change in behavior in 4.7.4 in handling in nc_def_var_deflate()

[edwardhartnett]

Another change that accompanied the recent filter changes was a change in the way nc_def_var_deflate() behaves when called more than once for the same var before enddef.

It used to be that nc_def_var_deflate() could be called repeatedly, and the settings could be changed, until enddef.

Now, the first call to nc_def_var_deflate() sets the deflate settings. Subsequent calls are ignored.

This was never tested in netCDF tests, but it does come up in PIO tests, so I'm sorry I didn't notice this before - I've been spending my time on the GFS weather model instead. ;-)

I'm putting together a test now to demonstrate the problem. @WardF if there is going to soon be a new release, with a reverse of some of the other changes in this code, then we probably want to get this fix in as well before that.

PIO counts on this behavior,. and its not hard to see how other programs may as well, especially fortran codes, which may set all vars to a default deflate, and then allow individual vars to be set to different levels as needed. This worked before, but with 4.7.4 all vars will receive the default deflate settings, and subsequent changes will be ignored.

[edwardhartnett]

@DennisHeimbigner in the old code, setting the deflate setting just changed values in the NC_VAR_INFO_T struct, and then, at enddef, whatever values were present were used.

But now the filter is put on a list as soon as def_var_deflate() is called(?). I think you want to hold off until enddef. That's how everything else works, and there are subtle side-effects when it comes to renaming.

Alternatively, have subsequent calls to nc_def_var_deflate() find the item in the list and change the settings. Then, like the old code, when enddef occurs you will be using the most recent settings, not the first settings.

[DennisHeimbigner]

Doh! It did not occur to me to consider the case where
nc_def_var_filter was called multiple times with different
filter values before nc_enddef was called. I will fix.
Thanks for point this out.

[DennisHeimbigner]

There is a potential issue though. THe order in which nc_def_var
is called determines the order in which the filters are applied.
If one calls nc_def_var_filter twice, should the filter application order change?
I would say no. That you need to remove the filter first before adding it
back in order to change the application order.

[edwardhartnett]

Prior to your filter work, the only two-filter combo was zlib + shuffle. So that's the only legacy case we have to worry about. And those two filters are set by the same function call, so no ordering is needed.

I think you need to handle what happens if they turn shuffle on then off again. The answer in all cases is that the most recent call to nc_def_var_deflate() overrides all previous calls, until enddef.

For other filters, I think you can define whatever rules you want; I suggest you document them in the code and in the doxygen docs, so it will be clear and others working on the filter code in the future can bear it in mind.

[DennisHeimbigner]

ok, Then the rules I propose are as follows:

Order of Invocation for Multiple Filters

When multiple filters are defined on a variable, the
order of application, when writing data to the file,
is same as the order in which nc_def_var_filter is called.
When reading a file the order of application is of necessity
the reverse.

There are some special cases.

1. If nc_def_var_filter or nc_def_var_deflate or nc_def_var_szip
   is called multiple times with the same filter id, but possibly
   with different sets of parameters, then the position of that filter
   in the sequence of applictions does not change. However the last set
   of parameters specified is used when actually writing the dataset.
2. Deflate and shuffle -- these two are inextricably linked in the
   current API. If you call nc_def_var_deflate multiple times, then
   the previous rule applies with respect to deflate. However,
   the shuffle filter, if enabled, is ''always'' applied just before
   applying deflate.
3. If you want to move the location of a filter in the application
   sequence, then you must remove it using nc_var_filter_remove
   or using nc_def_var_deflate. The next time you add the filter
   back, its position will be at the end of the current sequence.

[WardF]

Sounds good to me, @DennisHeimbigner

[DennisHeimbigner]

Minor changes.

1. fletcher32 is defined before any other filters
2. shuffle is defined before any filters but after fletcher32
   I think fletcher always needs to be first since it is a checksum.
   It is possible that shuffle should only be used with deflate
   and no other filters, but I am not sure.

[edwardhartnett]

Shuffle may be used with any compression filter, so deflate, szip, or with a third party compression filter.

I forgot about fletcher! ;-)

[edwardhartnett]

I believe this is fixed now, I will close this issue.