Query: Fix panic in ZLabelsToPromLabels due to unsafe pointer cast incompatible with stringlabels

CHANGELOG.md

@@ -15,6 +15,7 @@ It is recommend to upgrade the storage components first (Receive, Store, etc.) a
 ### Fixed
 
 - [#8128](https://github.com/thanos-io/thanos/issues/8128): Query-Frontend: Fix panic in `AnalyzesMerge` caused by indexing the wrong slice variable, leading to an out-of-range access when merging more than two query analyses.
+- [#8709](https://github.com/thanos-io/thanos/pull/8709): Query/Query-Frontend: fix panic in `ZLabelsToPromLabels` and `FromLabelAdaptersToLabels` caused by unsafe pointer casts incompatible with Prometheus `stringlabels` encoding.
 
 ### Added
 

internal/cortex/cortexpb/compat.go

@@ -19,21 +19,29 @@ import (
 	"github.com/thanos-io/thanos/internal/cortex/util"
 )
 
-// FromLabelAdaptersToLabels casts []LabelAdapter to labels.Labels.
-// It uses unsafe, but as LabelAdapter == labels.Label this should be safe.
-// This allows us to use labels.Labels directly in protos.
-//
-// Note: while resulting labels.Labels is supposedly sorted, this function
-// doesn't enforce that. If input is not sorted, output will be wrong.
+// FromLabelAdaptersToLabels converts []LabelAdapter to labels.Labels.
 func FromLabelAdaptersToLabels(ls []LabelAdapter) labels.Labels {
-	return *(*labels.Labels)(unsafe.Pointer(&ls))
+	if len(ls) == 0 {
+		return labels.EmptyLabels()
+	}
+	b := labels.NewScratchBuilder(len(ls))
+	for _, l := range ls {
+		b.Add(l.Name, l.Value)
+	}
+	b.Sort()
+	return b.Labels()
 }
 
-// FromLabelsToLabelAdapters casts labels.Labels to []LabelAdapter.
-// It uses unsafe, but as LabelAdapter == labels.Label this should be safe.
-// This allows us to use labels.Labels directly in protos.
+// FromLabelsToLabelAdapters converts labels.Labels to []LabelAdapter.
 func FromLabelsToLabelAdapters(ls labels.Labels) []LabelAdapter {
-	return *(*[]LabelAdapter)(unsafe.Pointer(&ls))
+	if ls.IsEmpty() {
+		return nil
+	}
+	result := make([]LabelAdapter, 0, ls.Len())
+	ls.Range(func(l labels.Label) {
+		result = append(result, LabelAdapter{Name: l.Name, Value: l.Value})
+	})
+	return result
 }
 
 // FromLabelAdaptersToMetric converts []LabelAdapter to a model.Metric.

pkg/store/labelpb/label.go

@@ -38,18 +38,25 @@ func noAllocBytes(buf string) []byte {
 	return *(*[]byte)(unsafe.Pointer(&buf))
 }
 
-// ZLabelsFromPromLabels converts Prometheus labels to slice of labelpb.ZLabel in type unsafe manner.
-// It reuses the same memory. Caller should abort using passed labels.Labels.
+// ZLabelsFromPromLabels converts Prometheus labels to slice of labelpb.ZLabel.
 func ZLabelsFromPromLabels(lset labels.Labels) []ZLabel {
-	return *(*[]ZLabel)(unsafe.Pointer(&lset))
+	if lset.IsEmpty() {
+		return nil
+	}
+	zlabels := make([]ZLabel, 0, lset.Len())
+	lset.Range(func(l labels.Label) {
+		zlabels = append(zlabels, ZLabel{Name: l.Name, Value: l.Value})
+	})
+	return zlabels
 }
 
-// ZLabelsToPromLabels convert slice of labelpb.ZLabel to Prometheus labels in type unsafe manner.
-// It reuses the same memory. Caller should abort using passed []ZLabel.
-// NOTE: Use with care. ZLabels holds memory from the whole protobuf unmarshal, so the returned
-// Prometheus Labels will hold this memory as well.
+// ZLabelsToPromLabels converts slice of labelpb.ZLabel to Prometheus labels.
 func ZLabelsToPromLabels(lset []ZLabel) labels.Labels {
-	return *(*labels.Labels)(unsafe.Pointer(&lset))
+	b := labels.NewScratchBuilder(len(lset))
+	for _, l := range lset {
+		b.Add(l.Name, l.Value)
+	}
+	return b.Labels()
 }
 
 // ReAllocAndInternZLabelsStrings re-allocates all underlying bytes for string, detaching it from bigger memory pool.

pkg/store/labelpb/label_zlabel_test.go

@@ -38,10 +38,9 @@ func TestLabelsToPromLabels_LabelsToPromLabels(t *testing.T) {
 		zlset[i].Value += "yolo"
 	}
 
+	// ZLabelsFromPromLabels now copies data, so modifying zlset does not affect the original.
 	m := lset.Map()
-	testutil.Equals(t, "1yolo", m["a"])
-
-	m["a"] = "1"
+	testutil.Equals(t, "1", m["a"])
 	testutil.Equals(t, testLsetMap, m)
 }