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HACKING add some thoughts on input/queries #1469

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nick black 2021-06-09 22:42:51 -04:00 committed by Nick Black
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@ -366,3 +366,35 @@ be a multiple of six pixels tall.
* always starts at the origin of its plane
* admits no other output to its plane, nor resizing
* greatly complicates rendering
## Input
Input is greatly complicated by rare but critical in-band signaling from the
terminal itself. This is the method by which, for instance, terminals
advertising Sixel indicate how many color registers they support. We must
ensure such responses never reach the user, and that we act on them quickly.
Such replies are generally distinguished by a (literal) escape. Unfortunately,
the user can (and often does) generate ESC themselves.
The primary instance of this signaling is on startup, when we query the
terminal as part of capability discovery. Until we process the reply, we
don't know what capabilities the terminal offers, particularly with regard
to bitmap graphics.
We have two potential input sources, both of which *might* correspond to
`stdin`. If we were spawned attached to the terminal, we receive both user and
terminal input on the same fd (corresponding to `stdin`). If our input was
redirected from somewhere else, we need open the controlling terminal, and
read from it. This has the happy side-effect of isolating the control plane
from the data plane (though you mustn't rely that this will make control
communication unforgeable; the user can likely write to the controlling
terminal themselves).
If a terminal doesn't understand or implement some query, there will typically
be no response. If a negative response is required, follow up the query (or
queries) with a Device Attributes (DA, `\e[c`) query, to which all known terminals
will respond. So long as a valid response cannot be confused with a response to
DA, this serves as a negative acknowledgement. Relying on this, at startup we
fire off two `XTSMGRAPHICS` queries followed by a DA query, all as one write. We
don't sit around waiting for the response, but instead continue initialization.
Ideally, by the time we're done and need the info, it's ready for us to read.