This PR introduces a mechanism via which DCS data strings can be passed
through directly to the dispatch method that will be handling them, so
the data can be processed as it is received, rather than being buffered
in the state machine. This also simplifies the way string termination is
handled, so it now more closely matches the behaviour of the original
DEC terminals.
* Initial support for DCS sequences was introduced in PR #6328.
* Handling of DCS (and other) C1 controls was added in PR #7340.
* This is a prerequisite for Sixel (#448) and Soft Font (#9164) support.
The way this now works, a `DCS` sequence is dispatched as soon as the
final character of the `VTID` is received. Based on that ID, the
`OutputStateMachineEngine` should forward the call to the corresponding
dispatch method, and its the responsibility of that method to return an
appropriate handler function for the sequence.
From then on, the `StateMachine` will pass on all of the remaining bytes
in the data string to the handler function. When a data string is
terminated (with `CAN`, `SUB`, or `ESC`), the `StateMachine` will pass
on one final `ESC` character to let the handler know that the sequence
is finished. The handler can also end a sequence prematurely by
returning false, and then all remaining data bytes will be ignored.
Note that once a `DCS` sequence has been dispatched, it's not possible
to abort the data string. Both `CAN` and `SUB` are considered valid
forms of termination, and an `ESC` doesn't necessarily have to be
followed by a `\` for the string terminator. This is because the data
string is typically processed as it's received. For example, when
outputting a Sixel image, you wouldn't erase the parts that had already
been displayed if the data string is terminated early.
With this new way of handling the string termination, I was also able to
simplify some of the `StateMachine` processing, and get rid of a few
states that are no longer necessary. These changes don't apply to the
`OSC` sequences, though, since we're more likely to want to match the
XTerm behavior for those cases (which requires a valid `ST` control for
the sequence to be accepted).
## Validation Steps Performed
For the unit tests, I've had to make a few changes to some of the
`OutputEngineTests` to account for the updated `StateMachine`
processing. I've also added a new `StateMachineTest` to confirm that the
data strings are correctly passed through to the string handler under
all forms of termination.
To test whether the framework is actually usable, I've been working on
DRCS Soft Font support branched off of this PR, and haven't encountered
any problems. To test the throughput speed, I also hacked together a
basic Sixel parser, and that seemed to perform reasonably well.
Closes#7316
2559ed6efa