Hi Junio,
On Mon, 16 Dec 2019, Junio C Hamano wrote:
> Johannes Schindelin <Johannes.Schindelin@xxxxxx> writes:
>
> > If you care to look at our very own `compat/win32/dirent.h`, you will see
> > this:
> >
> > struct dirent {
> > unsigned char d_type; /* file type to prevent lstat after readdir */
> > char *d_name; /* file name */
> > };
> >
> > And looking at
> > https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/dirent.h.html, I
> > do not see any guarantee of that `[256]` at all:
> >
> > The <dirent.h> header shall [...] define the structure dirent which shall
> > include the following members:
> >
> > [XSI][Option Start]
> > ino_t d_ino File serial number.
> > [Option End]
> > char d_name[] Filename string of entry.
> >
> > You will notice that not even `d_type` is guaranteed.
>
> I am reasonably sure that the code (without Elijah's patches anyway)
> takes the possibility of missing d_type into account already.
>
> Doesn't the above mean d_name[] has to be an in-place array of some
> size (i.e. even a flex-array is OK)? It does not look to me that it
> allows for it to be a pointer pointing at elsewhere (possibly on
> heap), which may be asking for trouble.
You are right, of course.
I also was not _quite_ spot on, as I had looked at Git for Windows'
`shears/pu` branch, not at the `pu` branch. Alas, we have patches in Git
for Windows that allow for switching to a faster, caching way to access
the stat() and readdir() data (it is called the "FSCache" and it is
responsible for some rather dramatic speed-ups). And these patches change
`struct dirent` to the form that is quoted above, to allow switching back
and forth between two _different_ backends, storing the actual `d_name`
not in `struct dirent` but in `DIR`.
Is this compliant with POSIX? I guess not. Does it work? Yes, it does.
I can't know for sure that it makes a dent, but FSCache is designed for
speed, and it actually does not even store the `d_name` in the `DIR`, but
directly in the cache structure, avoiding copying at all.
In short: if we can allow FSCache to keep operating that way (i.e. keep
`d_name` as a pointer), then that would be helpful to keep the performance
on Windows somewhat within acceptable boundaries.
Ciao,
Dscho
