fix(handlers): harden gacha_items and boost time limit ACK paths

Both bugs surfaced when running protbot against a live Erupe instance.

LoadColumnWithDefault now treats an empty bytea ('\x', len 0) the same
as NULL. The postgres driver returns a non-nil empty slice for empty
bytea, so the prior `data == nil` check let a zero-byte slice reach
handleMsgMhfReceiveGachaItem, which forwarded it to the client as a
malformed gacha_items response. The MHF client interprets a zero-byte
count field as a protocol error and crashes the gacha menu, matching
the #175 symptom class. The handler also gains a defensive fallback
for len(data) == 0 in case another caller hits the same edge.

handleMsgMhfGetBoostTimeLimit was sending two ACKs for a single
request: doAckBufSucceed with the real payload, then an unconditional
doAckSimpleSucceed on the same ack handle. The second ACK was dead on
arrival (the handle was already consumed) but is a latent protocol
bug. Drop it and update the regression test that was asserting the
buggy 2-packet behavior.

server/channelserver/handlers_cafe.go

@@ -227,7 +227,6 @@ func handleMsgMhfGetBoostTimeLimit(s *Session, p mhfpacket.MHFPacket) {
 		bf.WriteUint32(uint32(boostLimit.Unix()))
 	}
 	doAckBufSucceed(s, pkt.AckHandle, bf.Data())
-	doAckSimpleSucceed(s, pkt.AckHandle, make([]byte, 4))
 }
 
 func handleMsgMhfGetBoostRight(s *Session, p mhfpacket.MHFPacket) {

server/channelserver/handlers_cafe_test.go

@@ -231,7 +231,9 @@ func TestHandleMsgMhfGetBoostTimeLimit(t *testing.T) {
 
 	handleMsgMhfGetBoostTimeLimit(session, pkt)
 
-	// This handler sends two responses (doAckBufSucceed + doAckSimpleSucceed)
+	// One response: the stray second ACK (doAckSimpleSucceed) was removed
+	// because it fired with the same ack handle as the real buf ACK, which
+	// is a protocol bug.
 	count := 0
 	for {
 		select {
@@ -242,8 +244,8 @@ func TestHandleMsgMhfGetBoostTimeLimit(t *testing.T) {
 		}
 	}
 done:
-	if count != 2 {
+	if count != 1 {
-		t.Errorf("Expected 2 response packets, got %d", count)
+		t.Errorf("Expected 1 response packet, got %d", count)
 	}
 }
 

server/channelserver/handlers_gacha.go

@@ -84,7 +84,9 @@ func handleMsgMhfUseGachaPoint(s *Session, p mhfpacket.MHFPacket) {
 func handleMsgMhfReceiveGachaItem(s *Session, p mhfpacket.MHFPacket) {
 	pkt := p.(*mhfpacket.MsgMhfReceiveGachaItem)
 	data, err := s.server.charRepo.LoadColumnWithDefault(s.charID, "gacha_items", []byte{0x00})
-	if err != nil {
+	if err != nil || len(data) == 0 {
+		// The client requires at least one byte (the item count) or it
+		// treats the response as malformed and crashes (see #175).
 		data = []byte{0x00}
 	}
 

server/channelserver/repo_character.go

@@ -167,7 +167,11 @@ func (r *CharacterRepository) LoadColumnWithDefault(charID uint32, column string
 	if err != nil {
 		return defaultVal, err
 	}
-	if data == nil {
+	// Treat empty bytea ('\x', len 0) the same as NULL. The postgres driver
+	// returns a non-nil empty slice for empty bytea, so a bare `data == nil`
+	// check would send zero bytes to the client — which the MHF client
+	// interprets as a malformed response and crashes on (see #175).
+	if len(data) == 0 {
 		return defaultVal, nil
 	}
 	return data, nil

server/channelserver/repo_character_test.go

@@ -392,6 +392,27 @@ func TestLoadColumnWithDefaultExistingData(t *testing.T) {
 	}
 }
 
+// TestLoadColumnWithDefaultEmptyBytea verifies that an empty bytea ('\x', len 0)
+// is treated the same as NULL and returns the default value. Without this, the
+// postgres driver returns a non-nil empty slice that would reach the client
+// as a zero-byte ACK payload and crash the MHF gacha menu (see #175).
+func TestLoadColumnWithDefaultEmptyBytea(t *testing.T) {
+	repo, db, charID := setupCharRepo(t)
+
+	if _, err := db.Exec("UPDATE characters SET skin_hist=''::bytea WHERE id=$1", charID); err != nil {
+		t.Fatalf("Setup failed: %v", err)
+	}
+
+	defaultVal := []byte{0x00, 0x01, 0x02}
+	got, err := repo.LoadColumnWithDefault(charID, "skin_hist", defaultVal)
+	if err != nil {
+		t.Fatalf("LoadColumnWithDefault failed: %v", err)
+	}
+	if len(got) != 3 || got[0] != 0x00 || got[2] != 0x02 {
+		t.Errorf("Expected default value for empty bytea, got: %x", got)
+	}
+}
+
 func TestSetDeleted(t *testing.T) {
 	repo, db, charID := setupCharRepo(t)