[ffigen] Fix potential null access for _tmpDir when creating temporary directories

[Gurleen-kansray]

This PR fixes a potential null access issue for _tmpDir when creating a temporary directory. The assignment now safely uses the null-aware operator to ensure _tmpDir is initialized properly:

_tmpDir ??= Directory.systemTemp.createTempSync('temp dir ');

Verified with scope_test.dart, which directly tests this functionality.

The change is localized, so it should not affect unrelated functionality. This ensures safer initialization of _tmpDir and prevents potential runtime null errors.

[x] I’ve reviewed the contributor guide and applied the relevant portions to this PR.
Closes #3026

[liamappelbe]

PR description is inaccurate. The code was already null safe, you're just changing the name of the temp directory.

Setting that aside, I'd prefer if this change only affected the tests. Maybe change it from a getter to a function with an optional prefix param, and pass a path with a space in the tests.

But now that I'm looking at this code, I notice an issue with it (not related to your change, but it'd be nice if you could fix this at the same time). I recently tried to eliminate all the global variables from FFIgen, but it seems I missed one. _tmpDir should be deleted and moved to the Context object. The issue with having global variables like this is that if we want to do multiple concurrent FFIgen runs, they'll conflict with each other.

Moving tmpDir to Context will also make it easier to make the test paths contain spaces. Make the constructor of Context optionally take a tmpDir, and fall back to the existing logic if the user doesn't pass tmpDir. Then you can just look for all the places Context is constructed in the test dirs (ie, search test/ for Context( in your IDE), and pass a directory name with a space in it. I think most of the tests use a util to construct the context, so there shouldn't be many places to update.

[Gurleen-kansray]

Thanks for the feedback @liamappelbe ..you’re right, my PR description was inaccurate. The code was already null-safe, and the change effectively only altered the temp directory name.
Before reworking this, I wanted to clarify: is there an existing execution/context object in ffigen that you’d like the temp directory to live on, or would this involve introducing a new context abstraction and threading it through the call sites?

I want to make sure I’m aligning with the intended direction before attempting a larger refactor.

[liamappelbe]

Before reworking this, I wanted to clarify: is there an existing execution/context object in ffigen that you’d like the temp directory to live on, or would this involve introducing a new context abstraction and threading it through the call sites?

There's already a Context object plumbed through most of ffigen: https://github.com/dart-lang/native/blob/main/pkgs/ffigen/lib/src/context.dart#L18

[Gurleen-kansray]

Hi @liamappelbe, I updated _matchFileWithExpected to require a Context so it can use tmpDir from the context, avoiding conflicts in concurrent FFIgen runs. This affects all tests calling it. Should I go ahead and update the tests to pass Context, or keep backward compatibility for now?

[liamappelbe]

Hi @liamappelbe, I updated _matchFileWithExpected to require a Context so it can use tmpDir from the context, avoiding conflicts in concurrent FFIgen runs. This affects all tests calling it. Should I go ahead and update the tests to pass Context, or keep backward compatibility for now?

There's no need to maintain backwards compatibility in tests. Definitely update the tests.

[github-actions]

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[coveralls]

coverage: 81.802%. remained the same
when pulling 2128376 on Gurleen-kansray:fix-tmpdir-null
into 39e3e71 on dart-lang:main.

[liamappelbe]

What's with all these unrelated diffs? You've removed some important stuff.

[liamappelbe]

You've still got this global variable. The point was to move this into the context.

[Gurleen-kansray]

Thanks, that makes sense.

You’re right ......I added Context to _matchFileWithExpected but didn’t actually use it yet. I’ll update it to read the temp directory from context.tmpDir.

The unrelated diffs weren’t intentional; I’ll revert those and keep the change focused.

I also see the remaining global _tmpDir.....I’ll remove it and move all temp dir handling into Context as suggested.

I’ll update the PR accordingly.

[Gurleen-kansray]

Following the earlier review, I’ve updated _matchFileWithExpected to use context.tmpDir and removed the remaining global _tmpDir. Please let me know if any further adjustments are needed.