undefined | Better HN

0 pointsburfog7y ago0 comments

I'll go through what happens without replaying. The emulator runs, and the guest OS inside it boots up. For this example, suppose it is DOS running a multi-user BBS with 2 modems. This is not a UNIX clone; there is no /dev and there is no shell.

The emulator needs to pop up 2 terminal windows so that we can log in to the BBS. We do that, and a colorful menu is displayed on each of them. At this point, we save a snapshot and shut down the host computer. Later, we start the host computer, then start the emulator, and then try to resume the snapshot. Each terminal window remains blank, which is not the correct state. The guest OS (a DOS BBS) is in the state where it is showing a colorful menu, but we can't see it at all. Our terminals, being separate from the emulator and without an ability to save/restore snapshots, are inconsistent with the emulated guest OS.

In the above example, the guest OS (a DOS BBS) never did any kernel syscalls related to PTY operating mode. It directly acted on serial port hardware. There is no PTY, and adding one (where?) doesn't help.

Fundamentally, the emulator needs to be able to save/restore all terminal state. That even includes stuff like incomplete escape sequences, such as when just the first byte of the escape sequence has been passed to the terminal.

0 comments

laumars7y ago

Right, that's completely different to the thing originally described. It's also not possible without running a bespoke BBS server that can handle stateless connections. Again what you're trying to solve at the client side should really be solved at the server side.

I don't know why you even mentioned PTYs (now I understand your requirements). A PTY is just a detachable interface for local terminals, but since you're connecting via serial anyway there's no need for a PTY. There's also no need for a special emulated serial interface like you suggested because the DOS VM would have one already baked into it, just use that and have the BBS listen on a couple of COM ports.

Regarding storing the previous session, there are already terminal emulators out there that will restore the scroll back session from a previous instance, so you might find your idlogical terminal already exists.

It still seems like a daft problem you're trying to solve though. Putting aside you're suspending a server for no apparent reason (why would you even want to do this?), you're effectively running a forum for one local user. But I'm sure you also have your own reasons for these anti-patterns as well.

burfogOP7y ago

It's not different. A bespoke BBS server is not an option; the whole point is to run an existing piece of software for which there is no source code available. Suggesting a bespoke BBS server is like telling a person running Super Mario All-Stars in a Super Nintendo emulator that they should upgrade to a PC game.

I have implemented (wrote C code for) what you refer to as "a special emulated serial interface" and thus "the DOS VM would have one already baked into it". That is my code and it works fine, and I do indeed "have the BBS listen on a couple of COM ports".

I mentioned PTYs because I don't want them. Most terminal emulators will only accept data from a PTY, with themselves on the master side and a shell on the slave side. This alone makes these terminal emulators essentially worthless; to use them I'd really need to have my emulator act as a telnet server and then ask the user to telnet into the server! Perhaps a really screwy $SHELL could perform that automatically, but it still doesn't solve the replay/snapshot issue.

An xterm is very special because xterm supports the -S option. With this, my emulator can fork off a process and then exec an xterm with a file descriptor that I can feed data into. This works OK until I want to load a snapshot.

There may be "terminal emulators out there that will restore the scroll back session from a previous instance", but this gets unusable. My emulator supports multiple named snapshots. Using a terminal program with distinct snapshot functionality would require that the user save/restore snapshots both in my emulator and in each of the terminal programs. I have a case (other than a DOS BBS) where my emulator has dozens of serial ports, so this would place a heavy burden on the user.

Suspending a server is good for debugging it. I can go back to before a crash. I can load a snapshot to bypass slow and annoying start-up.

laumars7y ago

With the greatest of respect, the way you casually drop technical terms out of context suggests to me that you really don't understand how terminals work in the slightest. (That and the new revelation that you're restoring your BBS snapshots in random orders yet expecting your terminal to be smart enough to guess where to resume). This is why I think we are having such a high degree of difficulty agreeing on your problem.

The reason I commented about hacking the BBS software is because that's the only way to solve the weird requirements you're asking. You can complain about my suggestion all you like but it's your unusual requirements that are at fault The fault here, not my solution.

Maybe you need to stop and think for a moment that perhaps what you're asking is silly rather than just assuming that everyone else on the internet is stupid.

The closest work around you're going to get (and is buildable with your experience) would be to build a terminal server (like a minimal Linux distribution running on a VM or Raspberry Pi) which you can SSH onto from Linux or Windows (eg via PuTTY) and which has a serial interface to your BBS software (be that virtualized or physical). Install 'tmux' or 'screen' onto the terminal server and when you SSH onto the terminal server reattach to your screen session. That screen session will be connected to your BBS and you'll then have your detachable but persistent terminal sessions.

Edit: If your terminal server is a VM and the BBS software is a DOSbox instance running on the terminal server then you also only need to snapshot one host and have persistent scroll back PLUS the ability to restore from random snapshots. The only thing you would lose is your TCP/IP handshake to the xterm / PuTTY session but they're detachable anyway via tmux / screen so you'd just have to SSH back into the host and reattach to your multiplexer (screen / tmux).

1 more reply

j / k navigate · click thread line to collapse

0 comments

laumars7y ago

burfogOP7y ago

Suspending a server is good for debugging it. I can go back to before a crash. I can load a snapshot to bypass slow and annoying start-up.

laumars7y ago

Maybe you need to stop and think for a moment that perhaps what you're asking is silly rather than just assuming that everyone else on the internet is stupid.

1 more reply

j / k navigate · click thread line to collapse