Ensure buffer change cb does not read bad cursor

[axelf4]

In multiple editing operations the cursor may temporarily be out of bounds during calls into user code from buffer update listeners. This means doing anything in a change callback that depends on the cursor position - including calling any function from Vim script - can throw an error. Moving the cursor is also not possible, since there is no way to restore the cursor to its previous invalid position via the public APIs.

This pull request attempts to fix the issue by always saving the location of the cursor before running each change callback with the cursor in a valid spot.

For some background, I fixed one instance of this in #11782, but there are many more, as evidenced by these issue reports for the Neovim version of vim-strip-trailing-whitespace, all of which hit this issue:

• Neovim: Error with undo without EOL axelf4/vim-strip-trailing-whitespace#5

  (has reproduction steps)

• E5108 error when nvim-spectre is installed axelf4/vim-strip-trailing-whitespace#8

• Conflicts with floaterm in neovim axelf4/vim-strip-trailing-whitespace#9

[zeertzjq]

Related: #16729 #16732

[perrin4869]

@bfredl any chance you can have a look at this? thanks!

[bfredl]

Would it possible to write a simple test?

[zeertzjq]

I have a test in #16732.

[justinmk]

#16732 looks more complete so let's continue there. Thank you for getting this started @axelf4

[axelf4]

I agree! One thing though that seems to be missing from that other PR which I would like to see is resetting the cursor to e.g. firstline so that user callbacks do not ever see a bad cursor position.

Also, for completeness, here is the test case that I had lying around which currently fails on NVIM v0.7.0:

function! G()
endfunction
function! F()
	call G()
endfunction

let fname = tempname()
call writefile(['', '0'], fname)
silent execute 'edit!' fname
lua vim.api.nvim_buf_attach(0, false, {on_lines = function(...) vim.api.nvim_call_function("F", {}) end})
2delete _
1put ='0'
normal! u

[zeertzjq]

function! G()
endfunction
function! F()
	call G()
endfunction

let fname = tempname()
call writefile(['', '0'], fname)
silent execute 'edit!' fname
lua vim.api.nvim_buf_attach(0, false, {on_lines = function(...) vim.api.nvim_call_function("F", {}) end})
2delete _
1put ='0'
normal! u

This failure has already been fixed by #18139.