archiver: Shortcut for handling empty files

[greatroar]

What does this PR change? What problem does it solve?

Regular files that are empty according to stat are now not opened for reading their contents. Such files are quite common (in my homedir, at least) and we can save multiple system calls this way. On a network filesystem, that can mean round trips. Also, we can back up empty files that we cannot open for reading. Finally, fixes #4257.

Existing tests cover this case. fs.Reader now no longer has a meaningful Size. Nothing depended on that.

I tested restic backup --stdin on the command line and it still works. It also still gives the same error for empty stdin.

Was the change previously discussed in an issue or on the forum?

Fixes #4257.

Checklist

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• I have added tests for all code changes.
• I have added documentation for relevant changes (in the manual).
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• I'm done! This pull request is ready for review.

[HeikoSchlittermann]

Empty files may not be empty. What is about e.g. /proc/cpuinfo. Of course, if creating a backup of this directory is necessary, is another question. But anyway, I think, shortcuts like the one you suggest, are asking for trouble, sooner or later.

[pvgoran]

But anyway, I think, shortcuts like the one you suggest, are asking for trouble, sooner or later.

I second this opinion.

[greatroar]

I don't find the /proc example compelling at all, and we have at least one example were an empty file not being empty is exactly what is causing trouble.

[HeikoSchlittermann]

Programs should not try to be more clever than me ;)

[HeikoSchlittermann]

What about a broader approach like

restic backup --skip '{{ stat.Uid }} != 0'

or similiar. But, OTOH this duplicates things we can achive better using find and creating a file list then. Maybe this would help in your scenario too, using find ... -not -empty to create a list of files for the backup. (If I understand the options well, I'm new to restic, since about 3 days.)

[greatroar]

I'm not sure I get you completely, but I certainly want the empty files in, not skipped. I just want don't want to actually open them if lstat says they're empty.

[HeikoSchlittermann]

I'm not sure I fully understand you, but I definitely want the empty files to be there and not skipped. I just don't really want to open them when lstat says they are empty.

You are correct, I missed that point. Creating a list of files to back up would not help here. Just as the suggested filter would not help. So maybe you're suggesting a new option as --trust-file-size