Introduce a basic logger and some code for testing

README.md

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+# Milter experiment
+
+This project is for educational purposes.
+Any attempts to run this in production WILL FAIL!
+Use this code as inspiration, but do not blame me for your losses.
+
+
+# References
+
+* [The source code of sendmail-jilter](https://sourceforge.net/projects/sendmail-jilter/)
+* [The Sendmail source code](https://ftp.sendmail.org/)

logger.go

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+package main
+
+import (
+	"io"
+	"log"
+)
+
+// Loglevel is used to set the current loglevel of Logger.
+type Loglevel int
+
+const (
+	// NONE is a loglevel for Logger.
+	NONE Loglevel = 0
+	// ERROR is a loglevel for Logger.
+	ERROR Loglevel = 1
+	// WARN is a loglevel for Logger.
+	WARN Loglevel = 2
+	// INFO is a loglevel for Logger.
+	INFO Loglevel = 3
+	// DEBUG is a loglevel for Logger.
+	DEBUG Loglevel = 4
+)
+
+// Logger uses multiple instances of log.Logger to provide logging methods for multiple loglevels.
+type Logger struct {
+	debug    *log.Logger
+	info     *log.Logger
+	warn     *log.Logger
+	error    *log.Logger
+	logDebug bool
+	logInfo  bool
+	logWarn  bool
+	logError bool
+}
+
+// NewLogger prepares a new instance of Logger for custom use.
+func NewLogger(stdOut io.Writer, stdErr io.Writer, logLevel Loglevel) Logger {
+	newLogger := Logger{}
+	newLogger.debug = log.New(stdOut, "DEBUG ", 0)
+	newLogger.info = log.New(stdOut, "INFO ", 0)
+	newLogger.warn = log.New(stdErr, "WARN ", 0)
+	newLogger.error = log.New(stdErr, "ERROR ", 0)
+	newLogger.SetLogLevel(logLevel)
+	return newLogger
+}
+
+// SetLogLevel can be used to change the loglevel of Logger on the fly.
+func (l *Logger) SetLogLevel(logLevel Loglevel) {
+	l.logDebug = (logLevel >= DEBUG)
+	l.logInfo = (logLevel >= INFO)
+	l.logWarn = (logLevel >= WARN)
+	l.logError = (logLevel >= ERROR)
+}
+
+// Debug is an alias for log.Logger.Print() with severity DEBUG.
+func (l *Logger) Debug(v ...interface{}) {
+	if l.logDebug {
+		l.debug.Print(v...)
+	}
+}
+
+// Debugln is an alias for log.Logger.Println() with severity DEBUG.
+func (l *Logger) Debugln(v ...interface{}) {
+	if l.logDebug {
+		l.debug.Println(v...)
+	}
+}
+
+// Debugf is an alias for log.Logger.Printf() with severity DEBUG.
+func (l *Logger) Debugf(format string, v ...interface{}) {
+	if l.logDebug {
+		l.debug.Printf(format, v...)
+	}
+}
+
+// Info is an alias for log.Logger.Print() with severity INFO.
+func (l *Logger) Info(v ...interface{}) {
+	if l.logInfo {
+		l.info.Print(v...)
+	}
+}
+
+// Infoln is an alias for log.Logger.Println() with severity INFO.
+func (l *Logger) Infoln(v ...interface{}) {
+	if l.logInfo {
+		l.info.Println(v...)
+	}
+}
+
+// Infof is an alias for log.Logger.Printf() with severity INFO.
+func (l *Logger) Infof(format string, v ...interface{}) {
+	if l.logInfo {
+		l.info.Printf(format, v...)
+	}
+}
+
+// Warn is an alias for log.Logger.Print() with severity WARN.
+func (l *Logger) Warn(v ...interface{}) {
+	if l.logWarn {
+		l.warn.Print(v...)
+	}
+}
+
+// Warnln is an alias for log.Logger.Println() with severity WARN.
+func (l *Logger) Warnln(v ...interface{}) {
+	if l.logWarn {
+		l.warn.Println(v...)
+	}
+}
+
+// Warnf is an alias for log.Logger.Printf() with severity WARN.
+func (l *Logger) Warnf(format string, v ...interface{}) {
+	if l.logWarn {
+		l.warn.Printf(format, v...)
+	}
+}
+
+// Error is an alias for log.Logger.Print() with severity ERROR.
+func (l *Logger) Error(v ...interface{}) {
+	if l.logError {
+		l.error.Print(v...)
+	}
+}
+
+// Errorln is an alias for log.Logger.Println() with severity ERROR.
+func (l *Logger) Errorln(v ...interface{}) {
+	if l.logError {
+		l.error.Println(v...)
+	}
+}
+
+// Errorf is an alias for log.Logger.Printf() with severity ERROR.
+func (l *Logger) Errorf(format string, v ...interface{}) {
+	if l.logError {
+		l.error.Printf(format, v...)
+	}
+}

main.go

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+package main
+
+import (
+	"bufio"
+	"bytes"
+	"encoding/binary"
+	"encoding/hex"
+	"os"
+)
+
+var (
+	logger Logger
+)
+
+func handleMtaConnection(clientConnection *bufio.ReadWriter) {
+	buf := make([]byte, 4096)
+	for {
+		bytesRead, err := clientConnection.Read(buf)
+		if err != nil {
+			logger.Errorln("Error reading from clientConnection:", err.Error())
+			break
+		} else {
+			logger.Infof("Read %d bytes from connection\n", bytesRead)
+			logger.Debugf("%s", hex.Dump(buf[0:bytesRead]))
+
+			// First 4 Bytes will be some integer thing "length"
+			// Let's parse that
+			var packetLength uint32
+			bytesReader := bytes.NewReader(buf[0:4])
+			err := binary.Read(bytesReader, binary.BigEndian, &packetLength)
+			if err != nil {
+				logger.Errorln(err.Error())
+			}
+			logger.Debugf("Parsed length: %d\n", packetLength)
+		}
+	}
+}
+
+func main() {
+	logger = NewLogger(os.Stdout, os.Stderr, DEBUG)
+	runServer()
+}

socket.go

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+package main
+
+import (
+	"bufio"
+	"net"
+	"os"
+)
+
+func runServer() {
+	serverSocket, err := net.Listen("tcp", "127.0.0.1:7777")
+	if err != nil {
+		logger.Errorf("Could not create listening socket: %s\n", err.Error())
+		os.Exit(1)
+	} else {
+		logger.Infof("Now listening for incoming connections.\n")
+		defer serverSocket.Close()
+	}
+	for {
+		clientConnection, err := serverSocket.Accept()
+		if err != nil {
+			logger.Warnf("Could not accept client connection: %s\n", err.Error())
+		} else {
+			logger.Infof("Accepted client connection.\n")
+			milterReadWriter := bufio.NewReadWriter(bufio.NewReader(clientConnection), bufio.NewWriter(clientConnection))
+			go handleMtaConnection(milterReadWriter)
+		}
+	}
+}