Version 2.0.247 regression with structural editing, hangs at unbalance + structural delete

[PEZ]

Calva's structural editing hangs in some situations when there is unbalance in the document. Since structural editing was updated in 247, this is very probably a regression there.

This is my repro so far (the vertical bar is the cursor):

([{|)

backspace

Two things happen. 1, the new text is:

([|

Should be:

([|)

And, much, much worse: BOOM. Now Calva is essentially dead.

If you run with auto-closing parens on, this won't happen often. But of course might. Using Calva 2.0.246 is safer.

Update: Calva v2.0.249 reverts back to before the changes to the structural editing were merged.

[PEZ]

@corasaurus-hex While my changes to the structural editing are most likely to have caused this. In the same release we also did the no-floating-promises update. Is there a possibility that could cause this? There are some promises involved in the Paredit backspace function:

calva/src/cursor-doc/paredit.ts

Line 813 in 6947821

export function backspace(

[PEZ]

@corasaurus-hex I now tried with fd3be49 which has the no-floating-promise changes and then things worked, so this regression is not caused by that.

[PEZ]

Some expert digging by @corasaurus-hex shifted our attention to the formatter and formatPosition, which happens after paredit backspace. This function in particular.

I first suspected the cursor.rangeForCurrentForm(index) call, but e069624 did not agree.

Then I saw cursor.getFunctionName(), and had better luck with 3d4c381 Lookie:

calva-cursor-hang-1573-getfunctionname.mp4

[PEZ]

Closing in a bit more. This cursor.backwardListOfType test never finishes:

[PEZ]

Hmmm... backwardList returns true for this unbalance case, even though it doesn't move. That is just waiting to cause a hang!

[PEZ]

This fixes the issue.

    /**
     * Moves this cursor backwards to the open paren of the containing sexpr, or until the start of the document.
     */
    backwardList(): boolean {
        const cursor = this.clone();
        while (cursor.backwardSexp()) {
            // move backward until the cursor cannot move backward anymore
        }
-       if (cursor.getPrevToken().type === 'open') {
+       if (cursor.getPrevToken().type === 'open' && cursor.offsetStart !== this.offsetStart) {
            this.set(cursor);
            return true;
        }
        return false;
    }

However, it's very strange.... why did this not cause trouble until #1566?

[PEZ]

No, that broke backwardList totally. This is a much better fix:

    /**
     * Moves this cursor backwards to the open paren of the containing sexpr, or until the start of the document.
     */
    backwardList(): boolean {
        const cursor = this.clone();
        while (cursor.backwardSexp()) {
            // move backward until the cursor cannot move backward anymore
        }
        if (cursor.getPrevToken().type === 'open') {
            const checkCursor = cursor.clone();
            if (checkCursor.backwardUpList() && checkCursor.forwardSexp()) {
                this.set(cursor);
                return true;
            }
        }
        return false;
    }