Re: some Page/PageData const stuff

This has been committed.

On 09.12.24 16:44, Peter Eisentraut wrote:
> In [0] I wrote:
> 
> """
> I was fiddling a bit with making some Page-related APIs const-proof, 
> which might involve changing something like "Page p" to "const PageData 
> *p", but I was surprised that a type PageData exists but it's an 
> unrelated type local to generic_xlog.c.
> 
> This patch renames that type to a more specific name 
> [GenericXLogPageData].  This makes room for possibly adding another 
> PageData type with the earlier meaning, but that's not done here.
> 
> """
> 
> [0]: https://www.postgresql.org/message-id/flat/001d457e-c118-4219-8132- 
> e1846c2ae3c9%40eisentraut.org
> 
> This is now the follow-up that adds back PageData with the earlier 
> meaning and updates a few of the Page-related APIs to be const-proof. 
> That is all pretty straightforward, except one inline function that had 
> to be changed back to a macro, because it is used in a way that 
> sometimes it takes const and returns const and sometimes takes non-const 
> and returns non-const.  (We might be able to do that kind of thing 
> better with C23 in N years. ;-) )
> 
> Just a thought, I've been thinking it might be neat if PageData were 
> actually defined something like this:
> 
> typedef struct PageData
> {
>      union
>      {
>          PageHeaderData phdr;
>          PGAlignedBlock data;
>      };
> } PageData;
> 
> Then you could write all those (PageHeader) casts in a more elegant way, 
> and you don't get to randomly mix char * and Page, which has very weak 
> type safety.  But this currently totally breaks, because many places 
> assume you can do char-based pointer arithmetic with Page values.  So 
> this would need further analysis.