3])
+	}
+	// 至少 2 个不同的 ID 出现在首位（随机性验证）
+	require.GreaterOrEqual(t, len(seen), 2, "shuffle should produce different orderings")
+}
+
+func TestShuffleWithinSortGroups_NilLastUsedAt_SameGroup(t *testing.T) {
+	accounts := []accountWithLoad{
+		{account: &Account{ID: 1, Priority: 1, LastUsedAt: nil}, loadInfo: &AccountLoadInfo{LoadRate: 0}},
+		{account: &Account{ID: 2, Priority: 1, LastUsedAt: nil}, loadInfo: &AccountLoadInfo{LoadRate: 0}},
+		{account: &Account{ID: 3, Priority: 1, LastUsedAt: nil}, loadInfo: &AccountLoadInfo{LoadRate: 0}},
+	}
+
+	seen := map[int64]bool{}
+	for i := 0; i < 100; i++ {
+		cpy := make([]accountWithLoad, len(accounts))
+		copy(cpy, accounts)
+		shuffleWithinSortGroups(cpy)
+		seen[cpy[0].account.ID] = true
+	}
+	require.GreaterOrEqual(t, len(seen), 2, "nil LastUsedAt accounts should be shuffled")
+}
+
+func TestShuffleWithinSortGroups_MixedGroups(t *testing.T) {
+	now := time.Now()
+	earlier := now.Add(-1 * time.Hour)
+	sameAsNow := time.Unix(now.Unix(), 0)
+
+	// 组1: Priority=1, LoadRate=10, LastUsedAt=earlier (ID 1)  — 单元素组
+	// 组2: Priority=1, LoadRate=20, LastUsedAt=now (ID 2, 3)   — 双元素组
+	// 组3: Priority=2, LoadRate=10, LastUsedAt=earlier (ID 4)  — 单元素组
+	accounts := []accountWithLoad{
+		{account: &Account{ID: 1, Priority: 1, LastUsedAt: &earlier}, loadInfo: &AccountLoadInfo{LoadRate: 10}},
+		{account: &Account{ID: 2, Priority: 1, LastUsedAt: &now}, loadInfo: &AccountLoadInfo{LoadRate: 20}},
+		{account: &Account{ID: 3, Priority: 1, LastUsedAt: &sameAsNow}, loadInfo: &AccountLoadInfo{LoadRate: 20}},
+		{account: &Account{ID: 4, Priority: 2, LastUsedAt: &earlier}, loadInfo: &AccountLoadInfo{LoadRate: 10}},
+	}
+
+	for i := 0; i < 20; i++ {
+		cpy := make([]accountWithLoad, len(accounts))
+		copy(cpy, accounts)
+		shuffleWithinSortGroups(cpy)
+
+		// 组间顺序不变
+		require.Equal(t, int64(1), cpy[0].account.ID, "group 1 position fixed")
+		require.Equal(t, int64(4), cpy[3].account.ID, "group 3 position fixed")
+
+		// 组2 内部可以打乱，但仍在位置 1 和 2
+		mid := map[int64]bool{cpy[1].account.ID: true, cpy[2].account.ID: true}
+		require.True(t, mid[2] && mid[3], "group 2 elements should stay in positions 1-2")
+	}
+}
+
+// ============ shuffleWithinPriorityAndLastUsed 测试 ============
+
+func TestShuffleWithinPriorityAndLastUsed_Empty(t *testing.T) {
+	shuffleWithinPriorityAndLastUsed(nil)
+	shuffleWithinPriorityAndLastUsed([]*Account{})
+}
+
+func TestShuffleWithinPriorityAndLastUsed_SingleElement(t *testing.T) {
+	accounts := []*Account{{ID: 1, Priority: 1}}
+	shuffleWithinPriorityAndLastUsed(accounts)
+	require.Equal(t, int64(1), accounts[0].ID)
+}
+
+func TestShuffleWithinPriorityAndLastUsed_SameGroup_Shuffled(t *testing.T) {
+	accounts := []*Account{
+		{ID: 1, Priority: 1, LastUsedAt: nil},
+		{ID: 2, Priority: 1, LastUsedAt: nil},
+		{ID: 3, Priority: 1, LastUsedAt: nil},
+	}
+
+	seen := map[int64]bool{}
+	for i := 0; i < 100; i++ {
+		cpy := make([]*Account, len(accounts))
+		copy(cpy, accounts)
+		shuffleWithinPriorityAndLastUsed(cpy)
+		seen[cpy[0].ID] = true
+	}
+	require.GreaterOrEqual(t, len(seen), 2, "same group should be shuffled")
+}
+
+func TestShuffleWithinPriorityAndLastUsed_DifferentPriority_OrderPreserved(t *testing.T) {
+	accounts := []*Account{
+		{ID: 1, Priority: 1, LastUsedAt: nil},
+		{ID: 2, Priority: 2, LastUsedAt: nil},
+		{ID: 3, Priority: 3, LastUsedAt: nil},
+	}
+
+	for i := 0; i < 20; i++ {
+		cpy := make([]*Account, len(accounts))
+		copy(cpy, accounts)
+		shuffleWithinPriorityAndLastUsed(cpy)
+		require.Equal(t, int64(1), cpy[0].ID)
+		require.Equal(t, int64(2), cpy[1].ID)
+		require.Equal(t, int64(3), cpy[2].ID)
+	}
+}
+
+func TestShuffleWithinPriorityAndLastUsed_DifferentLastUsedAt_OrderPreserved(t *testing.T) {
+	now := time.Now()
+	earlier := now.Add(-1 * time.Hour)
+
+	accounts := []*Account{
+		{ID: 1, Priority: 1, LastUsedAt: nil},
+		{ID: 2, Priority: 1, LastUsedAt: &earlier},
+		{ID: 3, Priority: 1, LastUsedAt: &now},
+	}
+
+	for i := 0; i < 20; i++ {
+		cpy := make([]*Account, len(accounts))
+		copy(cpy, accounts)
+		shuffleWithinPriorityAndLastUsed(cpy)
+		require.Equal(t, int64(1), cpy[0].ID)
+		require.Equal(t, int64(2), cpy[1].ID)
+		require.Equal(t, int64(3), cpy[2].ID)
+	}
+}
+
+// ============ sameLastUsedAt 测试 ============
+
+func TestSameLastUsedAt(t *testing.T) {
+	now := time.Now()
+	sameSecond := time.Unix(now.Unix(), 0)
+	sameSecondDiffNano := time.Unix(now.Unix(), 999_999_999)
+	differentSecond := now.Add(1 * time.Second)
+
+	t.Run("both nil", func(t *testing.T) {
+		require.True(t, sameLastUsedAt(nil, nil))
+	})
+
+	t.Run("one nil one not", func(t *testing.T) {
+		require.False(t, sameLastUsedAt(nil, &now))
+		require.False(t, sameLastUsedAt(&now, nil))
+	})
+
+	t.Run("same second different nanoseconds", func(t *testing.T) {
+		require.True(t, sameLastUsedAt(&sameSecond, &sameSecondDiffNano))
+	})
+
+	t.Run("different seconds", func(t *testing.T) {
+		require.False(t, sameLastUsedAt(&now, &differentSecond))
+	})
+
+	t.Run("exact same time", func(t *testing.T) {
+		require.True(t, sameLastUsedAt(&now, &now))
+	})
+}
+
+// ============ sameAccountWithLoadGroup 测试 ============
+
+func TestSameAccountWithLoadGroup(t *testing.T) {
+	now := time.Now()
+	sameSecond := time.Unix(now.Unix(), 0)
+
+	t.Run("same group", func(t *testing.T) {
+		a := accountWithLoad{account: &Account{Priority: 1, LastUsedAt: &now}, loadInfo: &AccountLoadInfo{LoadRate: 10}}
+		b := accountWithLoad{account: &Account{Priority: 1, LastUsedAt: &sameSecond}, loadInfo: &AccountLoadInfo{LoadRate: 10}}
+		require.True(t, sameAccountWithLoadGroup(a, b))
+	})
+
+	t.Run("different priority", func(t *testing.T) {
+		a := accountWithLoad{account: &Account{Priority: 1, LastUsedAt: &now}, loadInfo: &AccountLoadInfo{LoadRate: 10}}
+		b := accountWithLoad{account: &Account{Priority: 2, LastUsedAt: &now}, loadInfo: &AccountLoadInfo{LoadRate: 10}}
+		require.False(t, sameAccountWithLoadGroup(a, b))
+	})
+
+	t.Run("different load rate", func(t *testing.T) {
+		a := accountWithLoad{account: &Account{Priority: 1, LastUsedAt: &now}, loadInfo: &AccountLoadInfo{LoadRate: 10}}
+		b := accountWithLoad{account: &Account{Priority: 1, LastUsedAt: &now}, loadInfo: &AccountLoadInfo{LoadRate: 20}}
+		require.False(t, sameAccountWithLoadGroup(a, b))
+	})
+
+	t.Run("different last used at", func(t *testing.T) {
+		later := now.Add(1 * time.Second)
+		a := accountWithLoad{account: &Account{Priority: 1, LastUsedAt: &now}, loadInfo: &AccountLoadInfo{LoadRate: 10}}
+		b := accountWithLoad{account: &Account{Priority: 1, LastUsedAt: &later}, loadInfo: &AccountLoadInfo{LoadRate: 10}}
+		require.False(t, sameAccountWithLoadGroup(a, b))
+	})
+
+	t.Run("both nil LastUsedAt", func(t *testing.T) {
+		a := accountWithLoad{account: &Account{Priority: 1, LastUsedAt: nil}, loadInfo: &AccountLoadInfo{LoadRate: 0}}
+		b := accountWithLoad{account: &Account{Priority: 1, LastUsedAt: nil}, loadInfo: &AccountLoadInfo{LoadRate: 0}}
+		require.True(t, sameAccountWithLoadGroup(a, b))
+	})
+}
+
+// ============ sameAccountGroup 测试 ============
+
+func TestSameAccountGroup(t *testing.T) {
+	now := time.Now()
+
+	t.Run("same group", func(t *testing.T) {
+		a := &Account{Priority: 1, LastUsedAt: nil}
+		b := &Account{Priority: 1, LastUsedAt: nil}
+		require.True(t, sameAccountGroup(a, b))
+	})
+
+	t.Run("different priority", func(t *testing.T) {
+		a := &Account{Priority: 1, LastUsedAt: nil}
+		b := &Account{Priority: 2, LastUsedAt: nil}
+		require.False(t, sameAccountGroup(a, b))
+	})
+
+	t.Run("different LastUsedAt", func(t *testing.T) {
+		later := now.Add(1 * time.Second)
+		a := &Account{Priority: 1, LastUsedAt: &now}
+		b := &Account{Priority: 1, LastUsedAt: &later}
+		require.False(t, sameAccountGroup(a, b))
+	})
+}
+
+// ============ sortAccountsByPriorityAndLastUsed 集成随机化测试 ============
+
+func TestSortAccountsByPriorityAndLastUsed_WithShuffle(t *testing.T) {
+	t.Run("same priority and nil LastUsedAt are shuffled", func(t *testing.T) {
+		accounts := []*Account{
+			{ID: 1, Priority: 1, LastUsedAt: nil},
+			{ID: 2, Priority: 1, LastUsedAt: nil},
+			{ID: 3, Priority: 1, LastUsedAt: nil},
+		}
+
+		seen := map[int64]bool{}
+		for i := 0; i < 100; i++ {
+			cpy := make([]*Account, len(accounts))
+			copy(cpy, accounts)
+			sortAccountsByPriorityAndLastUsed(cpy, false)
+			seen[cpy[0].ID] = true
+		}
+		require.GreaterOrEqual(t, len(seen), 2, "identical sort keys should produce different orderings after shuffle")
+	})
+
+	t.Run("different priorities still sorted correctly", func(t *testing.T) {
+		now := time.Now()
+		accounts := []*Account{
+			{ID: 3, Priority: 3, LastUsedAt: &now},
+			{ID: 1, Priority: 1, LastUsedAt: &now},
+			{ID: 2, Priority: 2, LastUsedAt: &now},
+		}
+
+		sortAccountsByPriorityAndLastUsed(accounts, false)
+		require.Equal(t, int64(1), accounts[0].ID)
+		require.Equal(t, int64(2), accounts[1].ID)
+		require.Equal(t, int64(3), accounts[2].ID)
+	})
+}