Re-explaining that errno thing....

[Uri Simchoni]

Jeremy,

I thought I understood commit cda6764f1a8db96182bfd1855440bc6a1ba1abee,
which is about correctness of errno in failures in vfs_shadow_copy2, but
yesterday, when reviewing Ralph / Metze's vfs_catia patch, I found out
that I can't explain to myself why always saving errno and restoring it
is incorrect (there's this pattern in that patch).

So, commit cda6764f1a8db96182bfd1855440bc6a1ba1abee says that the way to
deal with errors is:

	int save_errno = 0;
	
	if (error) {
		save_errno = errno;
		goto out;
	}

out:
...do some cleanup
	if (save_errno) {
		errno = save_errno;
	}
	return ret;


Whereas the previous pattern was:

	int save_errno;
	
	if (error) {
		goto out;
	}

out:
	save_errno = errno;
...do some cleanup
	errno = save_errno;
	return ret;


The commit said code shouldn't be *setting* errno if there's no error.
Fine. But the "old" pattern isn't setting errno - it's keeping it the
way it was.

The "old" pattern is less obviously correct, as errno has to be kept
untouched from the failure point up to the start of cleanup. But it
seems to me like the pattern itself doesn't break anything (it just lets
code around it break things).

Is that analysis correct or am I missing something again?

Thanks,
Uri.