This feature was accidentally removed when doing the initial import from vim. It
makes vim use pipes instead of temporary files for filtering buffers through
shell commands.
I found that this was missing when looking for references of
SHELL_READ/SHELL_WRITE outside mch_call_shell`.
When `mch_call_shell` is reimplemented on top of libuv process management
facilities, pipes will always be used for communication with child processes so
it makes sense to enable the feature permanently.
The functions `mch_inchar`, `mch_breakcheck`, `mch_char_avail` were
reimplemented on top of libuv. Here's how it works:
- When Neovim needs to wait for characters, it will transfer control to libuv
event loop.
- When the libuv event loop gets user input, it will transfer control back to
Neovim
- Neovim uses the `input_read` function to get the actual data read by libuv.
With this scheme its possible to keep Neovim single-threaded while enjoying the
benefits provided by libuv.
This commit leaves SIGWINCH broken for now
