fix(gacha): prevent infinite loop in getRandomEntries

Add guards for edge cases:
- Empty entries with rolls > 0
- Zero or negative rolls
- Zero total weight in non-box mode
- Box mode with more rolls than available entries

Previously these cases caused infinite loops or panics.

server/channelserver/handlers_shop_gacha.go

@@ -353,10 +353,16 @@ func addGachaItem(s *Session, items []GachaItem) {
 
 func getRandomEntries(entries []GachaEntry, rolls int, isBox bool) ([]GachaEntry, error) {
 	var chosen []GachaEntry
+	if len(entries) == 0 || rolls <= 0 {
+		return chosen, nil
+	}
 	var totalWeight float64
 	for i := range entries {
 		totalWeight += entries[i].Weight
 	}
+	if !isBox && totalWeight <= 0 {
+		return chosen, nil
+	}
 	for {
 		if rolls == len(chosen) {
 			break
@@ -371,6 +377,9 @@ func getRandomEntries(entries []GachaEntry, rolls int, isBox bool) ([]GachaEntry
 				}
 			}
 		} else {
+			if len(entries) == 0 {
+				break
+			}
 			result := rand.Intn(len(entries))
 			chosen = append(chosen, entries[result])
 			entries[result] = entries[len(entries)-1]

server/channelserver/handlers_shop_gacha_test.go

@@ -231,11 +231,9 @@ func TestGachaItemStruct(t *testing.T) {
 	}
 }
 
-func TestGetRandomEntries_EmptyEntriesZeroRolls(t *testing.T) {
+func TestGetRandomEntries_EmptyEntries(t *testing.T) {
-	// Note: getRandomEntries with empty entries and rolls > 0 causes infinite loop.
-	// Only test the valid case of 0 rolls with empty entries.
 	entries := []GachaEntry{}
-	result, err := getRandomEntries(entries, 0, false)
+	result, err := getRandomEntries(entries, 5, false)
 	if err != nil {
 		t.Errorf("unexpected error: %v", err)
 	}
@@ -244,6 +242,57 @@ func TestGetRandomEntries_EmptyEntriesZeroRolls(t *testing.T) {
 	}
 }
 
+func TestGetRandomEntries_EmptyEntriesBoxMode(t *testing.T) {
+	entries := []GachaEntry{}
+	result, err := getRandomEntries(entries, 5, true)
+	if err != nil {
+		t.Errorf("unexpected error: %v", err)
+	}
+	if len(result) != 0 {
+		t.Errorf("expected empty result, got %d entries", len(result))
+	}
+}
+
+func TestGetRandomEntries_NegativeRolls(t *testing.T) {
+	entries := []GachaEntry{{ID: 1, Weight: 1.0}}
+	result, err := getRandomEntries(entries, -5, false)
+	if err != nil {
+		t.Errorf("unexpected error: %v", err)
+	}
+	if len(result) != 0 {
+		t.Errorf("expected empty result, got %d entries", len(result))
+	}
+}
+
+func TestGetRandomEntries_ZeroTotalWeight(t *testing.T) {
+	entries := []GachaEntry{
+		{ID: 1, Weight: 0},
+		{ID: 2, Weight: 0},
+	}
+	result, err := getRandomEntries(entries, 5, false)
+	if err != nil {
+		t.Errorf("unexpected error: %v", err)
+	}
+	if len(result) != 0 {
+		t.Errorf("expected empty result with zero weight, got %d entries", len(result))
+	}
+}
+
+func TestGetRandomEntries_BoxModeMoreRollsThanEntries(t *testing.T) {
+	entries := []GachaEntry{
+		{ID: 1, Weight: 1.0},
+		{ID: 2, Weight: 1.0},
+	}
+	// Request 5 rolls but only 2 entries - should return only 2
+	result, err := getRandomEntries(entries, 5, true)
+	if err != nil {
+		t.Errorf("unexpected error: %v", err)
+	}
+	if len(result) != 2 {
+		t.Errorf("expected 2 results (all available), got %d", len(result))
+	}
+}
+
 func TestGetRandomEntries_ZeroRolls(t *testing.T) {
 	entries := []GachaEntry{
 		{ID: 1, Weight: 1.0},