Make B3 parser more readable. (#376)

No functional change.

Author: g-easy

opencensus/trace/internal/b3.cc

@@ -75,17 +75,23 @@ SpanContext FromB3Headers(absl::string_view b3_trace_id,
   std::string trace_id_binary = absl::HexStringToBytes(b3_trace_id);
   std::string span_id_binary = absl::HexStringToBytes(b3_span_id);
 
-  // Extend 64-bit trace_id to 128-bit.
   uint8_t extended_trace_id[16];
-  if (b3_trace_id.length() == 16) {
+
+  // trace_id_ptr must point to a 128-bit trace_id.
+  const uint8_t* trace_id_ptr;
+  if (trace_id_binary.length() == 16) {
+    trace_id_ptr = reinterpret_cast<const uint8_t*>(trace_id_binary.data());
+  } else if (trace_id_binary.length() == 8) {
+    // Extend 64-bit trace_id to 128-bit using the buffer.
     memset(extended_trace_id, 0, 8);
     memcpy(extended_trace_id + 8, trace_id_binary.data(), 8);
+    trace_id_ptr = extended_trace_id;
+  } else {
+    return invalid;
   }
 
   return SpanContext(
-      TraceId((b3_trace_id.length() == 16)
-                  ? extended_trace_id
-                  : reinterpret_cast<const uint8_t*>(trace_id_binary.data())),
+      TraceId(trace_id_ptr),
       SpanId(reinterpret_cast<const uint8_t*>(span_id_binary.data())),
       TraceOptions(&sampled));
 }