crypto/x509: update MaxPathLen & MaxPathLenZero docs

Fixes #19285

Change-Id: I11e864eff9980dec8247c47e4c600f76602e7ada
Reviewed-on: https://go-review.googlesource.com/46092Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/crypto/x509/x509.go

@@ -665,13 +665,28 @@ type Certificate struct {
 	ExtKeyUsage        []ExtKeyUsage           // Sequence of extended key usages.
 	UnknownExtKeyUsage []asn1.ObjectIdentifier // Encountered extended key usages unknown to this package.
 
-	BasicConstraintsValid bool // if true then the next two fields are valid.
+	// BasicConstraintsValid indicates whether IsCA, MaxPathLen,
+	// and MaxPathLenZero are valid.
+	BasicConstraintsValid bool
 	IsCA                  bool
+
+	// MaxPathLen and MaxPathLenZero indicate the presence and
+	// value of the BasicConstraints' "pathLenConstraint".
+	//
+	// When parsing a certificate, a positive non-zero MaxPathLen
+	// means that the field was specified, -1 means it was unset,
+	// and MaxPathLenZero being true mean that the field was
+	// explicitly set to zero. The case of MaxPathLen==0 with MaxPathLenZero==false
+	// should be treated equivalent to -1 (unset).
+	//
+	// When generating a certificate, an unset pathLenConstraint
+	// can be requested with either MaxPathLen == -1 or using the
+	// zero value for both MaxPathLen and MaxPathLenZero.
 	MaxPathLen int
-	// MaxPathLenZero indicates that BasicConstraintsValid==true and
-	// MaxPathLen==0 should be interpreted as an actual maximum path length
-	// of zero. Otherwise, that combination is interpreted as MaxPathLen
-	// not being set.
+	// MaxPathLenZero indicates that BasicConstraintsValid==true
+	// and MaxPathLen==0 should be interpreted as an actual
+	// maximum path length of zero. Otherwise, that combination is
+	// interpreted as MaxPathLen not being set.
 	MaxPathLenZero bool
 
 	SubjectKeyId   []byte
@@ -1149,7 +1164,7 @@ func parseCertificate(in *certificate) (*Certificate, error) {
 				out.IsCA = constraints.IsCA
 				out.MaxPathLen = constraints.MaxPathLen
 				out.MaxPathLenZero = out.MaxPathLen == 0
-
+				// TODO: map out.MaxPathLen to 0 if it has the -1 default value? (Issue 19285)
 			case 17:
 				out.DNSNames, out.EmailAddresses, out.IPAddresses, err = parseSANExtension(e.Value)
 				if err != nil {
@@ -1717,12 +1732,12 @@ func signingParamsForPublicKey(pub interface{}, requestedSigAlgo SignatureAlgori
 	return
 }
 
-// CreateCertificate creates a new certificate based on a template. The
-// following members of template are used: AuthorityKeyId,
-// BasicConstraintsValid, DNSNames, ExcludedDNSDomains, ExtKeyUsage, IsCA,
-// KeyUsage, MaxPathLen, NotAfter, NotBefore, PermittedDNSDomains,
-// PermittedDNSDomainsCritical, SerialNumber, SignatureAlgorithm, Subject,
-// SubjectKeyId, and UnknownExtKeyUsage.
+// CreateCertificate creates a new certificate based on a template.
+// The following members of template are used: AuthorityKeyId,
+// BasicConstraintsValid, DNSNames, ExcludedDNSDomains, ExtKeyUsage,
+// IsCA, KeyUsage, MaxPathLen, MaxPathLenZero, NotAfter, NotBefore,
+// PermittedDNSDomains, PermittedDNSDomainsCritical, SerialNumber,
+// SignatureAlgorithm, Subject, SubjectKeyId, and UnknownExtKeyUsage.
 //
 // The certificate is signed by parent. If parent is equal to template then the
 // certificate is self-signed. The parameter pub is the public key of the