Line break at file end can't be added by PyCharm when running the Ruff plugin

[simonpercivall]

Describe the bug
I think this is some weird interaction / race condition between PyCharm and the Ruff plugin running ruff on save.

Running the Ruff plugin on save removes the line break PyCharm is trying to add the the end of the file.

To Reproduce

1. I Have PyCharm with the setting Editor->General->Ensure every saved file ends with a line break,
2. and the Ruff plugin set to "Run ruff when the python file is saved".
3. Now, if I have a file with a \n at the end, everything works as expected (i.e. there's nothing weird going on). But, if I remove that line break, and (with the cursor at some other line) try to save, the line break gets added and then immediately removed, and the file is marked as having unsaved changes (see the flashing 4 in the gif).
4. If I disable "Run ruff when the python file is saved" everything works as expected.

Expected behavior
A line break is added at the file end, and the file is saved.

Screenshots

Environments (please complete the following information):

• IDE: PyCharm Professional 2023.1
• OS: macOS 13.3.1
• Ruff Version 0.0.261
• Plugin version 0.0.11

[koxudaxi]

@simonpercivall
Thank you for creating the issue.
I can't reproduce the problem.
Could you please share the ruff config like pyproject.toml?

[simonpercivall]

Weird, I've verified it on different computers with several projects. But it is inconsistent, in that in one case it does add the line break, but leaves the file in an "unsaved changes" state.

[tool.ruff]
select = ["E", "F", "B", "I", "UP", "C4", "DJ", "ISC", "G", "INP", "PT", "RET", "SIM", "TCH", "PTH", "PLE", "PLW", "RUF"]
line-length = 105
ignore = ["E731", "C408", "PLW2901", "RET504", "RET505"]

[koxudaxi]

@simonpercivall
Thank you for sharing the information.
I tested your config and examples. But I can't represent the problem :(

[simonpercivall]

Well, thanks for checking! I guess it's some interaction between different PyCharm plugins.

I do wonder, is the plugin getting the changed text back or running ruff to fix the files directly? If the latter, it'll always be susceptible to race conditions. But I don't know how complex that change would be, to run ruff with --diff on save (if it isn't already, of course).

[koxudaxi]

@simonpercivall

I do wonder, is the plugin getting the changed text back or running ruff to fix the files directly? If the latter,

PyCharm invokes the plugin's method with the file's text to reformat before saving the file.
I should check the order of calling plugin methods.

But I don't know how complex that change would be, to run ruff with --diff on save (if it isn't already, of course).

I feel it's big changes 😅