[#1770] logger: Support runtime level reloading

Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>

cmd/neofs-node/config.go

@@ -492,7 +492,7 @@ func initCfg(appCfg *config.Config) *cfg {
 	)
 	fatalOnErr(err)
 
-	log, err := logger.NewLogger(logPrm)
+	log, err := logger.NewLogger(&logPrm)
 	fatalOnErr(err)
 
 	var netAddr network.AddressGroup

pkg/util/logger/logger.go

@@ -1,8 +1,6 @@
 package logger
 
 import (
-	"errors"
-
 	"go.uber.org/zap"
 	"go.uber.org/zap/zapcore"
 )
@@ -15,23 +13,58 @@ type Logger struct {
 }
 
 // Prm groups Logger's parameters.
+// Successful passing non-nil parameters to the NewLogger (if returned
+// error is nil) connects the parameters with the returned Logger.
+// Parameters that have been connected to the Logger support its
+// configuration changing.
+//
+// Passing Prm after a successful connection via the NewLogger, connects
+// the Prm to a new instance of the Logger.
+//
+// See also Reload, SetLevelString.
 type Prm struct {
+	// link to the created Logger
+	// instance; used for a runtime
+	// reconfiguration
+	_log *Logger
+
+	// support runtime rereading
 	level zapcore.Level
+
+	// do not support runtime rereading
 }
 
-// ErrNilLogger is returned by functions that
-// expect a non-nil Logger but received nil.
-var ErrNilLogger = errors.New("logger is nil")
-
-// SetLevelString sets the minimum logging level.
+// SetLevelString sets the minimum logging level. Default is
+// "info".
 //
-// Returns error of s is not a string representation of zap.Level
-// value (see zapcore.Level docs).
+// Returns an error if s is not a string representation of a
+// supporting logging level.
+//
+// Supports runtime rereading.
 func (p *Prm) SetLevelString(s string) error {
 	return p.level.UnmarshalText([]byte(s))
 }
 
-// NewLogger constructs a new zap logger instance.
+// Reload reloads configuration of a connected instance of the Logger.
+// Returns ErrLoggerNotConnected if no connection has been performed.
+// Returns any reconfiguration error from the Logger directly.
+func (p Prm) Reload() error {
+	if p._log == nil {
+		// incorrect logger usage
+		panic("parameters are not connected to any Logger")
+	}
+
+	return p._log.reload(p)
+}
+
+func defaultPrm() *Prm {
+	return new(Prm)
+}
+
+// NewLogger constructs a new zap logger instance. Constructing with nil
+// parameters is safe: default values will be used then.
+// Passing non-nil parameters after a successful creation (non-error) allows
+// runtime reconfiguration.
 //
 // Logger is built from production logging configuration with:
 //   - parameterized level;
@@ -39,7 +72,11 @@ func (p *Prm) SetLevelString(s string) error {
 //   - ISO8601 time encoding.
 //
 // Logger records a stack trace for all messages at or above fatal level.
-func NewLogger(prm Prm) (*Logger, error) {
+func NewLogger(prm *Prm) (*Logger, error) {
+	if prm == nil {
+		prm = defaultPrm()
+	}
+
 	lvl := zap.NewAtomicLevelAt(prm.level)
 
 	c := zap.NewProductionConfig()
@@ -47,12 +84,20 @@ func NewLogger(prm Prm) (*Logger, error) {
 	c.Encoding = "console"
 	c.EncoderConfig.EncodeTime = zapcore.ISO8601TimeEncoder
 
-	l, err := c.Build(
+	lZap, err := c.Build(
 		zap.AddStacktrace(zap.NewAtomicLevelAt(zap.FatalLevel)),
 	)
 	if err != nil {
 		return nil, err
 	}
 
-	return &Logger{Logger: l, lvl: lvl}, nil
+	l := &Logger{Logger: lZap, lvl: lvl}
+	prm._log = l
+
+	return l, nil
+}
+
+func (l *Logger) reload(prm Prm) error {
+	l.lvl.SetLevel(prm.level)
+	return nil
 }