Fixes merge of empty samples in pprof (#3031)

Author: cyriltovena

pkg/pprof/merge.go

@@ -37,16 +37,18 @@ func (m *ProfileMerge) merge(p *profilev1.Profile, clone bool) error {
 		return nil
 	}
 	ConvertIDsToIndices(p)
+	var initial bool
 	if m.profile == nil {
 		m.init(p, clone)
+		initial = true
 	}
 
 	// We rewrite strings first in order to compare
 	// sample types and period type.
 	m.tmp = slices.GrowLen(m.tmp, len(p.StringTable))
 	m.stringTable.Index(m.tmp, p.StringTable)
 	RewriteStrings(p, m.tmp)
-	if len(m.profile.StringTable) == 0 {
+	if initial {
 		// Right after initialisation we need to make
 		// sure that the string identifiers are normalized
 		// among profiles.

pkg/pprof/merge_test.go

@@ -368,3 +368,60 @@ func Test_Merge_Sample(t *testing.T) {
 
 	testhelper.EqualProto(t, expected, m.Profile())
 }
+
+func TestMergeEmpty(t *testing.T) {
+	var m ProfileMerge
+
+	err := m.Merge(&profilev1.Profile{
+		SampleType: []*profilev1.ValueType{
+			{
+				Type: 2,
+				Unit: 1,
+			},
+		},
+		PeriodType: &profilev1.ValueType{
+			Type: 2,
+			Unit: 1,
+		},
+		StringTable: []string{"", "nanoseconds", "cpu"},
+	})
+	require.NoError(t, err)
+	err = m.Merge(&profilev1.Profile{
+		Sample: []*profilev1.Sample{
+			{
+				LocationId: []uint64{1},
+				Value:      []int64{1},
+			},
+		},
+		Location: []*profilev1.Location{
+			{
+				Id:        1,
+				MappingId: 1,
+				Line:      []*profilev1.Line{{FunctionId: 1, Line: 1}},
+			},
+		},
+		Function: []*profilev1.Function{
+			{
+				Id:   1,
+				Name: 1,
+			},
+		},
+		SampleType: []*profilev1.ValueType{
+			{
+				Type: 3,
+				Unit: 2,
+			},
+		},
+		PeriodType: &profilev1.ValueType{
+			Type: 3,
+			Unit: 2,
+		},
+		Mapping: []*profilev1.Mapping{
+			{
+				Id: 1,
+			},
+		},
+		StringTable: []string{"", "bar", "nanoseconds", "cpu"},
+	})
+	require.NoError(t, err)
+}