paste: handle "dangling paste" (missing stop-paste code)

[justinmk]

Since #4448 , paste is handled now by the vim.paste() handler (which is invoked by the UI via nvim_paste()).

There is a potential issue where the TUI could get stuck in paste-mode if the host terminal fails to send a stop-paste sequence (<Esc>[201~). In this state, the user could recover by RPC:

nvim_paste([''], 3)

Nvim GUIs using nvim_paste do not need such workarounds because they use an out-of-band channel (as opposed to the TUI where user input + paste are in-band).

Vim fixed a similar issue with its bracketed-paste implementation, but:

• the "infinite loop" problem does not apply because Nvim doesn't actively loop
• CTRL-C would not interrupt the paste in Nvim, so a "dangling paste" is less likely. (Could be caused by network failure instead.)
  • But CTRL-C would be useful for recovering from this "stuck in paste" state.
  • Related: <ctrl-c> not stopping text paste #13031 (comment)

Plan

1. TUI needs to check nvim_paste() result (e.g. revert 2c605d1 )
2. vim.paste() Lua handler could track some state to decide if a paste is "stuck". E.g. if the paste stream is very "slow", and CTRL-C is seen ~twice in a row, we can guess that (1) the paste was exhausted and (2) the user is trying to cancel the operation, thus false should be returned.

[e-kwsm]

Since #4448, Shift + Ins in insert mode does not work (like #10223), while "+/*p in normal mode works.

• nvim --version: NVIM v0.4.0-1856-g82d52b229df7
  Build type: RelWithDebInfo
  LuaJIT 2.0.5
• vim -u DEFAULTS (version: 8.1.1401) behaves differently? yes (+ and * registers are unablable though)
• Operating system/version: Debian 10.0
• Terminal name/version: GNOME Terminal 3.30.2 using VTE 0.54.2 +GNUTLS
• $TERM: xterm-256color

Steps to reproduce using nvim -u NORC

Copy the followuing two lines

a

and then nvim -u NORC
"+P
"*P
O Shift+Ins

Actual behaviour

a^M^M
a

a

od -x gives

0000000 0d61 0a0d 0a61 610a 0a0a 000a
0000013

Expected behaviour

a

a

a

[justinmk]

@e-kwsm please create a new ticket, that is off-topic for this one.