refactor: flush after calling write_all

[jarhodes314]

Proposed changes

Improve the file handling by:

• Making sure we flush any buffers once we have written to the file
• Making sure we use a BufWriter to improve file performance
• For cases where we write all the contents at once (often the case in tests, use fs::write(...) instead of manually opening a file

There are probably many cases here where it's not strictly necessary to do a flush (anywhere where we close a file before we rely on the data being persisted), but there are definitely quite a few cases where we this assumption doesn't hold for us.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvement (general improvements like code refactoring that doesn't explicitly fix a bug or add any new functionality)
• Documentation Update (if none of the other choices apply)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Paste Link to the issue

Checklist

• I have read the CONTRIBUTING doc
• I have signed the CLA (in all commits with git commit -s. You can activate automatic signing by running just prepare-dev once)
• I ran just format as mentioned in CODING_GUIDELINES
• I used just check as mentioned in CODING_GUIDELINES
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments

[codecov]

Codecov Report

Attention: Patch coverage is 61.90476% with 24 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/common/tedge_utils/src/file.rs	27.27%	3 Missing and 5 partials ⚠️
crates/common/tedge_utils/src/paths.rs	0.00%	7 Missing ⚠️
plugins/c8y_remote_access_plugin/src/lib.rs	0.00%	6 Missing ⚠️
crates/core/plugin_sm/src/plugin.rs	0.00%	1 Missing ⚠️
crates/core/tedge/src/cli/certificate/create.rs	0.00%	0 Missing and 1 partial ⚠️
...ensions/tedge_log_manager/src/manager/log_utils.rs	75.00%	1 Missing ⚠️

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[github-actions]

Robot Results

✅ Passed	❌ Failed	⏭️ Skipped	Total	Pass %	⏱️ Duration
633	0	3	633	100	1h48m9.756495s

[Bravo555]

Looks alright, some flushes could maybe be syncs, but I'm not sure if it's necessary either way, so approving.

[Bravo555]

note: this is an example where there's no functional change, because flush on a File is a noop, but still it's probably good to have for consistency or if somebody added a buffer and forgot to add a flush call.

If we actually want to ensure that all writes reach the file, I think sync_data or sync_all might be better here.

[Bravo555]

note: if the caller calls serialize multiple times, then adding flush here means some of the flushes will be redundant. The point is to flush the writer before it's dropped, so the caller should probably do it either way, but keeping it here is also fine.

[didier-wenzek]

Approved