Add support for configuration static logging fields

To allow flexibility in log message context information, this changeset
provides the ability to configure static fields that are included in the
context. Such fields can be set via configuration or environment variables.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

configuration/configuration_test.go

@@ -15,7 +15,14 @@ func Test(t *testing.T) { TestingT(t) }
 
 // configStruct is a canonical example configuration, which should map to configYamlV0_1
 var configStruct = Configuration{
-	Version:  "0.1",
+	Version: "0.1",
+	Log: struct {
+		Level     Loglevel          `yaml:"level"`
+		Formatter string            `yaml:"formatter"`
+		Fields    map[string]string `yaml:"fields"`
+	}{
+		Fields: map[string]string{"environment": "test"},
+	},
 	Loglevel: "info",
 	Storage: Storage{
 		"s3": Parameters{
@@ -57,6 +64,9 @@ var configStruct = Configuration{
 // configYamlV0_1 is a Version 0.1 yaml document representing configStruct
 var configYamlV0_1 = `
 version: 0.1
+log:
+  fields:
+    environment: test
 loglevel: info
 storage:
   s3:
@@ -136,6 +146,7 @@ func (suite *ConfigSuite) TestParseSimple(c *C) {
 func (suite *ConfigSuite) TestParseInmemory(c *C) {
 	suite.expectedConfig.Storage = Storage{"inmemory": Parameters{}}
 	suite.expectedConfig.Reporting = Reporting{}
+	suite.expectedConfig.Log.Fields = nil
 
 	config, err := Parse(bytes.NewReader([]byte(inmemoryConfigYamlV0_1)))
 	c.Assert(err, IsNil)
@@ -150,6 +161,7 @@ func (suite *ConfigSuite) TestParseIncomplete(c *C) {
 	_, err := Parse(bytes.NewReader([]byte(incompleteConfigYaml)))
 	c.Assert(err, NotNil)
 
+	suite.expectedConfig.Log.Fields = nil
 	suite.expectedConfig.Storage = Storage{"filesystem": Parameters{"rootdirectory": "/tmp/testroot"}}
 	suite.expectedConfig.Auth = Auth{"silly": Parameters{"realm": "silly"}}
 	suite.expectedConfig.Reporting = Reporting{}
@@ -303,6 +315,12 @@ func copyConfig(config Configuration) *Configuration {
 
 	configCopy.Version = MajorMinorVersion(config.Version.Major(), config.Version.Minor())
 	configCopy.Loglevel = config.Loglevel
+	configCopy.Log = config.Log
+	configCopy.Log.Fields = make(map[string]string, len(config.Log.Fields))
+	for k, v := range config.Log.Fields {
+		configCopy.Log.Fields[k] = v
+	}
+
 	configCopy.Storage = Storage{config.Storage.Type(): Parameters{}}
 	for k, v := range config.Storage.Parameters() {
 		configCopy.Storage.setParameter(k, v)