fix(rengoku): prevent Sky Corridor race condition from wiping skill data (#85)

The client's Sky Corridor area transition handler saves rengoku data
before the load response is parsed into the character data area,
producing saves with zeroed skill fields but preserved point totals.

Detect this pattern server-side and merge existing skill data from the
database. Also reject sentinel-zero saves when valid data already exists.

server/channelserver/handlers_rengoku.go

@@ -1,29 +1,106 @@
 package channelserver
 
 import (
+	"encoding/binary"
 	ps "erupe-ce/common/pascalstring"
 	"fmt"
-	"github.com/jmoiron/sqlx"
 	"os"
 	"path/filepath"
 
+	"github.com/jmoiron/sqlx"
+
 	"erupe-ce/common/byteframe"
 	"erupe-ce/network/mhfpacket"
 	"go.uber.org/zap"
 )
 
+// rengokuSkillsZeroed checks if the skill slot IDs (offsets 0x1B-0x20) and
+// equipped skill values (offsets 0x2E-0x39) are all zero in a rengoku save blob.
+func rengokuSkillsZeroed(data []byte) bool {
+	if len(data) < 0x3A {
+		return true
+	}
+	for _, b := range data[0x1B:0x21] {
+		if b != 0 {
+			return false
+		}
+	}
+	for _, b := range data[0x2E:0x3A] {
+		if b != 0 {
+			return false
+		}
+	}
+	return true
+}
+
+// rengokuHasPoints checks if any skill point allocation (offsets 0x3B-0x46) is nonzero.
+func rengokuHasPoints(data []byte) bool {
+	if len(data) < 0x47 {
+		return false
+	}
+	for _, b := range data[0x3B:0x47] {
+		if b != 0 {
+			return true
+		}
+	}
+	return false
+}
+
+// rengokuMergeSkills copies skill slot IDs (0x1B-0x20) and equipped skill
+// values (0x2E-0x39) from existing data into the incoming save payload,
+// preserving the skills that the client failed to populate due to a race
+// condition during area transitions (see issue #85).
+func rengokuMergeSkills(dst, src []byte) {
+	copy(dst[0x1B:0x21], src[0x1B:0x21])
+	copy(dst[0x2E:0x3A], src[0x2E:0x3A])
+}
+
 func handleMsgMhfSaveRengokuData(s *Session, p mhfpacket.MHFPacket) {
-	// saved every floor on road, holds values such as floors progressed, points etc.
+	// Saved every floor on road, holds values such as floors progressed, points etc.
-	// can be safely handled by the client
+	// Can be safely handled by the client.
 	pkt := p.(*mhfpacket.MsgMhfSaveRengokuData)
 	dumpSaveData(s, pkt.RawDataPayload, "rengoku")
-	_, err := s.server.db.Exec("UPDATE characters SET rengokudata=$1 WHERE id=$2", pkt.RawDataPayload, s.charID)
+
+	saveData := pkt.RawDataPayload
+
+	// Guard against a client race condition (issue #85): the Sky Corridor init
+	// path triggers a rengoku save BEFORE the load response has been parsed into
+	// the character data area. This produces a save with zeroed skill fields but
+	// preserved point totals. Detect this pattern and merge existing skill data.
+	if len(saveData) >= 0x47 && rengokuSkillsZeroed(saveData) && rengokuHasPoints(saveData) {
+		var existing []byte
+		if err := s.server.db.QueryRow("SELECT rengokudata FROM characters WHERE id=$1", s.charID).Scan(&existing); err == nil {
+			if len(existing) >= 0x47 && !rengokuSkillsZeroed(existing) {
+				s.logger.Info("Rengoku save has zeroed skills with invested points, preserving existing skills",
+					zap.Uint32("charID", s.charID))
+				merged := make([]byte, len(saveData))
+				copy(merged, saveData)
+				rengokuMergeSkills(merged, existing)
+				saveData = merged
+			}
+		}
+	}
+
+	// Also reject saves where the sentinel is 0 (no data) if valid data already exists.
+	if len(saveData) >= 4 && binary.BigEndian.Uint32(saveData[:4]) == 0 {
+		var existing []byte
+		if err := s.server.db.QueryRow("SELECT rengokudata FROM characters WHERE id=$1", s.charID).Scan(&existing); err == nil {
+			if len(existing) >= 4 && binary.BigEndian.Uint32(existing[:4]) != 0 {
+				s.logger.Warn("Refusing to overwrite valid rengoku data with empty sentinel",
+					zap.Uint32("charID", s.charID))
+				doAckSimpleSucceed(s, pkt.AckHandle, make([]byte, 4))
+				return
+			}
+		}
+	}
+
+	_, err := s.server.db.Exec("UPDATE characters SET rengokudata=$1 WHERE id=$2", saveData, s.charID)
 	if err != nil {
 		s.logger.Error("Failed to save rengokudata", zap.Error(err))
 		doAckSimpleSucceed(s, pkt.AckHandle, make([]byte, 4))
 		return
 	}
-	bf := byteframe.NewByteFrameFromBytes(pkt.RawDataPayload)
+	bf := byteframe.NewByteFrameFromBytes(saveData)
 	_, _ = bf.Seek(71, 0)
 	maxStageMp := bf.ReadUint32()
 	maxScoreMp := bf.ReadUint32()
@@ -48,7 +125,8 @@ func handleMsgMhfLoadRengokuData(s *Session, p mhfpacket.MHFPacket) {
 	var data []byte
 	err := s.server.db.QueryRow("SELECT rengokudata FROM characters WHERE id = $1", s.charID).Scan(&data)
 	if err != nil {
-		s.logger.Error("Failed to load rengokudata", zap.Error(err))
+		s.logger.Error("Failed to load rengokudata", zap.Error(err),
+			zap.Uint32("charID", s.charID))
 	}
 	if len(data) > 0 {
 		doAckBufSucceed(s, pkt.AckHandle, data)