Re: Add LZ4 compression in pg_dump

On Sun, Jan 15, 2023 at 07:56:25PM -0600, Justin Pryzby wrote:
> On Mon, Jan 16, 2023 at 10:28:50AM +0900, Michael Paquier wrote:
>> The functions changed by 0001 are cfopen[_write](),
>> AllocateCompressor() and ReadDataFromArchive().  Why is it a good idea
>> to change these interfaces which basically exist to handle inputs?
>
> I changed to pass pg_compress_specification as a pointer, since that's
> the usual convention for structs, as followed by the existing uses of
> pg_compress_specification.

Okay, but what do we gain here?  It seems to me that this introduces
the risk that a careless change in one of the internal routines if
they change slight;ly compress_spec, hence impacting any of their
callers?  Or is that fixing an actual bug (except if I am missing your
point, that does not seem to be the case)?

>> Is there some benefit in changing compression_spec within the
>> internals of these routines before going back one layer down to their
>> callers?  Changing the compression_spec on-the-fly in these internal
>> paths could be risky, actually, no?
>
> I think what you're saying is that if the spec is passed as a pointer,
> then the called functions shouldn't set spec->algorithm=something.

Yes.  HEAD makes sure of that, 0001 would not prevent that.  So I am a
bit confused in seeing how this is a benefit.
--
Michael