[git/2.1 regression] Fix opj_write_tile() failure when numresolutions=1

[rouault]

When trying the GDAL OpenJPEG driver against openjpeg current master HEAD,
I get failures when trying to create .jp2 files. The driver uses
opj_write_tile() and in some tests numresolutions = 1.

In openjp2/dwt.c:410, l_data_size = opj_dwt_max_resolution( tilec->resolutions,tilec->numresolutions) * (OPJ_UINT32)sizeof(OPJ_INT32);
is called and returns l_data_size = 0. Now in git opj_malloc() has a special case
for 0 to return a NULL pointer whereas previously it relied on system malloc(),
which in my case didn't return NULL.

So only test the pointer value if l_data_size != 0. This makes the GDAL
autotest suite to pass again.

[stweil]

Maybe it would be better to return a pointer to a fixed address which points to const memory instead of returning NULL. Then callers would not need special handling.

[rouault]

Maybe it would be better to return a pointer to a fixed address which points to const memory instead of returning NULL. Then callers would not need special handling.

Well you'd have also to make a special case in opj_free() to not free this address. Another solution would be to do malloc(1) when opj_malloc(0) is called

[stweil]

Sure, opj_free must ignore that address. Using malloc would only be an option if it had the same defined behavior on all platforms when called with 0.

[rouault]

I suggested using malloc(1)

[mayeut]

Isn't that linked to #239 , #215 ? I think that there's something wrong other than malloc returning NULL for size 0 which is, I think, sane behavior. c.f. #635 for the sane behavior of opj_malloc

[rouault]

@mayeut I don't think it's connected with #239. Whe comparing HEAD with 2.1, both return l_data_size = 0 but the difference is that HEAD now fails because malloc(0) now returns NULL which is considered as an error.

[malaterre]

Could you please handle the case l_data_size == 0 && bj == NULL

[rouault]

@malaterre Not sure to understand what you mean. It is the nominal case handled by the code after the if (l_data_size != 0 && ! bj) check. What I know is that with my patch things work again as in 2.1

[rouault]

Hum, or perhaps do you mean that if tilec->numresolutions == 1 we could do an early exit return OPJ_TRUE at the beginning of the function ? (since when looking closer the code after the if (l_data_size != 0 && ! bj) check ends up not entering the while (i--) loop )

[malaterre]

@rouault my bad. Could you please add comment stating that bj will not be used when l_data_size == 0.

[rouault]

Comment added per 6a1974d