net: followup to codereview for cl/7096014

net/base/asn1_util.h

Index: net/base/asn1_util.h
diff --git a/net/base/asn1_util.h b/net/base/asn1_util.h
index ea6277ac0345bac929552a9e7a4bbf5ea9101e24..dc9982be8d463a77798625d09da24dc53525ce58 100644
--- a/net/base/asn1_util.h
+++ b/net/base/asn1_util.h
@@ -24,10 +24,12 @@ static const unsigned kSEQUENCE = 0x30;
 
 // These are flags that can be ORed with the above tag numbers.
 static const unsigned kContextSpecific = 0x80;
-static const unsigned kCompound = 0x20;
+static const unsigned kConstructed = 0x20;
 
 // kAny matches any tag value;
 static const unsigned kAny = 0x10000;
+// kOptional denotes an optional element.
+static const unsigned kOptional = 0x20000;
 
 // ParseElement parses a DER encoded ASN1 element from |in|, requiring that
 // it have the given |tag_value|. It returns true on success. The following
@@ -38,6 +40,9 @@ static const unsigned kAny = 0x10000;
 //   |in| is advanced over the element
 //   |out| contains the element, including the tag and length bytes.
 //   |out_header_len| contains the length of the tag and length bytes in |out|.
+//
+// If |tag_value & kOptional| is true then *out_header_len can be zero after a
+// true return value if the element was not found.
 bool ParseElement(base::StringPiece* in,
                   unsigned tag_value,
                   base::StringPiece* out,
@@ -45,11 +50,13 @@ bool ParseElement(base::StringPiece* in,
 
 // GetElement performs the same actions as ParseElement, except that the header
 // bytes are not included in the output.
+//
+// If |tag_value & kOptional| is true then this function cannot distinguish
+// between a missing optional element and an empty one.
 bool GetElement(base::StringPiece* in,
                 unsigned tag_value,
                 base::StringPiece* out);
 
-
 // ExtractSPKIFromDERCert parses the DER encoded certificate in |cert| and
 // extracts the bytes of the SubjectPublicKeyInfo. On successful return,
 // |spki_out| is set to contain the SPKI, pointing into |cert|.
@@ -63,6 +70,8 @@ NET_TEST bool ExtractSPKIFromDERCert(base::StringPiece cert,
 // CRLs that only cover a subset of the reasons are omitted as the spec
 // requires that at least one CRL be included that covers all reasons.
 //
+// CRLs that use an alternative issuer are also omitted.
+//
 // The nested set of GeneralNames is flattened into a single list because
 // having several CRLs with one location is equivalent to having one CRL with
 // several locations as far as a CRL filter is concerned.