rename variables, struct fields and functions to include an "x" when they can panic for handling errors

and document the convention in develop.txt.
spurred by running errcheck again (it has been a while). it still has too many
false to enable by default.

smtpserver/server.go

@@ -323,14 +323,16 @@ type conn struct {
 	origConn net.Conn
 	conn     net.Conn
 
-	tls                   bool
-	extRequireTLS         bool // Whether to announce and allow the REQUIRETLS extension.
-	viaHTTPS              bool // Whether the connection came in via the HTTPS port (using TLS ALPN).
-	resolver              dns.Resolver
-	r                     *bufio.Reader
-	w                     *bufio.Writer
-	tr                    *moxio.TraceReader // Kept for changing trace level during cmd/auth/data.
-	tw                    *moxio.TraceWriter
+	tls           bool
+	extRequireTLS bool // Whether to announce and allow the REQUIRETLS extension.
+	viaHTTPS      bool // Whether the connection came in via the HTTPS port (using TLS ALPN).
+	resolver      dns.Resolver
+	// The "x" in the readers and writes indicate Read and Write errors use panic to
+	// propagate the error.
+	xbr                   *bufio.Reader
+	xbw                   *bufio.Writer
+	xtr                   *moxio.TraceReader // Kept for changing trace level during cmd/auth/data.
+	xtw                   *moxio.TraceWriter
 	slow                  bool      // If set, reads are done with a 1 second sleep, and writes are done 1 byte at a time, to keep spammers busy.
 	lastlog               time.Time // Used for printing the delta time since the previous logging for this connection.
 	submission            bool      // ../rfc/6409:19 applies
@@ -692,12 +694,12 @@ func (c *conn) xcheckAuth() {
 
 func (c *conn) xtrace(level slog.Level) func() {
 	c.xflush()
-	c.tr.SetTrace(level)
-	c.tw.SetTrace(level)
+	c.xtr.SetTrace(level)
+	c.xtw.SetTrace(level)
 	return func() {
 		c.xflush()
-		c.tr.SetTrace(mlog.LevelTrace)
-		c.tw.SetTrace(mlog.LevelTrace)
+		c.xtr.SetTrace(mlog.LevelTrace)
+		c.xtw.SetTrace(mlog.LevelTrace)
 	}
 }
 
@@ -780,7 +782,7 @@ func (c *conn) Read(buf []byte) (int, error) {
 var bufpool = moxio.NewBufpool(8, 2*1024)
 
 func (c *conn) readline() string {
-	line, err := bufpool.Readline(c.log, c.r)
+	line, err := bufpool.Readline(c.log, c.xbr)
 	if err != nil && errors.Is(err, moxio.ErrLineTooLong) {
 		c.writecodeline(smtp.C500BadSyntax, smtp.SeProto5Other0, "line too long, smtp max is 512, we reached 2048", nil)
 		panic(fmt.Errorf("%s (%w)", err, errIO))
@@ -834,12 +836,12 @@ func (c *conn) bwritecodeline(code int, secode string, msg string, err error) {
 // Buffered-write a formatted response line to connection.
 func (c *conn) bwritelinef(format string, args ...any) {
 	msg := fmt.Sprintf(format, args...)
-	fmt.Fprint(c.w, msg+"\r\n")
+	fmt.Fprint(c.xbw, msg+"\r\n")
 }
 
 // Flush pending buffered writes to connection.
 func (c *conn) xflush() {
-	c.w.Flush() // Errors will have caused a panic in Write.
+	c.xbw.Flush() // Errors will have caused a panic in Write.
 }
 
 // Write (with flush) a response line with codes and message. err is not written, used for logging and can be nil.
@@ -919,10 +921,10 @@ func serve(listenerName string, cid int64, hostname dns.Domain, tlsConfig *tls.C
 		}
 		return l
 	})
-	c.tr = moxio.NewTraceReader(c.log, "RC: ", c)
-	c.r = bufio.NewReader(c.tr)
-	c.tw = moxio.NewTraceWriter(c.log, "LS: ", c)
-	c.w = bufio.NewWriter(c.tw)
+	c.xtr = moxio.NewTraceReader(c.log, "RC: ", c)
+	c.xbr = bufio.NewReader(c.xtr)
+	c.xtw = moxio.NewTraceWriter(c.log, "LS: ", c)
+	c.xbw = bufio.NewWriter(c.xtw)
 
 	metricConnection.WithLabelValues(c.kind()).Inc()
 	c.log.Info("new connection",
@@ -1007,9 +1009,9 @@ func serve(listenerName string, cid int64, hostname dns.Domain, tlsConfig *tls.C
 
 		// If another command is present, don't flush our buffered response yet. Holding
 		// off will cause us to respond with a single packet.
-		n := c.r.Buffered()
+		n := c.xbr.Buffered()
 		if n > 0 {
-			buf, err := c.r.Peek(n)
+			buf, err := c.xbr.Peek(n)
 			if err == nil && bytes.IndexByte(buf, '\n') >= 0 {
 				continue
 			}
@@ -1246,9 +1248,9 @@ func (c *conn) cmdStarttls(p *parser) {
 	// but make sure any bytes already read and in the buffer are used for the TLS
 	// handshake.
 	conn := c.conn
-	if n := c.r.Buffered(); n > 0 {
+	if n := c.xbr.Buffered(); n > 0 {
 		conn = &moxio.PrefixConn{
-			PrefixReader: io.LimitReader(c.r, int64(n)),
+			PrefixReader: io.LimitReader(c.xbr, int64(n)),
 			Conn:         conn,
 		}
 	}
@@ -2121,7 +2123,7 @@ func (c *conn) cmdData(p *parser) {
 	}
 	defer store.CloseRemoveTempFile(c.log, dataFile, "smtpserver delivered message")
 	msgWriter := message.NewWriter(dataFile)
-	dr := smtp.NewDataReader(c.r)
+	dr := smtp.NewDataReader(c.xbr)
 	n, err := io.Copy(&limitWriter{maxSize: c.maxMessageSize, w: msgWriter}, dr)
 	c.xtrace(mlog.LevelTrace) // Restore.
 	if err != nil {