Why does (stat() ?) open files ?

[Ben RUBSON]

On 09 Apr 2018 12:52, Corinna Vinschen wrote:

> On Apr  9 12:28, Ben RUBSON wrote:
>> Hi,
>>
>> This follows the "Why does readdir() open files ?" discussion we had a few
>> days ago.
>> Thank you Corinna for your answers and suport there !
>>
>> So, context is Cygwin, especially rsync, working over a Fuse FS.
>> This Fuse FS is assumed to be mounted on `/cygdrive/x` below.
>>
>> I finally found that readdir() does not open every file.
>> `ls /cygdrive/x` does not fire any open() call. Perfect.
>>
>> However, `ls -l /cygdrive/x` does, every file is opened, with read access.
>> As `rsync -an /cygdrive/x /tmp/`, which is a dry-run just grabbing files'
>> attributes.
>>
>> I then went through Cygwin code and found that NtCreateFile/NtOpenFile  
>> calls
>> from symlink_info::check() in path.cc may be the culprits.
>> To demonstrate this I added write access to these calls, and found that
>> every file was then opened with write access.
>>
>> Later in this function we have a failback to NtQueryDirectoryFile call.
>> I assume (assume only, I may be wrong) this one does not open the  
>> requested
>> file.
>
> It's nice that you're testing all this, but you should ask *why* Cygwin
> does it in the first place.  The reason is that the information one can
> gather without opening the file on Windows is insufficient to fill in
> all of the stat struct.  The directory info returned by
> NtQueryDirectoryFile just isn't, thus it's only a fallback.

Corinna, thank you very much for your answer.
What info would be missing without opening the file ?

Do you know where the open call could come from, when only using  
NtQueryDirectoryFile in symlink_info::check() ?
(certainly related to the previous question)

Thank you !

Ben