fix: guard against nil chunk in bucket.Get to prevent panic on corrupted cache load

fastcache.go

@@ -393,9 +393,12 @@ func (b *bucket) Get(dst, k []byte, h uint64, returnDst bool) ([]byte, bool) {
 				goto end
 			}
 			chunk := chunks[chunkIdx]
+			if chunk == nil {
+				atomic.AddUint64(&b.corruptions, 1)
+				goto end
+			}
 			idx %= chunkSize
 			if idx+4 >= chunkSize {
-				// Corrupted data during the load from file. Just skip it.
 				atomic.AddUint64(&b.corruptions, 1)
 				goto end
 			}
@@ -404,7 +407,6 @@ func (b *bucket) Get(dst, k []byte, h uint64, returnDst bool) ([]byte, bool) {
 			valLen := (uint64(kvLenBuf[2]) << 8) | uint64(kvLenBuf[3])
 			idx += 4
 			if idx+keyLen+valLen >= chunkSize {
-				// Corrupted data during the load from file. Just skip it.
 				atomic.AddUint64(&b.corruptions, 1)
 				goto end
 			}

fastcache_test.go

@@ -6,6 +6,8 @@ import (
 	"sync"
 	"testing"
 	"time"
+
+	xxhash "github.com/cespare/xxhash/v2"
 )
 
 func TestCacheSmall(t *testing.T) {
@@ -222,6 +224,38 @@ func testCacheGetSet(c *Cache, itemsCount int) error {
 	return nil
 }
 
+func TestBucketGetNilChunkNoCorruptionPanic(t *testing.T) {
+	var b bucket
+	b.Init(64 * 1024)
+
+	k := []byte("testkey")
+	v := []byte("testvalue")
+	h := xxhash.Sum64(k)
+	b.Set(k, v, h)
+
+	got, found := b.Get(nil, k, h, true)
+	if !found || string(got) != "testvalue" {
+		t.Fatalf("expected to find key before corruption; found=%v val=%q", found, got)
+	}
+
+	b.mu.Lock()
+	mapVal := b.m[h]
+	chunkIdx := (mapVal & ((1 << bucketSizeBits) - 1)) / chunkSize
+	b.chunks[chunkIdx] = nil
+	b.mu.Unlock()
+
+	got, found = b.Get(nil, k, h, true)
+	if found {
+		t.Fatalf("expected miss for nil chunk, but got found=true val=%q", got)
+	}
+
+	var s Stats
+	b.UpdateStats(&s)
+	if s.Corruptions != 1 {
+		t.Fatalf("expected 1 corruption; got %d", s.Corruptions)
+	}
+}
+
 func TestCacheResetUpdateStatsSetConcurrent(t *testing.T) {
 	c := New(12334)