pkg/normalizer: populate function name for v2 arrow locations

pkg/normalizer/arrow_v2.go

@@ -520,7 +520,9 @@ func encodeStacktraceListViewV2(mem memory.Allocator, stacktrace *array.ListView
 //	  if hasFunction: uvarint(startLine), string(name), string(systemName), string(filename)
 //
 // v2 has no mapping_start/limit/offset, so they are written as zero. v2 has no
-// separate function name vs. system_name, so name is written as the empty string.
+// separate function name vs. system_name, so system_name is written as both;
+// readers that only look at the name (flame graph, table) would otherwise see
+// every pre-symbolized frame (e.g. kernel frames) as unnamed.
 func encodeV2Location(locIdx int, r *v2LocationReader) []byte {
 	address := r.address.Value(locIdx)
 
@@ -572,9 +574,10 @@ func encodeV2Location(locIdx int, r *v2LocationReader) []byte {
 			if !ok {
 				continue
 			}
+			systemName := v2FunctionSystemName(r, fIdx)
 			size += uvarintSize(r.startLine.Value(fIdx))
-			size += sizeOfString("") // function name (not present in v2)
-			size += sizeOfString(v2FunctionSystemName(r, fIdx))
+			size += sizeOfString(systemName) // name
+			size += sizeOfString(systemName)
 			size += sizeOfString(v2FunctionFilename(r, fIdx))
 		}
 	}
@@ -613,9 +616,10 @@ func encodeV2Location(locIdx int, r *v2LocationReader) []byte {
 			}
 			buf[offset] = 0x1
 			offset++
+			systemName := v2FunctionSystemName(r, fIdx)
 			offset += binary.PutUvarint(buf[offset:], r.startLine.Value(fIdx))
-			offset = writeStringV2(buf, offset, "")
-			offset = writeStringV2(buf, offset, v2FunctionSystemName(r, fIdx))
+			offset = writeStringV2(buf, offset, systemName)
+			offset = writeStringV2(buf, offset, systemName)
 			offset = writeStringV2(buf, offset, v2FunctionFilename(r, fIdx))
 		}
 	}

pkg/normalizer/arrow_v2_test.go

@@ -14,6 +14,7 @@
 package normalizer
 
 import (
+	"bytes"
 	"context"
 	"testing"
 
@@ -70,6 +71,13 @@ func TestAddSampleRecordV2(t *testing.T) {
 		require.NotEmpty(t, schema.FieldIndices(name), "expected internal record to have column %q", name)
 	}
 
+	// v2 only carries system_name; it must also be written as the function
+	// name, which is what the flame graph and table read.
+	stacks := out.Column(schema.FieldIndices(profile.ColumnStacktrace)[0]).(*array.List)
+	locs := stacks.ListValues().(*array.Dictionary)
+	encoded := locs.Dictionary().(*array.Binary).Value(locs.GetValueIndex(0))
+	require.Equal(t, 2, bytes.Count(encoded, []byte("do_thing")), "expected name and system_name to both be do_thing")
+
 	// Verify the timestamp ms / time_nanos split.
 	timestampIdx := schema.FieldIndices(profile.ColumnTimestamp)[0]
 	timeNanosIdx := schema.FieldIndices(profile.ColumnTimeNanos)[0]

pkg/profile/decode.go

@@ -159,6 +159,15 @@ func DecodeInto(lw LocationsWriter, data []byte, demangler Demangler) (DecodeRes
 				name, n := decodeString(data[offset:])
 				offset += n
 
+				systemName, n := decodeString(data[offset:])
+				offset += n
+
+				// Data written by the v2 ingest path before it populated the
+				// name only carries system_name.
+				if len(name) == 0 {
+					name = systemName
+				}
+
 				if demangler != nil {
 					name = []byte(demangler.Demangle(name))
 				}
@@ -167,9 +176,6 @@ func DecodeInto(lw LocationsWriter, data []byte, demangler Demangler) (DecodeRes
 					return DecodeResult{}, fmt.Errorf("append function name: %w", err)
 				}
 
-				systemName, n := decodeString(data[offset:])
-				offset += n
-
 				if err := lw.FunctionSystemName.Append(systemName); err != nil {
 					return DecodeResult{}, fmt.Errorf("append function system name: %w", err)
 				}
@@ -257,7 +263,11 @@ func DecodeFunctionName(data []byte) ([]byte, error) {
 				_, n = varint.Uvarint(data[offset:])
 				offset += n
 
-				name, _ := decodeString(data[offset:])
+				name, n := decodeString(data[offset:])
+				if len(name) == 0 {
+					// Fall back to system_name, matching DecodeInto.
+					name, _ = decodeString(data[offset+n:])
+				}
 				return name, nil
 			}
 		}

pkg/profile/encode_test.go

@@ -16,7 +16,9 @@ package profile
 import (
 	"testing"
 
+	"github.com/apache/arrow-go/v18/arrow/array"
 	"github.com/apache/arrow-go/v18/arrow/memory"
+	"github.com/stretchr/testify/require"
 
 	pprofpb "github.com/parca-dev/parca/gen/proto/go/google/pprof"
 )
@@ -72,3 +74,29 @@ func TestEncodeDecode(t *testing.T) {
 		})
 	}
 }
+
+// Locations written by the v2 ingest path before it populated the function
+// name (e.g. agent-symbolized kernel frames) only carry a system name.
+func TestDecodeFallsBackToSystemName(t *testing.T) {
+	t.Parallel()
+
+	buf := EncodePprofLocation(
+		&pprofpb.Location{Line: []*pprofpb.Line{{Line: 1, FunctionId: 1}}},
+		nil,
+		[]*pprofpb.Function{{SystemName: 1, Filename: 2}},
+		[]string{"", "do_syscall_64", "vmlinux"},
+	)
+
+	name, err := DecodeFunctionName(buf)
+	require.NoError(t, err)
+	require.Equal(t, "do_syscall_64", string(name))
+
+	lw := NewLocationsWriter(memory.DefaultAllocator)
+	defer lw.RecordBuilder.Release()
+	_, err = DecodeInto(lw, buf, nil)
+	require.NoError(t, err)
+
+	arr := lw.FunctionName.NewArray().(*array.Dictionary)
+	defer arr.Release()
+	require.Equal(t, "do_syscall_64", arr.Dictionary().(*array.Binary).ValueString(arr.GetValueIndex(0)))
+}