Cache the PROTOCOLINFO response if it was issued before AUTHENTICATE.

Control clients get one PROTOCOLINFO before they must issue an
AUTHCHALLENGE or AUTHENTICATE, and the Authenticate() implementation
uses PROTOCOLINFO.  This allows users to call ProtocolInfo() before
Authenticate() without mysterious failures.

cmd_authenticate.go

@@ -132,7 +132,6 @@ func (c *Conn) Authenticate(password string) error {
 		_, err = c.Request("%s %s", cmdAuthenticate, passwordStr)
 		c.isAuthenticated = err == nil
 		return err
-	} else {
-		return newProtocolError("no supported authentication methods")
 	}
+	return newProtocolError("no supported authentication methods")
 }

cmd_protocolinfo.go

@@ -23,6 +23,13 @@ type ProtocolInfo struct {
 
 // ProtocolInfo issues a PROTOCOLINFO command and returns the parsed response.
 func (c *Conn) ProtocolInfo() (*ProtocolInfo, error) {
+	// In the pre-authentication state, only one PROTOCOLINFO command
+	// may be issued.  Cache the value returned so that subsequent
+	// calls continue to work.
+	if !c.isAuthenticated && c.cachedPI != nil {
+		return c.cachedPI, nil
+	}
+
 	resp, err := c.Request("PROTOCOLINFO")
 	if err != nil {
 		return nil, err
@@ -79,5 +86,8 @@ func (c *Conn) ProtocolInfo() (*ProtocolInfo, error) {
 		default: // MUST ignore unsupported InfoLines.
 		}
 	}
+	if !c.isAuthenticated {
+		c.cachedPI = pi
+	}
 	return pi, nil
 }

conn.go

@@ -32,6 +32,7 @@ type Conn struct {
 	conn            *textproto.Conn
 	isAuthenticated bool
 	debugLog        bool
+	cachedPI        *ProtocolInfo
 
 	asyncReaderLock    sync.Mutex
 	asyncReaderRunning bool