s/apparmor: use os.WriteFile in apparmor_test.go

[natibek]

Replace several separate os.OpenFile, c.Check(err, IsNil), c.Check(file.Close(), IsNil) calls with a single os.WriteFile call.

Pointed out in #15624 (comment)

Tracked with: SNAPDENG-35228

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.11%. Comparing base (8152be4) to head (858b2ef).

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #17008      +/-   ##
==========================================
+ Coverage   79.06%   79.11%   +0.05%     
==========================================
  Files        1375     1364      -11     
  Lines      191085   190918     -167     
  Branches     2465     2465              
==========================================
- Hits       151075   151051      -24     
+ Misses      30917    30779     -138     
+ Partials     9093     9088       -5     

Flag	Coverage Δ
unittests	79.11% <ø> (+0.05%)	⬆️

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[github-actions]

Tue May 5 20:00:15 UTC 2026
The following results are from: https://github.com/canonical/snapd/actions/runs/25396485467

Failures:

Skipped tests from snapd-testing-skip

If you wish to have any of the below tests run in your PR, in your PR description, add 'unskip:' followed by a copy-and-pasted list (without variants) of the below tests you wish to run (unskip plus test list must be valid yaml)

• garden:ubuntu-25.10-64:tests/main/apparmor-prompting-support

[olivercalder]

Thanks, these changes look good! Running a quick rg, I see several other places in the codebase where os.OpenFile, write, then close is used. Sometimes it's for good reason, like appending or writing at a particular offset, but I think there are other cases where we could replace it with os.WriteFile. This would probably be a good task to practice throwing a cheap and/or local LLM at, see what it can find.

[natibek]

Thanks! I noticed that too but I thought we wanted to limit it to just the package. Definitely the most LLM friendly task.

[olivercalder]

Looks good! I went rging for some more potential cases:

snap/squashfs/delta_test.go
721:	f, err := os.OpenFile(deltaPath, os.O_APPEND|os.O_WRONLY, 0644)
722-	c.Assert(err, IsNil)
723-	_, err = f.Write(expectedData)
724-	f.Close()
725-	c.Assert(err, IsNil)
726-
--
829:	f, err := os.OpenFile(deltaPath, os.O_RDWR, 0644)
830-	c.Assert(err, IsNil)
831-	_, err = f.WriteAt([]byte{0x63, 0x00}, 6) // 0x0063 = 99
832-	f.Close()
833-	c.Assert(err, IsNil)
834-

osutil/syncdir_test.go
399:	file, err := os.OpenFile(testPath, os.O_RDWR, 0)
400-	c.Assert(err, IsNil)
401-	defer file.Close()

### mayyybe this one, not sure ###
sandbox/cgroup/freezer.go
181:	f, err := os.OpenFile(where, os.O_WRONLY|os.O_TRUNC, 0644)
182-	if err != nil {
183-		return err
184-	}
185-	_, errW := f.Write(data)
186-	errC := f.Close()

[natibek]

I found these as well when searching but none of them can be updated.

• snap/squashfs/delta_test.go: the first case needs to append to the deltaPath file and os.WriteFile truncates. The second case needs to write at an offset which os.WriteFile does not support.
• osutil/syncdir_test.go: the file needs to be left open for

	fref = osutil.FileReference{Path: testPath}
	_, _, _, err = fref.State()

• sandbox/cgroup/freezer.go: it needs to throw an error if the file does not exist.

[Rnfudge02]

When I used 'rg' I noticed there are a few additional cases, not sure if they would be candidates or not.

• bootloader/ubootenv/env_test.go 615:617
• bootloader/ubootenv/env/go 365:369

These two are tagged RDWR but it seems its just the nil check

• bootloader/lkenv/lkenv.go 348:352
• image/image_test.go 5646:5648
• osutil/disks/gpt_test.go - Multiple

The rg expression I used to check is:

rg -U '(?s)os\.OpenFile.*?Close\(\)'

This surfaces a lot more cases than I've stated, but some are clearly not good candidates.

[Rnfudge02]

This looks a lot better than the existing syntax, a few other potential cases, but besides that this is looking good!

[natibek]

I took a look at these cases and they all do more than just writing with the *File returned by the call to os.OpenFile. They either avoid truncating, use the file pointer in another function call, write at an offset, or check if the file is created already.

[Rnfudge02]

LGTM, thanks!

[olivercalder]

Thanks!