logging: first outlet receives logger error message

Abandons stderr special-casing:

* looks weird on shell and IO redirection to same file because of
interleaving of stdout and stderr
* better than a separate dedicated outlet because it does not require
additional configuration

fixes #28

BREAK SEMANTICS CONFIG

cmd/config_logging.go

@@ -13,7 +13,7 @@ import (
 )
 
 type LoggingConfig struct {
-	Outlets *logger.Outlets
+	Outlets logger.Outlets
 }
 
 type MetadataFlags int64
@@ -26,10 +26,9 @@ const (
 	MetadataAll  MetadataFlags = ^0
 )
 
-func LoggerErrorOutlet() logger.Outlet {
-
+func StdoutOutlet() logger.Outlet {
 	formatter := &HumanFormatter{}
-	writer := os.Stderr
+	writer := os.Stdout
 	formatter.SetMetadataFlags(MetadataAll)
 	return WriterOutlet{
 		formatter, writer,
@@ -39,8 +38,7 @@ func LoggerErrorOutlet() logger.Outlet {
 func parseLogging(i interface{}) (c *LoggingConfig, err error) {
 
 	c = &LoggingConfig{}
-
-	c.Outlets = logger.NewOutlets(LoggerErrorOutlet())
+	c.Outlets = logger.NewOutlets()
 
 	var asList []interface{}
 	if err = mapstructure.Decode(i, &asList); err != nil {

cmd/test.go

@@ -65,7 +65,7 @@ func init() {
 
 func testCmdGlobalInit(cmd *cobra.Command, args []string) {
 
-	out := logger.NewOutlets(WriterOutlet{&NoFormatter{}, os.Stderr})
+	out := logger.NewOutlets()
 	out.Add(WriterOutlet{&NoFormatter{}, os.Stdout}, logger.Info)
 	log := logger.NewLogger(out, 1*time.Second)
 	testCmdGlobal.log = log

docs/configuration/logging.rst

@@ -27,6 +27,10 @@ Check out :sampleconf:`random/logging.yml` for an example on how to configure mu
     
     jobs: ...
 
+.. ATTENTION::
+    The **first outlet is special**: if an error writing to any outlet occurs, the first outlet receives the error and can print it.
+    Thus, the first outlet must be the one that always works and does not block, e.g. ``stdout``, which is the default.
+
 Default Configuration
 ---------------------
 
@@ -41,10 +45,6 @@ By default, the following logging configuration is used
           level:  "warn"
           format: "human"
 
-.. ATTENTION::
-    Output to **stderr** should always be considered a **critical error**.
-    Only errors in the logging infrastructure itself, e.g. IO errors when writing to an outlet, are sent to stderr.
-
 Building Blocks
 ---------------
 

logger/datastructures.go

@@ -78,28 +78,32 @@ type Outlet interface {
 	WriteEntry(ctx context.Context, entry Entry) error
 }
 
-type Outlets struct {
-	reg map[Level][]Outlet
-	e   Outlet
+type Outlets map[Level][]Outlet
+
+func NewOutlets() Outlets {
+	return make(map[Level][]Outlet, len(AllLevels))
 }
 
-func NewOutlets(loggerErrorOutlet Outlet) *Outlets {
-	return &Outlets{
-		make(map[Level][]Outlet, len(AllLevels)),
-		loggerErrorOutlet,
-	}
-}
-
-func (os *Outlets) Add(outlet Outlet, minLevel Level) {
+func (os Outlets) Add(outlet Outlet, minLevel Level) {
 	for _, l := range AllLevels[minLevel:] {
-		os.reg[l] = append(os.reg[l], outlet)
+		os[l] = append(os[l], outlet)
 	}
 }
 
-func (os *Outlets) Get(level Level) []Outlet {
-	return os.reg[level]
+func (os Outlets) Get(level Level) []Outlet {
+	return os[level]
 }
 
-func (os *Outlets) GetLoggerErrorOutlet() Outlet {
-	return os.e
+// Return the first outlet added to this Outlets list using Add()
+// with minLevel <= Error.
+// If no such outlet is in this Outlets list, a discarding outlet is returned.
+func (os Outlets) GetLoggerErrorOutlet() Outlet {
+	if len(os[Error]) < 1 {
+		return nullOutlet{}
+	}
+	return os[Error][0]
 }
+
+type nullOutlet struct{}
+
+func (nullOutlet) WriteEntry(ctx context.Context, entry Entry) error { return nil }

logger/logger.go

@@ -17,13 +17,13 @@ const DefaultUserFieldCapacity = 5
 
 type Logger struct {
 	fields        Fields
-	outlets       *Outlets
+	outlets       Outlets
 	outletTimeout time.Duration
 
 	mtx *sync.Mutex
 }
 
-func NewLogger(outlets *Outlets, outletTimeout time.Duration) *Logger {
+func NewLogger(outlets Outlets, outletTimeout time.Duration) *Logger {
 	return &Logger{
 		make(Fields, DefaultUserFieldCapacity),
 		outlets,