WithLogger connOption and Conn.logInfo bool (samuel#170)

* add WithLogger connOption

This allows setting the logger as the connection is created, rather
than racing to call SetLogger while the connection's goroutine might
already be logging a successful (or failed) connection.

In addition using WithLogger works around the data race between SetLogger
and the connection's goroutine.

The data race is NOT fixed by this commit. AFAICS doing so is a nontrivial.
Every access to conn.logger would need to be atomic.

* add Conn.logInfo bool controling informational messages

By default it is enabled for backwards compatability, but
by setting logInfo=false you can have silence on stderr
when no errors are in fact occurring.

This is desireable for commandline tools, where the convention
I prefer is that silence indicates success.

zk/conn.go

@@ -102,7 +102,8 @@ type Conn struct {
 	setWatchLimit    int
 	setWatchCallback func([]*setWatchesRequest)
 
-	logger Logger
+	logger  Logger
+	logInfo bool // true if information messages are logged; false if only errors are logged
 
 	buf []byte
 }
@@ -200,6 +201,7 @@ func Connect(servers []string, sessionTimeout time.Duration, options ...connOpti
 		watchers:       make(map[watchPathType][]chan Event),
 		passwd:         emptyPassword,
 		logger:         DefaultLogger,
+		logInfo:        true, // default is true for backwards compatability
 		buf:            make([]byte, bufferSize),
 	}
 
@@ -237,6 +239,21 @@ func WithHostProvider(hostProvider HostProvider) connOption {
 	}
 }
 
+// WithLogger returns a connection option specifying a non-default Logger
+func WithLogger(logger Logger) connOption {
+	return func(c *Conn) {
+		c.logger = logger
+	}
+}
+
+// WithLogInfo returns a connection option specifying whether or not information messages
+// shoud be logged.
+func WithLogInfo(logInfo bool) connOption {
+	return func(c *Conn) {
+		c.logInfo = logInfo
+	}
+}
+
 // EventCallback is a function that is called when an Event occurs.
 type EventCallback func(Event)
 
@@ -361,7 +378,9 @@ func (c *Conn) connect() error {
 		if err == nil {
 			c.conn = zkConn
 			c.setState(StateConnected)
-			c.logger.Printf("Connected to %s", c.Server())
+			if c.logInfo {
+				c.logger.Printf("Connected to %s", c.Server())
+			}
 			return nil
 		}
 
@@ -375,8 +394,10 @@ func (c *Conn) resendZkAuth(reauthReadyChan chan struct{}) {
 
 	defer close(reauthReadyChan)
 
-	c.logger.Printf("Re-submitting `%d` credentials after reconnect",
-		len(c.creds))
+	if c.logInfo {
+		c.logger.Printf("Re-submitting `%d` credentials after reconnect",
+			len(c.creds))
+	}
 
 	for _, cred := range c.creds {
 		resChan, err := c.sendRequest(
@@ -444,7 +465,9 @@ func (c *Conn) loop() {
 			c.logger.Printf("Authentication failed: %s", err)
 			c.conn.Close()
 		case err == nil:
-			c.logger.Printf("Authenticated: id=%d, timeout=%d", c.SessionID(), c.sessionTimeoutMs)
+			if c.logInfo {
+				c.logger.Printf("Authenticated: id=%d, timeout=%d", c.SessionID(), c.sessionTimeoutMs)
+			}
 			c.hostProvider.Connected()        // mark success
 			c.closeChan = make(chan struct{}) // channel to tell send loop stop
 			reauthChan := make(chan struct{}) // channel to tell send loop that authdata has been resubmitted
@@ -454,15 +477,19 @@ func (c *Conn) loop() {
 			go func() {
 				<-reauthChan
 				err := c.sendLoop()
-				c.logger.Printf("Send loop terminated: err=%v", err)
+				if err != nil || c.logInfo {
+					c.logger.Printf("Send loop terminated: err=%v", err)
+				}
 				c.conn.Close() // causes recv loop to EOF/exit
 				wg.Done()
 			}()
 
 			wg.Add(1)
 			go func() {
 				err := c.recvLoop(c.conn)
-				c.logger.Printf("Recv loop terminated: err=%v", err)
+				if err != io.EOF || c.logInfo {
+					c.logger.Printf("Recv loop terminated: err=%v", err)
+				}
 				if err == nil {
 					panic("zk: recvLoop should never return nil error")
 				}