Re: fstatat + AT_NO_AUTOMOUNT

[Paul Eggert]

On 3/7/22 13:56, Pádraig Brady wrote:
> $ git show 571f63f50 | grep -B3 fstatat

Ah, I now see the problem: I suppressed some of GitHub's JavaScript code 
when looking at the URL you sent. I'll have to remember not to do that.

After thinking about it a bit more, how about the following idea?

* Normally, do not use AT_NO_AUTOMOUNT.

* If a file is known to exist but its type is not known, it's OK to use 
fstatat with AT_NO_AUTOMOUNT to determine the file's type, under the 
theory that mounting should merely replace directories with directories 
and so won't change the type, so that AT_NO_AUTOMOUNT is merely an 
optimization here. A use case might be fts.c wanting to know whether a 
directory entry with type DT_UNKNOWN is itself a directory.

* I don't yet see any other use for AT_NO_AUTOMOUNT.

'cp -x' is an example where we can't use AT_NO_AUTOMOUNT, since cp needs 
to read the source file (not just stat it) and so we want the automount 
so that the stat result is consistent with the other accesses.

'du -x' is a more-plausible use of AT_NO_AUTOMOUNT but I don't see how 
it could work correctly.

'df' likely needs to use both fstatat and statvfs and the latter lacks 
AT_NO_AUTOMOUNT, which means the former shouldn't use AT_NO_AUTOMOUNT.