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Conpty "pass-through" doesn't maintain order of operations #11948

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opened 2026-01-31 03:01:53 +00:00 by claunia · 11 comments
claunia commented 2026-01-31 03:01:53 +00:00
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Originally created by @j4james on GitHub (Jan 3, 2021).

Originally assigned to: @zadjii-msft on GitHub.

Environment

Windows build number: Version 10.0.18363.1198
Windows Terminal version (if applicable): Version: 1.5.3242.0

Steps to reproduce

  1. Open a bash shell in Window Terminal.
  2. Execute the following command:
printf "\e[40m\e[2J\e]4;0;rgb:00/00/80\e\\"; sleep 1; printf "\e#8\e]4;0;rgb:FF/00/00\e\\"

Expected behavior

This should erase the screen with background color 0, but with the 0 color palette set to blue. It then pauses for a second, before filling the screen with the alignment test pattern (which uses default colors), and changes the 0 color palette to bright red.

Since the screen is filled with the default colors of the test pattern before the palette is changed to red, you should never see that red.

Actual behavior

There's a brief moment when the screen flashes bright red before the test pattern appears.

If you enable the "debug tap" you can see what's going wrong. The palette change is passed through to the conpty connection as soon as it's encountered, whereas the test pattern is written to the conhost buffer and only passed through to conpty in the next paint cycle. You thus end up having the operations arrive out of order.

I'm guessing it's timing related, so it may not happen all the time, but I'm definitely seeing it a lot.

Originally created by @j4james on GitHub (Jan 3, 2021). Originally assigned to: @zadjii-msft on GitHub. # Environment Windows build number: Version 10.0.18363.1198 Windows Terminal version (if applicable): Version: 1.5.3242.0 # Steps to reproduce 1. Open a bash shell in Window Terminal. 2. Execute the following command: ``` printf "\e[40m\e[2J\e]4;0;rgb:00/00/80\e\\"; sleep 1; printf "\e#8\e]4;0;rgb:FF/00/00\e\\" ``` # Expected behavior This should erase the screen with background color 0, but with the 0 color palette set to blue. It then pauses for a second, before filling the screen with the alignment test pattern (which uses default colors), and changes the 0 color palette to bright red. Since the screen is filled with the default colors of the test pattern before the palette is changed to red, you should never see that red. # Actual behavior There's a brief moment when the screen flashes bright red before the test pattern appears. If you enable the "debug tap" you can see what's going wrong. The palette change is passed through to the conpty connection as soon as it's encountered, whereas the test pattern is written to the conhost buffer and only passed through to conpty in the next paint cycle. You thus end up having the operations arrive out of order. I'm guessing it's timing related, so it may not happen all the time, but I'm definitely seeing it a lot.
claunia commented 2026-01-31 03:01:56 +00:00
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@DHowett commented on GitHub (Jan 3, 2021):

Oh yeah, this is absolutely a problem. ConPTY renders SGR and text during frame render, but passthrough is instant at time of dispatch failure. We've hit this in a number of places. @zadjii-msft knows more.

@DHowett commented on GitHub (Jan 3, 2021): Oh yeah, this is absolutely a problem. ConPTY renders SGR and text during _frame render_, but passthrough is instant at time of dispatch failure. We've hit this in a number of places. @zadjii-msft knows more.
claunia commented 2026-01-31 03:01:57 +00:00
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@j4james commented on GitHub (Jan 4, 2021):

I thought this had come up before, and I also thought there was an API somewhere that would force the frame to render before pass-through so we could trigger that on operations that needed it. I couldn't find anything like that, though, so maybe I imagined it.

@j4james commented on GitHub (Jan 4, 2021): I thought this had come up before, and I also thought there was an API somewhere that would force the frame to render before pass-through so we could trigger that on operations that needed it. I couldn't find anything like that, though, so maybe I imagined it.
claunia commented 2026-01-31 03:01:57 +00:00
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@DHowett commented on GitHub (Jan 4, 2021):

Actually, yeah... hm. https://github.com/microsoft/terminal/pull/4896

@DHowett commented on GitHub (Jan 4, 2021): Actually, yeah... hm. https://github.com/microsoft/terminal/pull/4896
claunia commented 2026-01-31 03:01:58 +00:00
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@j4james commented on GitHub (Jan 4, 2021):

OK, that's probably what I was thinking of, but that doesn't solve this problem. The content being flushed there is just the VT engine's output buffer. That includes the pass-through content that was just written, but it won't include any frame changes that haven't been painted yet. We need a paint to occur before the pass-through content is even added to that buffer. I think.

@j4james commented on GitHub (Jan 4, 2021): OK, that's probably what I was thinking of, but that doesn't solve this problem. The content being flushed there is just the VT engine's output buffer. That includes the pass-through content that was just written, but it won't include any frame changes that haven't been painted yet. We need a paint to occur before the pass-through content is even added to that buffer. I think.
claunia commented 2026-01-31 03:01:58 +00:00
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@DHowett commented on GitHub (Jan 28, 2021):

I'm not sure what to do with this one, but I am going to mark it up as a correctness issue in conpty and put it on the backlog. There will always be cases of this with our approach (especially when the control sequences in question apply to spans of text, ugh), and we need to revisit our approach if we want to get 100% of them.

Full passthrough for ENABLE_VIRTUAL_TERMINAL_PROCESSING apps with a low-fidelity text buffer would serve a great number of use cases. Perhaps this is worth factoring into that design.

@DHowett commented on GitHub (Jan 28, 2021): I'm not sure what to do with this one, but I _am_ going to mark it up as a correctness issue in conpty and put it on the backlog. There will always be cases of this with our approach (especially when the control sequences in question apply to spans of text, ugh), and we need to revisit our approach if we want to get 100% of them. Full passthrough for `ENABLE_VIRTUAL_TERMINAL_PROCESSING` apps with a low-fidelity text buffer would serve a great number of use cases. Perhaps this is worth factoring into that design.
claunia commented 2026-01-31 03:01:59 +00:00
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@zadjii-msft commented on GitHub (Mar 23, 2022):

Some of the work in #12561 might help here. In that PR, I've got a helper that the alt buffer handling can use to manually flush the current frame and start buffering a new one. We may want to do that always when conpty is flushing through to the terminal side.

(we'll probably want to also do the thing where we don't actually WriteFile on the flush, for perf reasons. This would certainly exacerbate that hot path)

@zadjii-msft commented on GitHub (Mar 23, 2022): Some of the work in #12561 might help here. In that PR, I've got a helper that the alt buffer handling can use to manually flush the current frame and start buffering a new one. We may want to do that _always_ when conpty is flushing through to the terminal side. (we'll probably want to also do the thing where we don't actually `WriteFile` on the flush, for perf reasons. This would certainly exacerbate that hot path)
claunia commented 2026-01-31 03:01:59 +00:00
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@zadjii-msft commented on GitHub (Jun 23, 2022):

Discussed more with xterm.js folks - we should definitely fix this. Should be as trivial as doing the frame flush, then the sequence flush.

See also https://github.com/microsoft/vscode/issues/151143

@zadjii-msft commented on GitHub (Jun 23, 2022): Discussed more with xterm.js folks - we should definitely fix this. _Should_ be as trivial as doing the frame flush, then the sequence flush. See also https://github.com/microsoft/vscode/issues/151143
claunia commented 2026-01-31 03:01:59 +00:00
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@zadjii-msft commented on GitHub (Jul 7, 2022):

Okay I've got a really dumb solution for this in e5fbc1bb1

import sys
import time

sys.stdout.write(f'foo\nbar\n\x1b]663;A\x1b\\baz\nqwer\n')
sys.stdout.flush()

image

I need to sit and actually think about this more, but I don't know why this wouldn't work

@zadjii-msft commented on GitHub (Jul 7, 2022): Okay I've got a really dumb solution for this in e5fbc1bb1 ```python3 import sys import time sys.stdout.write(f'foo\nbar\n\x1b]663;A\x1b\\baz\nqwer\n') sys.stdout.flush() ``` ![image](https://user-images.githubusercontent.com/18356694/177839440-6e0287cc-7ad5-4bcc-9a0a-f4cfc76703ea.png) I need to sit and actually think about this more, but I don't know why this _wouldn't_ work
claunia commented 2026-01-31 03:02:00 +00:00
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@j4james commented on GitHub (Jul 7, 2022):

That seems reasonable to me. As I understand it, that's the equivalent of what we're doing manually in a couple of places in AdaptDispatch when we call TriggerFlush before returning false like this:

c754f4d22d/src/terminal/adapter/adaptDispatch.cpp (L2481-L2486)

Assumedly those manual calls to TriggerFlush could now be dropped with this new approach.

@j4james commented on GitHub (Jul 7, 2022): That seems reasonable to me. As I understand it, that's the equivalent of what we're doing manually in a couple of places in `AdaptDispatch` when we call `TriggerFlush` before returning false like this: https://github.com/microsoft/terminal/blob/c754f4d22da47d93ba6e1401496605eba45248d6/src/terminal/adapter/adaptDispatch.cpp#L2481-L2486 Assumedly those manual calls to `TriggerFlush` could now be dropped with this new approach.
claunia commented 2026-01-31 03:02:01 +00:00
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@zadjii-msft commented on GitHub (Jul 12, 2022):

I don't know why this wouldn't work

there were lots of reasons

@zadjii-msft commented on GitHub (Jul 12, 2022): > I don't know why this _wouldn't_ work _there were lots of reasons_
claunia commented 2026-01-31 03:02:01 +00:00
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@zadjii-msft commented on GitHub (Apr 25, 2023):

A note regarding #14677

That PR doesn't fix this. I that's because the _stateMachine->FlushToTerminal() call is still sticking the unknown sequence at the start of the frame, instead of at the end. The diagrams in https://github.com/microsoft/terminal/pull/14677#issuecomment-1499470956 are helpful. We still need something like #13462 for "paint the current frame, then append this sequence", or.... just... insert this sequence as something to add at the end of the current frame (and start the frame now). Hmm.

@zadjii-msft commented on GitHub (Apr 25, 2023): A note regarding #14677 That PR doesn't fix this. I that's because the `_stateMachine->FlushToTerminal()` call is still sticking the unknown sequence at the start of the frame, instead of at the end. The diagrams in https://github.com/microsoft/terminal/pull/14677#issuecomment-1499470956 are helpful. We still need something _like_ #13462 for "paint the current frame, then append this sequence", ~or.... just... insert this sequence as something to add at the end of the current frame (and start the frame now). Hmm.~
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Reference: starred/terminal#11948
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