[logdog/common/types] custom JSON encoding to allow streamaddr to be zero.

logdog/common/types/streamaddr.go

Index: logdog/common/types/streamaddr.go
diff --git a/logdog/common/types/streamaddr.go b/logdog/common/types/streamaddr.go
index f952ff1bd4ffbaf419c2421935561cef70cc1a44..193e3d51801821b85a2bb686dca612222deb2c02 100644
--- a/logdog/common/types/streamaddr.go
+++ b/logdog/common/types/streamaddr.go
@@ -5,6 +5,8 @@
 package types
 
 import (
+	"bytes"
+	"encoding/json"
 	"flag"
 	"net/url"
 	"strings"
@@ -18,13 +20,13 @@ const logDogURLScheme = "logdog"
 // StreamAddr is a fully-qualified LogDog stream address.
 type StreamAddr struct {
 	// Host is the LogDog host.
-	Host string
+	Host string `json:"host"`
 
 	// Project is the LUCI project name that this log belongs to.
-	Project cfgtypes.ProjectName
+	Project cfgtypes.ProjectName `json:"project"`
 
 	// Path is the LogDog stream path.
-	Path StreamPath
+	Path StreamPath `json:"path"`
 }
 
 var _ flag.Value = (*StreamAddr)(nil)
@@ -70,6 +72,45 @@ func (s *StreamAddr) URL() *url.URL {
 	}
 }
 
+// jsonStreamAddr is used to allow type checking during deserialization, but
+// special casing IsZero == 'null'.
+type jsonStreamAddr struct {

[dnj, 2017/05/23 15:28:19]

This is unfortunate. Are you opposed to using "omitempty" in all of the fields
and settling for an empty object instead of null?

addr: null
addr: {}

The latter isn't any less descriptive to me, and null will still unmarshal just
fine.

+	Host    string               `json:"host"`
+	Project cfgtypes.ProjectName `json:"project"`
+	Path    StreamPath           `json:"path"`
+}
+
+// MarshalJSON implements json.Marshaler
+func (s StreamAddr) MarshalJSON() ([]byte, error) {
+	if s.IsZero() {
+		return []byte(`null`), nil
+	}
+	// TODO(iannucci): when we support go 1.8 on appengine, this can be written as
+	//   json.Marshal((*jsonStreamAddr)(s)).
+	jsa := jsonStreamAddr{s.Host, s.Project, s.Path}
+	return json.Marshal(jsa)
+}
+
+// UnmarshalJSON implements json.Marshaler
+func (s *StreamAddr) UnmarshalJSON(d []byte) error {
+	if bytes.Equal(d, []byte(`null`)) {
+		s.Host = ""
+		s.Project = ""
+		s.Path = ""
+		return nil
+	}
+	// TODO(iannucci): when we support go 1.8 on appengine, this can be written as
+	//   return json.Unmarshal(d, (*jsonStreamAddr)(s))
+	jsa := jsonStreamAddr{}
+	if err := json.Unmarshal(d, &jsa); err != nil {
+		return err
+	}
+	s.Host = jsa.Host
+	s.Project = jsa.Project
+	s.Path = jsa.Path
+	return nil
+}
+
 // ParseURL parses a LogDog URL into a Stream. If the URL is malformed, or
 // if the host, project, or path is invalid, an error will be returned.
 //