cmd/cue: cue fix writing files with zero changes updates mtime for no reason

[stevenj]

What version of CUE are you using (cue version)?

$ cue version

cue version 0.12.0

go version go1.24.1
      -buildmode pie
       -compiler gc
       -trimpath true
  DefaultGODEBUG asynctimerchan=1,gotestjsonbuildtext=1,gotypesalias=0,httpservecontentkeepheaders=1,multipathtcp=0,randseednop=0,rsa1024min=0,tls3des=1,tlsmlkem=0,x509keypairleaf=0,x509negativeserial=1,x509rsacrt=0,x509usepolicies=0
     CGO_ENABLED 1
          GOARCH amd64
            GOOS linux
         GOAMD64 v1
cue.lang.version v0.12.0

Does this issue reproduce with the latest stable release?

I guess so. There is no noted fix in the changelog for 0.12.1.

What did you do?

$ echo "package test" > test.cue
$ cue fix test.cue
$ cp test.cue test2.cue
$ touch -d "2 days ago" test.cue
$ ls -al test*.cue
-rw-r--r-- 1 steven steven 13 Apr  7 08:38 test.cue
-rw-r--r-- 1 steven steven 13 Apr  9 08:38 test2.cue
$  diff -q test.cue test2.cue; echo $?
0
$ cue fix test.cue
$ ls -al test*.cue
-rw-r--r-- 1 steven steven 13 Apr  9 08:39 test.cue
-rw-r--r-- 1 steven steven 13 Apr  9 08:38 test2.cue
$  diff -q test.cue test2.cue; echo $?
0

What did you expect to see?

The modified timestamp of the file would not change the second time cue fix was run on it because the contents did not change.

What did you see instead?

The contents did not change, but the modified timestamp of the file was set to the time cue fix was run.

[mvdan]

Thanks - we should probably not write/update files if the contents are entirely unchanged. This applies to any rewrite commands - cue fmt, cue fix, cue trim, cue mod tidy, and so on.