Re: Patch to allow pg_filedump to support reading of pg_filenode.map

Thanks for the feedback, Justin. I've gone ahead and switched to use
memcmp. I also refactored it to:

1. Don't assume that any file with first 4 bytes matching the
relmapper magic number is a pg_relnode.map file
2. Don't assume the pg_relnode.map file is uncorrupted and intact; perform
a check of the first 4 bytes against the reference magic number
3. Provide a flag (-m) for users to have their file interpreted as a
pg_relnode.map file

I hope this is more palatable to everyone :)

--Richard

On Wed, Apr 28, 2021 at 9:42 PM Justin Pryzby <pryzby(at)telsasoft(dot)com> wrote:

> This is separate from the postgresql server repo.
> https://git.postgresql.org/gitweb/?p=pg_filedump.git
>
> +#define RELMAPPER_FILEMAGIC 0x592717
> +char magic_buffer[8];
>
> ...
>
> + if ( (int) magic_buffer & RELMAPPER_FILEMAGIC ) {
>
> This is doing bitwise arithmetic on a pointer, which seems badly wrong.
> I think it breaks normal use of pg_filedump - unless you happen to get a
> magic_buffer without those bits set. The segfault seems to confirm that,
> as
> does gcc:
>
> pg_filedump.c:2041:8: warning: cast from pointer to integer of different
> size [-Wpointer-to-int-cast]
> 2041 | if ( (int) magic_buffer & RELMAPPER_FILEMAGIC ) {
>
> I think it probably means to do memcmp, instead ??
>
> --
> Justin
>