Create a temporary, readonly buffer for test output - neovim

Question

I want to create a Neovim plugin that automatically runs a test suite whenever a file is saved. Here's an overview:
If I save a Rust file, run cargo test
If there's not a buffer opened for the test output, automatically create the buffer
Mark that buffer as readonly
If I save another file, reuse the existing buffer
If the buffer is quit (e.g., :q), create a new buffer on the next test run
However, I'm currently facing three issues:
I cannot detect if when I quit the buffer
When the test output is longer than the buffer height, the buffer will not scroll down
When I want to quit Neovim, it asks me to save all these temporary buffers (which I don't want to do)
How can I resolve these issues? For reference, here's my code:
local buffer_number = -1
local function log(_, data)
if data then
vim.api.nvim_buf_set_lines(buffer_number, -1, -1, true, data)
end
end
local function open_buffer()
if buffer_number == -1 then
vim.api.nvim_command('botright vnew')
buffer_number = vim.api.nvim_get_current_buf()
end
end
local function autotest(pattern, command)
vim.api.nvim_create_autocmd("BufWritePost", {
group = vim.api.nvim_create_augroup("autotest", { clear = true }),
pattern = pattern,
callback = function()
open_buffer()
vim.api.nvim_buf_set_lines(buffer_number, 0, -1, true, {})
vim.fn.jobstart(command, {
stdout_buffered = true,
on_stdout = log,
on_stderr = log,
})
end
})
end
autotest("*.rs", { "cargo", "test" })
P.S. I know there are several existing plugins for testing. I'm creating my own because I want to learn how to write neovim plugins.

Answer 1

I gave it my best shot. I believe this addresses your questions and does what you're aiming to do.
local buffer_number = -1
local function log(_, data)
if data then
-- Make it temporarily writable so we don't have warnings.
vim.api.nvim_buf_set_option(buffer_number, "readonly", false)
-- Append the data.
vim.api.nvim_buf_set_lines(buffer_number, -1, -1, true, data)
-- Make readonly again.
vim.api.nvim_buf_set_option(buffer_number, "readonly", true)
-- Mark as not modified, otherwise you'll get an error when
-- attempting to exit vim.
vim.api.nvim_buf_set_option(buffer_number, "modified", false)
-- Get the window the buffer is in and set the cursor position to the bottom.
local buffer_window = vim.api.nvim_call_function("bufwinid", { buffer_number })
local buffer_line_count = vim.api.nvim_buf_line_count(buffer_number)
vim.api.nvim_win_set_cursor(buffer_window, { buffer_line_count, 0 })
end
end
local function open_buffer()
-- Get a boolean that tells us if the buffer number is visible anymore.
--
-- :help bufwinnr
local buffer_visible = vim.api.nvim_call_function("bufwinnr", { buffer_number }) ~= -1
if buffer_number == -1 or not buffer_visible then
-- Create a new buffer with the name "AUTOTEST_OUTPUT".
-- Same name will reuse the current buffer.
vim.api.nvim_command("botright vsplit AUTOTEST_OUTPUT")
-- Collect the buffer's number.
buffer_number = vim.api.nvim_get_current_buf()
-- Mark the buffer as readonly.
vim.opt_local.readonly = true
end
end
function autotest(pattern, command)
vim.api.nvim_create_autocmd("BufWritePost", {
pattern = pattern,
callback = function()
-- Open our buffer, if we need to.
open_buffer()
-- Clear the buffer's contents incase it has been used.
vim.api.nvim_buf_set_lines(buffer_number, 0, -1, true, {})
-- Run the command.
vim.fn.jobstart(command, {
stdout_buffered = true,
on_stdout = log,
on_stderr = log,
})
end
})
end
autotest("*.rs", { "cargo", "test" })
I added some comments to help explain each step but specifically for each of your problems:
I cannot detect if when I quit the buffer
Since you already have the buffer number, you can query to see if it's still visible. Just by using the same name when creating the new buffer we an reuse any existing ones.
When the test output is longer than the buffer height, the buffer will not scroll down
This was tricky, and I'm not sure if it's the best way, but I was able to query how many lines the buffer had and set the cursor of the buffer's window to the bottom.
When I want to quit Neovim, it asks me to save all these temporary buffers (which I don't want to do)
This is because the buffers are modified. Neovim by default doesn't want to make it easy to accidentally lose your work if you have active modifications. To get around that we can easily mark that it isn't modified!
Hope that helps.