feat(savedata): add tier 1 data integrity protections

Prevent savedata corruption and denial-of-service by adding four layers
of protection to the save pipeline:

- Bounded decompression (nullcomp.DecompressWithLimit): caps output size
  to prevent OOM from crafted payloads that expand to exhaust memory
- Bounds-checked delta patching (deltacomp.ApplyDataDiffWithLimit):
  validates offsets before writing, returns errors for negative offsets,
  truncated patches, and oversized output; ApplyDataDiff now returns
  original data on error instead of partial corruption
- Size limits on save handlers: rejects compressed payloads >512KB and
  decompressed data >1MB before processing; applied to main savedata,
  platedata, and platebox diff paths
- Rotating savedata backups: 3 slots per character with 30-minute
  interval, snapshots the previous state before overwriting, backed by
  new savedata_backups table (migration 0007)

server/channelserver/compression/deltacomp/deltacomp.go

@@ -2,6 +2,7 @@ package deltacomp
 
 import (
 	"bytes"
+	"fmt"
 	"io"
 
 	"go.uber.org/zap"
@@ -49,8 +50,21 @@ func readCount(r *bytes.Reader) (int, error) {
 
 // ApplyDataDiff applies a delta data diff patch onto given base data.
 func ApplyDataDiff(diff []byte, baseData []byte) []byte {
-	// Make a copy of the base data to return,
-	// (probably just make this modify the given slice in the future).
+	result, err := ApplyDataDiffWithLimit(diff, baseData, 0)
+	if err != nil {
+		zap.L().Error("ApplyDataDiff failed", zap.Error(err))
+		// Return original data on error to avoid corruption
+		out := make([]byte, len(baseData))
+		copy(out, baseData)
+		return out
+	}
+	return result
+}
+
+// ApplyDataDiffWithLimit applies a delta data diff patch onto given base data.
+// If maxOutput > 0, the result is capped at that size; exceeding it returns an error.
+// If maxOutput == 0, no limit is enforced (backwards-compatible behavior).
+func ApplyDataDiffWithLimit(diff []byte, baseData []byte, maxOutput int) ([]byte, error) {
 	baseCopy := make([]byte, len(baseData))
 	copy(baseCopy, baseData)
 
@@ -76,32 +90,35 @@ func ApplyDataDiff(diff []byte, baseData []byte) []byte {
 		}
 		differentCount--
 
-		// Grow slice if it's required
-		if len(baseCopy) < dataOffset {
-			zap.L().Warn("Slice smaller than data offset, growing slice")
-			baseCopy = append(baseCopy, make([]byte, (dataOffset+differentCount)-len(baseData))...)
-		} else {
-			length := len(baseCopy[dataOffset:])
-			if length < differentCount {
-				length -= differentCount
-				baseCopy = append(baseCopy, make([]byte, length)...)
-			}
+		if dataOffset < 0 {
+			return nil, fmt.Errorf("negative data offset %d", dataOffset)
+		}
+		if differentCount < 0 {
+			return nil, fmt.Errorf("negative different count %d at offset %d", differentCount, dataOffset)
+		}
+
+		endOffset := dataOffset + differentCount
+		if maxOutput > 0 && endOffset > maxOutput {
+			return nil, fmt.Errorf("patch writes to offset %d, exceeds limit %d", endOffset, maxOutput)
+		}
+
+		// Grow slice if required
+		if endOffset > len(baseCopy) {
+			baseCopy = append(baseCopy, make([]byte, endOffset-len(baseCopy))...)
 		}
 
 		// Apply the patch bytes.
 		for i := 0; i < differentCount; i++ {
 			b, err := checkReadUint8(patch)
 			if err != nil {
-				zap.L().Error("Invalid or misunderstood patch format", zap.Int("dataOffset", dataOffset))
-				return baseCopy
+				return nil, fmt.Errorf("truncated patch at offset %d+%d: %w", dataOffset, i, err)
 			}
 
 			baseCopy[dataOffset+i] = b
 		}
 
 		dataOffset += differentCount - 1
-
 	}
 
-	return baseCopy
+	return baseCopy, nil
 }

server/channelserver/compression/deltacomp/deltacomp_test.go

@@ -75,6 +75,83 @@ func readTestDataFile(filename string) []byte {
 	return data
 }
 
+func TestApplyDataDiffWithLimit_BoundsCheck(t *testing.T) {
+	// Base data: 10 bytes
+	baseData := []byte{0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0A}
+
+	// Build a patch that tries to write at offset 8 with 5 different bytes,
+	// which would extend to offset 13 (beyond 10-byte base).
+	// Format: matchCount=9 (first is +1), differentCount=6 (is -1 = 5 bytes)
+	diff := []byte{
+		0x09,                               // matchCount (first is +1, so offset becomes -1+9=8)
+		0x06,                               // differentCount (6-1=5 different bytes)
+		0xAA, 0xBB, 0xCC, 0xDD, 0xEE,      // 5 patch bytes
+	}
+
+	t.Run("within_limit", func(t *testing.T) {
+		// Limit of 20 allows the growth
+		result, err := ApplyDataDiffWithLimit(diff, baseData, 20)
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		if len(result) < 13 {
+			t.Errorf("expected result length >= 13, got %d", len(result))
+		}
+	})
+
+	t.Run("exceeds_limit", func(t *testing.T) {
+		// Limit of 10 doesn't allow writing past the base
+		_, err := ApplyDataDiffWithLimit(diff, baseData, 10)
+		if err == nil {
+			t.Error("expected error for write past limit, got none")
+		}
+	})
+
+	t.Run("no_limit", func(t *testing.T) {
+		// maxOutput=0 means no limit (backwards compatible)
+		result, err := ApplyDataDiffWithLimit(diff, baseData, 0)
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		if len(result) < 13 {
+			t.Errorf("expected result length >= 13, got %d", len(result))
+		}
+	})
+}
+
+func TestApplyDataDiffWithLimit_TruncatedPatch(t *testing.T) {
+	baseData := []byte{0x01, 0x02, 0x03, 0x04}
+
+	// Patch claims 3 different bytes but only provides 1
+	diff := []byte{
+		0x02, // matchCount (offset = -1+2 = 1)
+		0x04, // differentCount (4-1=3 different bytes)
+		0xAA, // only 1 byte provided (missing 2)
+	}
+
+	_, err := ApplyDataDiffWithLimit(diff, baseData, 100)
+	if err == nil {
+		t.Error("expected error for truncated patch, got none")
+	}
+}
+
+func TestApplyDataDiff_ReturnsOriginalOnError(t *testing.T) {
+	baseData := []byte{0x01, 0x02, 0x03, 0x04}
+
+	// Truncated patch
+	diff := []byte{
+		0x02,
+		0x04,
+		0xAA, // only 1 of 3 expected bytes
+	}
+
+	result := ApplyDataDiff(diff, baseData)
+	// On error, ApplyDataDiff should return the original data unchanged
+	if !bytes.Equal(result, baseData) {
+		t.Errorf("expected original data on error, got %v", result)
+	}
+}
+
 func TestDeltaPatch(t *testing.T) {
 	for k, tt := range tests {
 		testname := fmt.Sprintf("delta_patch_test_%d", k)