Add a function for parsing RFC 5280's "TBSCertificate".

net/cert/internal/test_helpers.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef NET_CERT_INTERNAL_TEST_HELPERS_H_
 #define NET_CERT_INTERNAL_TEST_HELPERS_H_
 
 #include <ostream>
 #include <string>
 #include <vector>
@@ skipping 18 matching lines @@
 // The returned der::Input() is only valid so long as the input string is alive
 // and is not mutated.
 //
 // Note that the input parameter has been made a pointer to prevent callers
 // from accidentally passing an r-value.
 der::Input InputFromString(const std::string* s);
 
 // Helper structure that maps a PEM block header (for instance "CERTIFICATE") to
 // the destination where the value for that block should be written.
 struct PemBlockMapping {
+  // The name of the PEM header. Example "CERTIFICATE".
   const char* block_name;
+
+  // The destination where the read value should be written to.
   std::string* value;
+
+  // True to indicate that the block is not required to be present. If the
+  // block is optional and is not present, then |value| will not be modified.
+  bool optional;
 };
 
 // ReadTestDataFromPemFile() is a helper function that reads a PEM test file
 // rooted in the "src/" directory.
 //
 //   * file_path_ascii:
 //       The path to the PEM file, relative to src. For instance
 //       "net/data/verify_signed_data_unittest/foopy.pem"
 //
 //   * mappings:
 //       An array of length |mappings_length| which maps the expected PEM
 //       headers to the destination to write its data.
 //
 // The function ensures that each of the chosen mappings is satisfied exactly
-// once. In other words, the header must be present, have valid data, and
-// appear no more than once.
+// once. In other words, the header must be present (unless marked as
+// optional=true), have valid data, and appear no more than once.
 ::testing::AssertionResult ReadTestDataFromPemFile(
     const std::string& file_path_ascii,
     const PemBlockMapping* mappings,
     size_t mappings_length);
 
 // This is the same as the variant above, however it uses template magic so an
 // mappings array can be passed in directly (and the correct length is
 // inferred).
 template <size_t N>
 ::testing::AssertionResult ReadTestDataFromPemFile(
     const std::string& file_path_ascii,
     const PemBlockMapping(&mappings)[N]) {
   return ReadTestDataFromPemFile(file_path_ascii, mappings, N);
 }
 
 }  // namespace net
 
 #endif  // NET_CERT_INTERNAL_TEST_HELPERS_H_