fix: SSHConfig: atomically write ssh config

[johnstcn]

Relates to #504

To avoid race conditions, write to a tempfile and then rename.

Smoke-tested by:

• Setting immutable flag on /Users/cian/.ssh/config (sudo chflags uchg $PATH) and validating that the rename failed:
  

• Setting immutable flag on /Users/cian/.ssh and validating that writing the temp file failed:
  

• Once immutable flag removed (sudo chflags nouchg $PATH), connecting to workspace is successful.

[code-asher]

I am probably overthinking this, but if there is somehow a file with this suffix already, would we want to error instead of overwrite? We could use flag: "wx" on the writeFile call to error if the path already exists.

Vanishingly unlikely, but maybe there could be a freak collision where a user has a ~/.ssh/config.conf and the random suffix was also conf.

[johnstcn]

1. We could just write to /tmp
2. We could instead name it something like config.vscode-coder.${timestamp}.${randSuffix} to minimize the likelihood of collisions

Thoughts?

[code-asher]

I like 2 personally because I think it gives us a better chance to get the temp files for debugging, with /tmp they could be long gone.

But I could see an argument for /tmp to avoid "clutter".

[bcpeinhardt]

What about a ULID? It would be sufficiently random and you could sort them by time to determine write order when debugging.

[mafredri]

If we want to guarantee atomicity, same folder is best (/tmp for instance could be on another filesystem).

To hide clutter in the case of error, I'd suggest a . prefix on the filename, perhaps .tmp.[filename].[rand]. At this point guaranteeing uniqueness beyond this would seem like overkill.

I.e. my suggestion is just /home/user/.ssh/config -> /home/user/.ssh/.tmp.config.XXXXXX.

One use-case that won't work here is if /home/user/.ssh/config is bind-mounted. In that situation we either error out or try to write the file directly. Erroring is obviously safest so perhaps we go with that.

[johnstcn]

What about a ULID?

I like the idea in theory, but it would be another import.

[mafredri]

Forgot to write down one more thing, we should also respect the existing file modes and permissions and make sure they're copied over.

[johnstcn]

Done. I've also adjusted the tempfile naming convention to ~/.ssh/.config.vscode-coder-tmp.abc123 to make it abundantly clear that

1. This refers to ~/.ssh/config
2. It was created by vscode-coder

[mafredri]

Nice work!