fs::Permissions set_readonly is odd on Unix.

[twmb]

• set_readonly(true) wipes out write permissions, leaving existing executable permissions in place. This seems contrary to what I would expect this function to do: make something only readable. I would expect set_readonly(true) to set the permissions to 0o444.

• set_readonly(false) adds write permissions to make permissions not readonly, as opposed to wiping out read permissions. Setting permissions to not readonly itself is a bit odd to begin with. Any combination of permissions that are not just read is technically not read only. I don't have a good solution to this, but I think more clarifying documentation would suffice.

[retep998]

Remember that because this is a cross platform API, it should ideally behave the same on all platforms. The only permission option exposed on Windows, besides full blown ACLs, is the readonly file attribute which isn't even a permission but a simple bit that can be trivially toggled. What it does is control whether you can write to the file; it has no effect on being able to read or execute the file. Changing the behavior on Unix would thus cause a significant difference in behavior across platforms, nevermind being a breaking behavior change in a stable function. However, improving the documentation to clarify the current behavior is a worthwhile goal.

[frewsxcv]

Opened a PR for this #43883