[pkg/stanza] panic when converting empty trace_flags []byte (#21031)

.chloggen/stanza_converter_panic.yaml

@@ -0,0 +1,16 @@
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: bug_fix
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: filelogreceiver
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: stanza panic when trace_flags is an empty string
+
+# One or more tracking issues related to the change
+issues: [20924]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:

pkg/stanza/adapter/converter.go

@@ -265,7 +265,7 @@ func convertInto(ent *entry.Entry, dest plog.LogRecord) {
 		copy(buffer[0:8], ent.SpanID)
 		dest.SetSpanID(buffer)
 	}
-	if ent.TraceFlags != nil {
+	if ent.TraceFlags != nil && len(ent.TraceFlags) > 0 {
 		// The 8 least significant bits are the trace flags as defined in W3C Trace
 		// Context specification. Don't override the 24 reserved bits.
 		flags := uint32(ent.TraceFlags[0])

pkg/stanza/adapter/converter_test.go

@@ -796,6 +796,27 @@ func TestConvertTrace(t *testing.T) {
 	require.Equal(t, uint32(0x01), uint32(record.Flags()))
 }
 
+func TestConvertTraceEmptyFlags(t *testing.T) {
+	record := convertAndDrill(&entry.Entry{
+		TraceID: []byte{
+			0x48, 0x01, 0x40, 0xf3, 0xd7, 0x70, 0xa5, 0xae, 0x32, 0xf0, 0xa2, 0x2b, 0x6a, 0x81, 0x2c, 0xff,
+		},
+		SpanID: []byte{
+			0x32, 0xf0, 0xa2, 0x2b, 0x6a, 0x81, 0x2c, 0xff,
+		},
+		TraceFlags: []byte{}})
+
+	require.Equal(t, pcommon.TraceID(
+		[16]byte{
+			0x48, 0x01, 0x40, 0xf3, 0xd7, 0x70, 0xa5, 0xae, 0x32, 0xf0, 0xa2, 0x2b, 0x6a, 0x81, 0x2c, 0xff,
+		}), record.TraceID())
+	require.Equal(t, pcommon.SpanID(
+		[8]byte{
+			0x32, 0xf0, 0xa2, 0x2b, 0x6a, 0x81, 0x2c, 0xff,
+		}), record.SpanID())
+	require.Equal(t, uint32(0x00), uint32(record.Flags()))
+}
+
 func BenchmarkConverter(b *testing.B) {
 	const (
 		entryCount = 1_000_000