fix: guard against nil chunk in bucket.Get to prevent panic

When cache data is loaded from a corrupted file, map entries may point
to chunk indices that are allocated but nil (beyond the saved chunksLen).
The existing bounds check verified chunkIdx against len(chunks) but did
not verify the chunk itself was non-nil before slicing it, which could
cause a runtime panic instead of being counted as a corruption miss.

Add a nil check for the chunk immediately after retrieval from the
chunks slice, before any slice expressions are evaluated.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: claude

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)