Fix: EncodeAggrChunk created corrupt block if the last chunk is empty

CHANGELOG.md

@@ -13,6 +13,7 @@ We use *breaking :warning:* to mark changes that are not backward compatible (re
 It is recommend to upgrade the storage components first (Receive, Store, etc.) and then Queriers. This will enable batching support. Otherwise, you risk high memory usage in the Querier component if gRPC compression is enabled.
 
 ### Fixed
+- [#8684](https://github.com/thanos-io/thanos/pull/8684): Fix: EncodeAggrChunk created corrupt block if the last chunk is empty
 
 ### Added
 

pkg/compact/downsample/aggr.go

@@ -23,6 +23,7 @@ type AggrChunk []byte
 func EncodeAggrChunk(chks [5]chunkenc.Chunk) *AggrChunk {
 	var b []byte
 	buf := [8]byte{}
+	lastNil := chks[AggrCounter] == nil
 
 	for _, c := range chks {
 		// Unset aggregates are marked with a zero length entry.
@@ -37,6 +38,11 @@ func EncodeAggrChunk(chks [5]chunkenc.Chunk) *AggrChunk {
 		b = append(b, byte(c.Encoding()))
 		b = append(b, c.Bytes()...)
 	}
+	// Keep one trailing byte when the last aggregate is missing so decoders that
+	// validate size before handling zero-length entries don't fail on the final field.
+	if lastNil {
+		b = append(b, 0)
+	}
 	chk := AggrChunk(b)
 	return &chk
 }

pkg/compact/downsample/aggr_test.go

@@ -58,3 +58,17 @@ func TestAggrChunk(t *testing.T) {
 	}
 	testutil.Equals(t, input, res)
 }
+
+func TestAggrChunk_TrailingNilAggregate(t *testing.T) {
+	var chks [5]chunkenc.Chunk
+
+	chks[AggrCount] = chunkenc.NewXORChunk()
+	a, err := chks[AggrCount].Appender()
+	testutil.Ok(t, err)
+	a.Append(100, 1)
+
+	// Keep AggrCounter (the last aggregate) nil. It should decode as missing aggregate.
+	ac := EncodeAggrChunk(chks)
+	_, err = ac.Get(AggrCounter)
+	testutil.Equals(t, ErrAggrNotExist, err)
+}