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0 pointslaumars7y ago0 comments

Your arguements don't really make any sense with regards to the technology you're discusing. Eg why would we replay past TTY output? Also escape are inlined so they would also be captured via the parents method. In fact contrary to your point, TTYs effectively are just text on a screen.

The only thing that wouldn't be inlined via control characters nor escape sequences would be the PTY operating mode (which are defined via kernal syscalls). Im talking about stuff like local echo, flow control, how CR/LF characters are handled, etc. However that's solved with a multiplexer (screen or tmux type thing) or a custom shell, thus you could save that running state and reset it when needed (shells actually already need to do this to some extent so they can pass control of the TTY to forked processes. But we'd obviously need to take things a step further and store any state changes made by forked processes as well). However now we are back to my original solution you dismissed.

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burfog7y ago

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.

laumarsOP7y 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.

burfog7y 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.

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burfog7y ago

laumarsOP7y ago

burfog7y 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.

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