fix: regex usage in w3c extraction (#121)

Usage of std::regex_match was problematic for traceparent with trailing
spaces. The overload use is `std::regex_match(str, str + std::char_traits<CharT>::length(str), m, e, flags)`,
which resolves as the whole input instead of the trimmed input.

Changes:
  - Fix std::regex_match usage.
  - Add unit test.

src/datadog/w3c_propagation.cpp

@@ -17,6 +17,18 @@ namespace datadog {
 namespace tracing {
 namespace {
 
+// Note that match group 0 is the entire match.
+constexpr StringView k_traceparent_pattern =
+    "([0-9a-f]{2})"  // hex version number (match group 1)
+    "-"
+    "([0-9a-f]{32})"  // hex trace ID (match group 2)
+    "-"
+    "([0-9a-f]{16})"  // hex parent span ID (match group 3)
+    "-"
+    "([0-9a-f]{2})"  // hex "trace-flags" (match group 4)
+    "($|-.*)";  // either the end, or a hyphen preceding further fields (match
+                // group 5)
+
 // Return a predicate that returns whether its `char` argument is any of the
 // following:
 //
@@ -45,32 +57,23 @@ Optional<std::string> extract_traceparent(ExtractedData& result,
 
   const auto traceparent = trim(*maybe_traceparent);
 
-  // Note that leading and trailing whitespace was already removed above.
-  // Note that match group 0 is the entire match.
-  static const auto& pattern =
-      "([0-9a-f]{2})"  // hex version number (match group 1)
-      "-"
-      "([0-9a-f]{32})"  // hex trace ID (match group 2)
-      "-"
-      "([0-9a-f]{16})"  // hex parent span ID (match group 3)
-      "-"
-      "([0-9a-f]{2})"  // hex "trace-flags" (match group 4)
-      "($|-.*)";  // either the end, or a hyphen preceding further fields (match
-                  // group 5)
-  static const std::regex regex{pattern};
+  static const std::regex regex{k_traceparent_pattern.data()};
 
   std::cmatch match;
-  if (!std::regex_match(traceparent.data(), match, regex)) {
+  if (!std::regex_match(traceparent.data(),
+                        traceparent.data() + traceparent.size(), match,
+                        regex)) {
     return "malformed_traceparent";
   }
 
   assert(match.ready());
-  assert(match.size() == 5 + 1);
+  assert(match.size() == 6);
 
-  const auto to_string_view = [traceparent](const std::cmatch& match,
-                                            const std::size_t index) {
+  const auto to_string_view = [traceparent_beg = traceparent.data()](
+                                  const std::cmatch& match,
+                                  const std::size_t index) {
     assert(index < match.size());
-    return StringView(traceparent.data() + match.position(index),
+    return StringView(traceparent_beg + match.position(index),
                       std::size_t(match.length(index)));
   };
 

test/test_tracer.cpp

@@ -622,6 +622,13 @@ TEST_CASE("span extraction") {
          67667974448284343ULL, // expected_parent_id
          0}, // expected_sampling_priority
 
+        {__LINE__, "valid: leading and trailing spaces",
+         "    00-4bf92f3577b34da6a3ce929d0e0e4736-00f067aa0ba902b7-01 \t", // traceparent
+         nullopt,
+         *TraceID::parse_hex("4bf92f3577b34da6a3ce929d0e0e4736"), // expected_trace_id
+         67667974448284343ULL, // expected_parent_id
+         1}, // expected_sampling_priority
+
         {__LINE__, "no traceparent",
          nullopt}, // traceparent