Validate labels (#528)

* validate label set coming through WAL Appender

Closes #527

* changelog

* tests

CHANGELOG.md

@@ -4,6 +4,10 @@ can be found at [#317](https://github.com/grafana/agent/issues/317).
 
 # Main (unreleased)
 
+- [BUGFIX] Validate that incoming scraped metrics do not have an empty label
+  set or a label set with duplicate labels, mirroring the behavior of
+  Prometheus. (@rfratto)
+
 # v0.13.0 (2021-02-25)
 
 The primary branch name has changed from `master` to `main`. You may have to

pkg/prom/wal/wal.go

@@ -15,6 +15,7 @@ import (
 	"github.com/prometheus/prometheus/pkg/timestamp"
 	"github.com/prometheus/prometheus/pkg/value"
 	"github.com/prometheus/prometheus/storage"
+	"github.com/prometheus/prometheus/tsdb"
 	"github.com/prometheus/prometheus/tsdb/record"
 	"github.com/prometheus/prometheus/tsdb/wal"
 )
@@ -557,6 +558,17 @@ func (a *appender) Add(l labels.Labels, t int64, v float64) (uint64, error) {
 		return series.ref, a.AddFast(series.ref, t, v)
 	}
 
+	// Ensure no empty or duplicate labels have gotten through. This mirrors the
+	// equivalent validation code in the TSDB's headAppender.
+	l = l.WithoutEmpty()
+	if len(l) == 0 {
+		return 0, errors.Wrap(tsdb.ErrInvalidSample, "empty labelset")
+	}
+
+	if lbl, dup := l.HasDuplicateLabelNames(); dup {
+		return 0, errors.Wrap(tsdb.ErrInvalidSample, fmt.Sprintf(`label name "%s" is not unique`, lbl))
+	}
+
 	a.w.mtx.Lock()
 	ref := a.w.nextRef
 	a.w.nextRef++

pkg/prom/wal/wal_test.go

@@ -17,6 +17,30 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
+func TestStorage_InvalidSeries(t *testing.T) {
+	walDir, err := ioutil.TempDir(os.TempDir(), "wal")
+	require.NoError(t, err)
+	defer os.RemoveAll(walDir)
+
+	s, err := NewStorage(log.NewNopLogger(), nil, walDir)
+	require.NoError(t, err)
+	defer func() {
+		require.NoError(t, s.Close())
+	}()
+
+	app := s.Appender(context.Background())
+
+	_, err = app.Add(labels.Labels{}, 0, 0)
+	require.Error(t, err, "should reject empty labels")
+
+	_, err = app.Add(labels.Labels{{Name: "a", Value: "1"}, {Name: "a", Value: "2"}}, 0, 0)
+	require.Error(t, err, "should reject duplicate labels")
+
+	// Sanity check: valid series
+	_, err = app.Add(labels.Labels{{Name: "a", Value: "1"}}, 0, 0)
+	require.NoError(t, err, "should not reject valid series")
+}
+
 func TestStorage(t *testing.T) {
 	walDir, err := ioutil.TempDir(os.TempDir(), "wal")
 	require.NoError(t, err)