feat: collect new up metric from fail2ban socket

Add support for connecting the exporter directly to the fail2ban server's
socket to send requests and receive data. The path to the socket file is
optional and specified on startup.
Export a new metric based on the response of the `ping` command sent to the
fail2ban server. The metric is set to 1 if the server responds with `pong`
and 0 in any other case. This metric is only shown if the path to the
socket file was provided on startup.

src/socket/decoder.go

@@ -0,0 +1,8 @@
+package socket
+
+// Py_builtins_str is used by the pickle decoder to parse the server response into a format Go can understand
+type Py_builtins_str struct{}
+
+func (c Py_builtins_str) Call(args ...interface{}) (interface{}, error) {
+	return args[0], nil
+}

src/socket/fail2banSocket.go

@@ -0,0 +1,42 @@
+package socket
+
+import (
+	"log"
+	"net"
+
+	"github.com/kisielk/og-rek"
+	"github.com/nlpodyssey/gopickle/types"
+)
+
+type Fail2BanSocket struct {
+	socket  net.Conn
+	encoder *ogórek.Encoder
+}
+
+func MustConnectToSocket(path string) *Fail2BanSocket {
+	c, err := net.Dial("unix", path)
+	if err != nil {
+		log.Fatalf("failed to open fail2ban socket: %v", err)
+	}
+	return &Fail2BanSocket{
+		socket:  c,
+		encoder: ogórek.NewEncoder(c),
+	}
+}
+
+func (s *Fail2BanSocket) Ping() bool {
+	response, err := s.sendCommand([]string{pingCommand, "100"})
+	if err != nil {
+		log.Printf("server ping failed: %v", err)
+		return false
+	}
+
+	if t, ok := response.(*types.Tuple); ok {
+		if (*t)[1] == "pong" {
+			return true
+		}
+		log.Printf("unexpected response data: %s", t)
+	}
+	log.Printf("unexpected response format - cannot parse: %v", response)
+	return false
+}

src/socket/protocol.go

@@ -0,0 +1,53 @@
+package socket
+
+import (
+	"bytes"
+	"fmt"
+	"github.com/nlpodyssey/gopickle/pickle"
+)
+
+const (
+	commandTerminator    = "<F2B_END_COMMAND>"
+	pingCommand          = "ping"
+	socketReadBufferSize = 10000
+)
+
+func (s *Fail2BanSocket) sendCommand(command []string) (interface{}, error) {
+	err := s.write(command)
+	if err != nil {
+		return nil, err
+	}
+	return s.read()
+}
+
+func (s *Fail2BanSocket) write(command []string) error {
+	err := s.encoder.Encode(command)
+	if err != nil {
+		return err
+	}
+	_, err = s.socket.Write([]byte(commandTerminator))
+	if err != nil {
+		return err
+	}
+	return nil
+}
+
+func (s *Fail2BanSocket) read() (interface{}, error) {
+	buf := make([]byte, socketReadBufferSize)
+	_, err := s.socket.Read(buf)
+	if err != nil {
+		return nil, err
+	}
+
+	bufReader := bytes.NewReader(buf)
+	unpickler := pickle.NewUnpickler(bufReader)
+
+	unpickler.FindClass = func(module, name string) (interface{}, error) {
+		if module == "builtins" && name == "str" {
+			return &Py_builtins_str{}, nil
+		}
+		return nil, fmt.Errorf("class not found: " + module + " : " + name)
+	}
+
+	return unpickler.Load()
+}