Re: PgStat_HashKey padding issue when passed by reference

Hi,

On 2025-09-04 11:14:53 -0500, Sami Imseih wrote:
> GDB when setting a breakpoint at pgstat_entry_ref_hash_insert (which
> is called inside
> pgstat_get_entry_ref_cached) shows that the padding has some unexpected
> values, see the 12-15th byte "0x00 0x23 0x21 0x21" which based on the
> memset done earlier should be 0.
>
> ```
> (gdb) x/24xb d
> 0x7fff68cc16f0: 0x02 0x00 0x00 0x00 0x05 0x00 0x00 0x00
> 0x7fff68cc16f8: 0x00 0x00 0x00 0x00 0x00 0x23 0x21 0x21
> 0x7fff68cc1700: 0x67 0x0a 0x00 0x00 0x00 0x00 0x00 0x00
> (gdb)
> ```
>
> Now, if we pass the key by reference to pgstat_entry_ref_hash_insert as is
> patched, we can see the padding is in fact cleared.
>
> ```
> (gdb) x/24xb d
> 0x7ffe1bf6c640: 0x01 0x00 0x00 0x00 0x05 0x00 0x00 0x00
> 0x7ffe1bf6c648: 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
> 0x7ffe1bf6c650: 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
> ```
>
> This looks like a compiler bug, we tested multiple ways to workaround:

Padding bytes aren't guaranteed to be zeroed, unless you take care to zero
them out with memset or such. That's not a compiler bug.

Greetings,

Andres Freund