Allow users to pass in their own logger.

For making raft log to github.com/golang/glog, it is necessary to
specify a custom logger, because glog expects you to set the Lshortfile
flag.

Aside from this specific example, I think in general it’s better to give
users more control over the logger instead of creating it for the user —
after all, it’s still just a single line of code.

config.go

@@ -3,6 +3,7 @@ package raft
 import (
 	"fmt"
 	"io"
+	"log"
 	"time"
 )
 
@@ -65,8 +66,13 @@ type Config struct {
 	// step down as leader.
 	LeaderLeaseTimeout time.Duration
 
-	// LogOutput is used as a sink for logs. Defaults to os.Stderr.
+	// LogOutput is used as a sink for logs, unless Logger is specified.
+	// Defaults to os.Stderr.
 	LogOutput io.Writer
+
+	// Logger is a user-provided logger. If nil, a logger writing to LogOutput
+	// is used.
+	Logger *log.Logger
 }
 
 // DefaultConfig returns a Config with usable defaults.

raft.go

@@ -158,8 +158,14 @@ func NewRaft(conf *Config, fsm FSM, logs LogStore, stable StableStore, snaps Sna
 	}
 
 	// Ensure we have a LogOutput
-	if conf.LogOutput == nil {
-		conf.LogOutput = os.Stderr
+	var logger *log.Logger
+	if conf.Logger != nil {
+		logger = conf.Logger
+	} else {
+		if conf.LogOutput == nil {
+			conf.LogOutput = os.Stderr
+		}
+		logger = log.New(conf.LogOutput, "", log.LstdFlags)
 	}
 
 	// Try to restore the current term
@@ -200,7 +206,7 @@ func NewRaft(conf *Config, fsm FSM, logs LogStore, stable StableStore, snaps Sna
 		fsmSnapshotCh: make(chan *reqSnapshotFuture),
 		leaderCh:      make(chan bool),
 		localAddr:     localAddr,
-		logger:        log.New(conf.LogOutput, "", log.LstdFlags),
+		logger:        logger,
 		logs:          logs,
 		peerCh:        make(chan *peerFuture),
 		peers:         peers,