From 4aee47a770dddcf74f4564fc25b48adcd7cfe943 Mon Sep 17 00:00:00 2001
From: Marco Maccaferri <maccaf@sourceforge.net>
Date: Sun, 9 Feb 2020 19:41:55 -0800
Subject: [PATCH] Fix handling of escape characters in softmodem

When running BBS DOSBox's softmodem does not correctly handle
escape characters properly nor does it properly transition
to command mode:

Characters are swallowed instead of passed-through, this causes
issue when transmitting strings with the plus character.
Additional characters before and after the escape sequence don't
reset the counters causing switch to command mode even if not
intended. For example, connect to a remote and type a+++ it
switches to command mode, it should not. The attached proposed
patch fixes the escape handling:

Transmission pause is checked before and after the escape
sequence, so <pause>+++<pause> is the correct
sequence.</pause></pause> Extra characters cause the counters to
be reset so no unwanted switch is triggered. Use register S12 to
set the pause timer in 1/50th of seconds, default is 50 = 1 sec.
Plus characters are passed to the remote. With this fix I'm able
to run a BBS software and do file transfers with XModem and ZModem
without problems.

Imported-from: https://sourceforge.net/p/dosbox/patches/287/
---
 src/hardware/serialport/softmodem.cpp | 43 ++++++++++++++-------------
 src/hardware/serialport/softmodem.h   |  1 +
 2 files changed, 23 insertions(+), 21 deletions(-)

diff --git a/src/hardware/serialport/softmodem.cpp b/src/hardware/serialport/softmodem.cpp
index bbacd685..2548c771 100644
--- a/src/hardware/serialport/softmodem.cpp
+++ b/src/hardware/serialport/softmodem.cpp
@@ -258,6 +258,7 @@ void CSerialModem::Reset(){
 	reg[MREG_CR_CHAR]          = '\r';
 	reg[MREG_LF_CHAR]          = '\n';
 	reg[MREG_BACKSPACE_CHAR]   = '\b';
+	reg[MREG_GUARD_TIME]       = 50;
 
 	cmdpause = 0;
 	echo = true;
@@ -688,14 +689,26 @@ void CSerialModem::TelnetEmulation(Bit8u * data, Bitu size) {
 }
 
 void CSerialModem::Timer2(void) {
-
-	bool sendbyte = true;
 	Bitu usesize;
 	Bit8u txval;
 	Bitu txbuffersize =0;
 
 	// Check for eventual break command
-	if (!commandmode) cmdpause++;
+	if (!commandmode) {
+		cmdpause++;
+		if (cmdpause > (20 * reg[MREG_GUARD_TIME])) {
+			if (plusinc == 0) {
+				plusinc = 1;
+			}
+			else if (plusinc == 4) {
+				LOG_MSG("Modem: Entering command mode(escape sequence)");
+				commandmode = true;
+				SendRes(ResOK);
+				plusinc = 0;
+			}
+		}
+	}
+
 	// Handle incoming data from serial port, read as much as available
 	CSerial::setCTS(true);	// buffer will get 'emptier', new data can be received
 	while (tqueue->inuse()) {
@@ -716,29 +729,18 @@ void CSerialModem::Timer2(void) {
 			}
 		}
 		else {// + character
-			// 1000 ticks have passed, can check for pause command
-			if (cmdpause > 1000) {
-				if (txval == reg[MREG_ESCAPE_CHAR]) // +
-				{
-					plusinc++;
-					if (plusinc >= 3) {
-						LOG_MSG("Modem: Entering command mode(escape sequence)");
-						commandmode = true;
-						SendRes(ResOK);
-						plusinc = 0;
-					}
-					sendbyte=false;
-				} else {
-					plusinc=0;
-				}
-	// If not a special pause command, should go for bigger blocks to send
+			if (plusinc >= 1 && plusinc <= 3 && txval == reg[MREG_ESCAPE_CHAR]) // +
+				plusinc++;
+			else {
+				plusinc = 0;
 			}
+			cmdpause = 0;
 			tmpbuf[txbuffersize] = txval;
 			txbuffersize++;
 		}
 	} // while loop
 
-	if (clientsocket && sendbyte && txbuffersize) {
+	if (clientsocket && txbuffersize) {
 		// down here it saves a lot of network traffic
 		if (!clientsocket->SendArray(tmpbuf,txbuffersize)) {
 			SendRes(ResNOCARRIER);
@@ -759,7 +761,6 @@ void CSerialModem::Timer2(void) {
 				TelnetEmulation(tmpbuf, usesize);
 			else
 				rqueue->adds(tmpbuf,usesize);
-			cmdpause = 0;
 		}
 	}
 	// Check for incoming calls
diff --git a/src/hardware/serialport/softmodem.h b/src/hardware/serialport/softmodem.h
index c3e01d61..22df203b 100644
--- a/src/hardware/serialport/softmodem.h
+++ b/src/hardware/serialport/softmodem.h
@@ -165,6 +165,7 @@ private:
 #define MREG_CR_CHAR 3
 #define MREG_LF_CHAR 4
 #define MREG_BACKSPACE_CHAR 5
+#define MREG_GUARD_TIME 12
 
 
 class CSerialModem : public CSerial {