x/term: does not recognize delete key

[josharian]

terminal.ReadLine() does not recognize the forward-delete key escape sequence \x1b[3~; it gets silently discarded, which makes the readline experience weird for folks who use the delete key.

Reproduce, as of x/term at 3863673230bd2f654d5c3749dc90e6d130067dd6:

package main

import (
	"fmt"
	"os"

	"golang.org/x/term"
)

func main() {
	s, err := term.MakeRaw(int(os.Stdin.Fd()))
	if err != nil {
		panic(err)
	}
	defer term.Restore(int(os.Stdin.Fd()), s)

	t := term.NewTerminal(os.Stdin, "> ")

	fmt.Fprintln(t, "type text, try out delete/backspace, hit enter at end of line to exit:")
	t.ReadLine()
}

I'm willing to fix, if somone will review. (I don't see an owner at https://dev.golang.org/owners.)

[mknyszek]

I'm happy to review.

Maybe you know @josharian: is parity with readline a goal for ReadLine? The docs are unfortunately terse, I don't know what the original intent was. (It kind of looks like that was the intent, the package is just quite old.) Still, this seems like a reasonable improvement (not eating your input when using the delete key).

[ldemailly]

Just to clarify the current behavior is that delete (unlike backspace) is noop (nothing is eaten)
The current way to delete under the cursor is ^D

[josharian]

I'm happy to review.

Yay, thanks! I'll put together a patch.

Maybe you know @josharian: is parity with readline a goal for ReadLine?

I have no idea.

What I personally want is for using x/term to feel natural (which is how I hit this case). By that metric, it is pretty close, but then, I'm not a heavy user of advanced readline features.

I set up a differential fuzz test of term.ReadLine vs gnu readline and...it's not even close. But then I looked at the details and they weren't compelling. Lots of what looked like gnu readline quirks.

I just kicked off a differential fuzz vs only cases in which gnu readline and bsd libedit respond identically, figuring that this would avoid a bunch of implementation details and capture only "this really should work" behavior, such as a functional Delete key. I'll report back when that has stabilized enough to draw some conclusions.

[gopherbot]

Change https://go.dev/cl/730440 mentions this issue: x/term: handle delete key

[josharian]

Turns out the gap between x/term and readline/libedit is...not small. Things uncovered by fuzzing includes ctrl+t, ctrl+z, ctrl+x, a bunch of CSI sequence handling, and more.

I personally care about ctrl+t but none of the rest, so unless someone raises their hand and asks for other things to be supported, I'll mail delete and ctrl+t and stop there.

I'm happy to share the fuzzing harness on demand, but it's rather ugly.