How do I configure null-ls.nvim to ignore a formatter's output if it returns an error exit code?

[gregorias]

I have a formating utility that prints an error message to stdout even when it fails and exits with 1. Is there a way to configure the binding to this utility to ignore its stdout if it returns a non-zero exit code? I've checked the documentation, and I couldn't seem anything directly relevant.

[jose-elias-alvarez]

You can use check_exit_code to determine which codes correspond to success / failure. Combining this with ignore_stderr will result in ignoring formatter output on a bad exit code (which is the default for null-ls built-in formatting sources).

If this is for a null-ls built-in source, I'd also consider putting in a PR so other users can benefit from the exit code fix.

[gregorias]

Thank you for the quick response. I don't see how it helps here, because problem is that the tool outputs something on STDOUT not STDERR:

• The tool exits with 1, indicating an error.
• ignore_stderr doesn't matter, because the issue is about the tool writing an error message on STDOUT.

What's perhaps needed is something like ignore_stdout to ignore STDOUT on error exit codes.

[jose-elias-alvarez]

Given the following null-ls source:

local bad_formatter = {
    name = "bad_formatter",
    method = null_ls.methods.FORMATTING,
    filetypes = {},
    generator = null_ls.formatter({
        -- by default, we assume that non-zero exit codes are bad
        command = vim.loop.cwd() .. "/bad_formatter.sh",
    }),
}

And the following script in bad_formatter.sh:

#!/bin/bash

echo "stdout"
exit 1

null-ls will correctly handle this case, as well as the more common stderr case:

#!/bin/bash

>&2 echo "stderr"
exit 1

If your formatter outputs to both stdout and stderr, that is indeed an edge case:

#!/bin/bash

# will replace file's content with stdout
echo "stdout"
>&2 echo "stderr"
exit 1

But since it hasn't come up before, I'm not sure the behavior is common enough to warrant a dedicated option. formatter is ultimately meant to handle the most common cases, and you can work around this case easily enough using a more general-purpose helper:

local bad_formatter = {
    name = "bad_formatter",
    method = null_ls.methods.FORMATTING,
    filetypes = {},
    generator = null_ls.generator({
        command = vim.loop.cwd() .. "/bad_formatter.sh",
        on_output = function(params, done)
            local err, output = params.err, params.output
            if err or not output then
                return done()
            end

            return done({ { text = output } })
        end,
    }),
}

[gregorias]

Thank you for an extensive answer. Your last code snippet is indeed what I needed to do to address my issue.