Re: Dereferenced pointers checked as NULL in btree_utils_var.c

On 01/21/2015 07:14 AM, Michael Paquier wrote:
> Also,
> looking at the code of gist, gbt_var_same is called through
> gistKeyIsEQ, which is used for an insertion or for a split. The point
> is that when doing an insertion, a call to gistgetadjusted is done and
> we look if one of the keys is NULL. If one of them is, code does not
> go through gistKeyIsEQ, so the NULL checks do not seem necessary in
> gbt_var_same.

I think you are confusing NULL pointers with an SQL NULL. 
gistgetadjusted checks that it's not an SQL NULL (!oldisnull[i]), but it 
does not check if it's a NULL pointer 
(DatumGetPointer(oldentries[i].key) != NULL). The case we're worried 
about is that the value is not an SQL NULL, i.e. isnull flag is false, 
but the Datum is a NULL pointer.

Nevertheless, looking at the code, I don't that a NULL pointer can ever 
be passed to the same-method, for any of the built-in or contrib 
opclasses, but it's very confusing because some functions check for 
that. My proof goes like this:

1. The key value passed as argument must've been originally formed by 
the compress, union, or picksplit methods.

1.1. Some compress methods do nothing, and just return the passed-in 
key, which comes from the table and cannot be a NULL pointer (the 
compress method is never called for SQL NULLs). Other compress methods 
don't check for a NULL pointer, and would crash if there was one. 
gist_poly_compress() and gist_circle_compress() do check for a NULL, but 
they only return a NULL key if the input key is NULL, which cannot 
happen because the input comes from a table. So I believe the 
NULL-checks in those functions are dead code, and none of the compress 
methods ever return a NULL key.

1.2. None of the union methods return a NULL pointer (nor expect one as 
input).

1.3. None of the picksplit methods return a NULL pointer. They all 
return one of the original values, or one formed with the union method. 
The picksplit method can return a NULL pointer in the spl_ldatum or 
spl_rdatum field, in the degenerate case that it puts all keys on the 
same halve. However, the caller, gistUserPickSplit checks for that and 
immediately overwrites spl_ldatum and spl_rdatum with sane values in 
that case.


I don't understand what this sentence in the documentation on the 
compress method means:

> Depending on your needs, you could also need to care about
> compressing NULL values in there, storing for example (Datum) 0 like
> gist_circle_compress does.

The compress method is never called for NULLs, so the above is nonsense. 
I think it should be removed, as well as the checks in 
gist_circle_compress and gist_poly_compress. According to git history, 
the check in gist_circle_compress been there ever since the module was 
imported into contrib/rtree_gist, in 2001. The documentation was added 
later:

commit a0a3883dd977d6618899ccd14258a0696912a9d2
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date:   Fri Jun 12 19:48:53 2009 +0000
    Improve documentation about GiST opclass support functions.    Dimitri Fontaine

I'm guessing that Tom added that sentence (it was not in the patch that 
Dimitri submitted originally) just because there was that check in the 
existing function, without realizing that the check was in fact dead code.

- Heikki