Re: Expanding the use of FLEXIBLE_ARRAY_MEMBER for declarations like foo[1]

On 2015-02-18 17:15:18 +0900, Michael Paquier wrote:
> >> - I don't think that the t_bits fields in htup_details.h should be
> >> updated either.
> >
> > Why not? Any not broken code should already use MinHeapTupleSize and
> > similar macros.
> 
> Changing t_bits impacts HeapTupleHeaderData, ReorderBufferTupleBuf and
> similarly a couple of redo routines in heapam.c using
> HeapTupleHeaderData in a couple of structures not placing it at the
> end (compiler complains).

The compiler will complain if you use a FLEXIBLE_ARRAY_MEMBER in the
middle of a struct but not when when you embed a struct that uses it
into the middle another struct. At least gcc doesn't and I think it'd be
utterly broken if another compiler did that. If there's a compiler that
does so, we need to make it define FLEXIBLE_ARRAY_MEMBER to 1.

Code embedding structs using *either* [FLEXIBLE_ARRAY_MEMBER] or [1] for
variable length obviously has to take care where the variable length
part goes. And that what the structs you removed where doing - that's
where the t_bits et al go:
{
...HeapTupleHeaderData header;char        data[MaxHeapTupleSize];
...
}
the 'data' bit is just the t_bits + data together. And similar in
-    struct
-    {
-        struct varlena hdr;
-        char        data[TOAST_MAX_CHUNK_SIZE]; /* make struct big enough */
-        int32        align_it;    /* ensure struct is aligned well enough */
-    }            chunk_data;

The 'data' is where the varlena's vl_dat stuff is stored.
> >> Places using a variable-length variable not at the end of a structure
> >> are changed with workaround, without impacting what those features do.
> >
> > I vote for rejecting most of this, except a (corrected version) of the
> > pg_authid change. Which doesn't really belong to the rest of the patch
> > anyway ;)x
> 
> Changing bytea to use FLEXIBLE_ARRAY_MEMBER requires those changes, or
> at least some changes in this area as something with
> FLEXIBLE_ARRAY_MEMBER can only be placed at the end of a structure.

Again, I think you're confusing things that may not be be done in a
single struct, and structs that are embedded in other places.

Greetings,

Andres Freund

-- Andres Freund                       http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training &
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